Seedling root responses to soil moisture and the identification of a belowground trait spectrum across three growth forms

The root strategy of the C4 grasses tends to be 'do-it-yourself'

Root resource acquisition strategies play a crucial role in understanding plant water uptake and drought adaptation. However, the interrelationships among mycorrhizal associations, root hair development, and fine root strategies, as well as the disparities between C3 and C4 grasses, remain largely unknown. A pot experiment was conducted to determine leaf gas exchange, root morphology, root hair, mycorrhizal fungi, and biomass allocation of three C4 grasses and four C3 grasses, common species of grasslands in Northeast China, under the control and drought conditions. Compared to the C3 grasses, the C4 grasses increased specific surface area by decreasing tissue density, yet exhibited root hair factor at only 21 % of the C3 grasses. Under the drought conditions, the C4 grasses exhibited more intense and extensive adjustments in root traits, characterized by shifts toward a more conservative morphology with increased root diameter and tissue density, as well as reduced mycorrhizal colonization rates. These adaptations led to a decrease in root absorptive function, which was compensated in the C4 grasses by greater root biomass partitioning and root hair factor. Variances in root strategies between plants functional groups were closely related to leaf photosynthetic rate, water and nitrogen use efficiency. We observed that the C4 grasses prefer direct acquisition of soil resources through the fine root pathway over the root hair or mycorrhizal pathway, suggesting a 'do-it-yourself' approach. These findings provide valuable insights into how plant communities of different photosynthetic types might respond to future climate change.

Naturalized species drive functional trait shifts in plant communities

Despite decades of research documenting the consequences of naturalized and invasive plant species on ecosystem functions, our understanding of the functional underpinnings of these changes remains rudimentary. This is partially due to ineffective scaling of trait differences between native and naturalized species to whole plant communities. Working with data from over 75,000 plots and over 5,500 species from across the United States, we show that changes in the functional composition of communities associated with increasing abundance of naturalized species mirror the differences in traits between native and naturalized plants. We find that communities with greater abundance of naturalized species are more resource acquisitive aboveground and belowground, shorter, more shallowly rooted, and increasingly aligned with an independent strategy for belowground resource acquisition via thin fine roots with high specific root length. We observe shifts toward herbaceous-dominated communities but shifts within both woody and herbaceous functional groups follow community-level patterns for most traits. Patterns are remarkably similar across desert, grassland, and forest ecosystems. Our results demonstrate that the establishment and spread of naturalized species, likely in combination with underlying environmental shifts, leads to predictable and consistent changes in community-level traits that can alter ecosystem functions.

Belowground plant allocation regulates rice methane emissions from degraded peat soils

Carbon-rich peat soils have been drained and used extensively for agriculture throughout human history, leading to significant losses of their soil carbon. One solution for rewetting degraded peat is wet crop cultivation. Crops such as rice, which can grow in water-saturated conditions, could enable agricultural production to be maintained whilst reducing CO2 and N2O emissions from peat. However, wet rice cultivation can release considerable methane (CH4). Water table and soil management strategies may enhance rice yield and minimize CH4 emissions, but they also influence plant biomass allocation strategies. It remains unclear how water and soil management influences rice allocation strategies and how changing plant allocation and associated traits, particularly belowground, influence CH4-related processes. We examined belowground biomass (BGB), aboveground biomass (AGB), belowground:aboveground ratio (BGB:ABG), and a range of root traits (root length, root diameter, root volume, root area, and specific root length) under different soil and water treatments; and evaluated plant trait linkages to CH4. Rice (Oryza sativa L.) was grown for six months in field mesocosms under high (saturated) or low water table treatments, and in either degraded peat soil or degraded peat covered with mineral soil. We found that BGB and BGB:AGB were lowest in water saturated conditions where mineral soil had been added to the peat, and highest in low-water table peat soils. Furthermore, CH4 and BGB were positively related, with BGB explaining 60% of the variation in CH4 but only under low water table conditions. Our results suggest that a mix of low water table and mineral soil addition could minimize belowground plant allocation in rice, which could further lower CH4 likely because root-derived carbon is a key substrate for methanogenesis. Minimizing root allocation, in conjunction with water and soil management, could be explored as a strategy for lowering CH4 emissions from wet rice cultivation in degraded peatlands.

Reproductive height determines the loss of clonal grasses with nitrogen enrichment in a temperate grassland

Tall clonal grasses commonly display competitive advantages with nitrogen (N) enrichment. However, it is currently unknown whether the height is derived from the vegetative or reproductive module. Moreover, it is unclear whether the height of the vegetative or reproductive system regulates the probability of extinction and colonization, and determines species diversity. In this study, the impacts on clonal grasses were studied in a field experiment employing two frequencies (twice a year vs. monthly) crossing with nine N addition rates in a temperate grassland, China. We found that the N addition decreased species frequency and increased extinction probability, but did not change the species colonization probability. A low frequency of N addition decreased species frequency and colonization probability, but increased extinction probability. Moreover, we found that species reproductive height was the best index to predict the extinction probability of clonal grasses in N-enriched conditions. The low frequency of N addition may overestimate the negative effect from N deposition on clonal grass diversity, suggesting that a higher frequency of N addition is more suitable in assessing the ecological effects of N deposition. Overall, this study illustrates that reproductive height was associated with the clonal species extinction probability under N-enriched environment.

Biomass allocation and plant morphology explain the difference in shrub species abundance in a temperate forest

In forested ecosystems, shrubs must succeed in persisting in low-light environments, while simultaneously having the ability to rapidly expand and occupy newly created canopy openings, yet little is known about the traits that make this possible. We hypothesize that shrub species that are abundant in the understory exhibit a specific set of functional traits that define their ability to persist during unfavorable periods and to rapidly exploit newly created habitats. We tested this by comparing field-measured functional traits such as biomass allocation, leaf display, crown morphology, and leaf traits, across individual size classes and two gap-forest environments of five shrub species. We observed significant differences in traits between species, size classes, and gap-forest environments. These differences were primarily related to biomass allocation traits, followed by leaf display, crown morphology, and leaf traits. Abundant shrubs like mountain maple (Acer spicatum) and hazelnut (Corylus cornuta) invested significantly more biomass in roots, had a larger total leaf area, and displayed leaves in a more efficient manner to intercept light. The high investment in root biomass can be interpreted as shrubs exploiting the persistence and colonization strategy through resprouting. Permanent sub-canopy status likely explains the importance of efficient leaf display, wherein abundant shrubs had a large leaf area with minimal support structures.

Recovering from trampling: The role of dauciform roots to functional traits response of Carex filispica in alpine meadow

In the natural habitats of China, dauciform roots were only described in degraded alpine meadows. It was found that the presence of dauciform roots of Carex filispica was related to the advantage of multiple functional traits after trampling, reflecting short-term resistance. However, the long-term response of dauciform roots to trampling and the recovery of C. filispica with and without dauciform roots to trampling require further studies. In this study, different intensities of trampling (0, 50, 200 and 500 passages) were performed in an alpine meadow. One year later, individuals with and without dauciform roots were separated and their functional traits related to the economic spectrum of leaves and roots were measured as a reflection of recovery from trampling. The results showed that: (1) 1 year after trampling, the number of dauciform roots showed an increase with trampling intensity; (2) 1 year later, there was no significant difference in the response of economic spectrum traits among trampling intensities, or between plants with and without dauciform roots; (3) the number of dauciform roots was positively correlated with the leaf area of both individuals with and without dauciform roots, as well as with the biomass of those without dauciform roots; and (4) plants with more resource-conservative roots showed an advantage after trampling recovery: specifically, plants with dauciform roots showed such an advantage in the control group, which was lost with a leaning towards resource-acquisitive roots and an increased density of dauciform roots once trampled. In contrast, plants without dauciform roots showed a significant advantage of conservative roots only after trampling. In conclusion, the presence of dauciform roots is related to the plants' position on the root economic spectrum, thereby influencing the recovery of C. filispica from trampling. Carex filispica showed strong recovery from trampling after 1 year, which makes it an adequate choice for ecological restoration in alpine meadows. Dauciform roots showed a positive correlation with the aboveground growth of both plants with and without them, however, it requires a lab-controlled study to confirm whether there is indeed a positive effect on the growth of neighbouring plants.

Clinal variations in seedling traits and responses to water availability correspond to seed-source environmental gradients in a foundational dryland tree species

BACKGROUND AND AIMS

In dryland ecosystems, conifer species are threatened by more frequent and severe droughts, which can push species beyond their physiological limits. Adequate seedling establishment will be critical for future resilience to global change. We used a common garden glasshouse experiment to determine how seedling functional trait expression and plasticity varied among seed sources in response to a gradient of water availability, focusing on a foundational dryland tree species of the western USA, Pinus monophylla. We hypothesized that the expression of growth-related seedling traits would show patterns consistent with local adaptation, given clinal variation among seed source environments.

METHODS

We collected P. monophylla seeds from 23 sites distributed across rangewide gradients of aridity and seasonal moisture availability. A total of 3320 seedlings were propagated with four watering treatments representing progressively decreasing water availability. Above- and below-ground growth-related traits of first-year seedlings were measured. Trait values and trait plasticity, here representing the degree of variation among watering treatments, were modelled as a function of watering treatment and environmental conditions at the seed source locations (i.e. water availability, precipitation seasonality).

KEY RESULTS

We found that, under all treatments, seedlings from more arid climates had larger above- and below-ground biomass compared to seedlings from sites experiencing lower growing-season water limitation, even after accounting for differences in seed size. Additionally, trait plasticity in response to watering treatments was greatest for seedlings from summer-wet sites that experience periodic monsoonal rain events.

CONCLUSIONS

Our results show that P. monophylla seedlings respond to drought through plasticity in multiple traits, but variation in trait responses suggests that different populations are likely to respond uniquely to changes in local climate. Such trait diversity will probably influence the potential for future seedling recruitment in woodlands that are projected to experience extensive drought-related tree mortality.

Short wind pulses consistently change the morphology of roots, but not of shoots, across young plants of different growth forms

Wind is an environmental stimulus that stresses plants of all growth forms at all life-stages by influencing the development, architecture, and morphology of roots and shoots. However, comparative studies are scarce and no study directly investigated whether shoot and root morphological traits of trees, grasses and forbs differ in their response to short wind pulses of different wind intensity. In this study, we found that across species, wind stress by short wind pulses of increasing intensity consistently changed root morphology, but did not affect shoot morphological traits, except plant height in four species. Wind effects in roots were generally weak in tree species but consistent across growth forms. Furthermore, plant height of species was correlated with changes in specific root length and average diameter.Our results indicate that short-pulse wind treatments affect root morphology more than shoot morphology across growth forms. They further suggest that wind stress possibly promotes root anchorage in young plants and that these effects might depend on plant height.

Interacting ecological filters influence success and functional composition in restored plant communities over time

A trait-based community assembly framework has great potential to direct ecological restoration, but uncertainty over how traits and environmental factors interact to influence community composition over time limits the widespread application of this approach. In this study, we examined how the composition of seed mixes and environment (north- vs. south-facing slope aspect) influence functional composition and native plant cover over time in restored grassland and shrubland communities. Variation in native cover over 4 years was primarily driven by species mix, slope aspect, and a species mix by year interaction rather than an interaction between species mix and slope aspect as predicted. Although native cover was higher on wetter, north-facing slopes for most of the study, south-facing slopes achieved a similar cover (65%-70%) by year 4. While community-weighted mean (CWM) values generally became more resource conservative over time, we found shifts in particular traits across community types and habitats. For example, CWM for specific leaf area increased over time in grassland mixes. Belowground, CWM for root mass fraction increased while CWM for specific root length decreased across all seed mixes. Multivariate functional dispersion remained high in shrub-containing mixes throughout the study, which could enhance invasion resistance and recovery following disturbance. Functional diversity and species richness were initially higher in drier, south-facing slopes compared to north-facing slopes, but these metrics were similar across north- and south-facing slopes by the end of the 4-year study. Our finding that different combinations of traits were favored in south- and north-facing slopes and over time demonstrates that trait-based approaches can be used to identify good restoration candidate species and, ultimately, enhance native plant cover across community types and microhabitat. Changing the composition of planting mixes based on traits could be a useful strategy for restoration practitioners to match species to specific environmental conditions and may be more informative than using seed mixes based on growth form, as species within functional groups can vary tremendously in leaf and root traits.

Drought Eliminates the Difference in Root Trait Plasticity and Mycorrhizal Responsiveness of Two Semiarid Grassland Species with Contrasting Root System

Root traits and arbuscular mycorrhizal (AM) fungi are important in determining the access of plants to soil resources. However, whether plants with different root systems (i.e., taproot vs. fibrous-root) exhibit different root trait plasticity and mycorrhizal responsiveness under drought remains largely unexplored. Tap-rooted Lespedeza davurica and fibrous-rooted Stipa bungeana were grown in monocultures in sterilized and live soils, followed by a drought treatment. Biomass, root traits, root colonization by AM fungi, and nutrient availability were evaluated. Drought decreased biomass and root diameter but increased the root:shoot ratio (RSR), specific root length (SRL), soil NO₃^--N, and available P for the two species. Under control and drought conditions, soil sterilization significantly increased the RSR, SRL, and soil NO₃^--N for L. davurica, but this only occurs under drought condition for S. bungeana. Soil sterilization significantly reduced AM fungal root colonization of both species, but drought significantly increased it in live soil. In water-abundant conditions, tap-rooted L. davurica may depend more on AM fungi than fibrous-rooted S. bungeana; however, under drought conditions, AM fungi are of equal importance in favoring both plant species to forage soil resources. These findings provide new insights for understanding the resource utilization strategies under climate change.

Complementary belowground strategies underlie species coexistence in an early successional forest

Unravelling belowground strategies is critical for understanding species coexistence and successional dynamics; yet, our knowledge of nutrient acquisition strategies of forest species at different successional stages remains limited. We measured morphological (diameter, specific root length, and root tissue density), architectural (branching ratio), physiological (ammonium, nitrate, and glycine uptake rates) root traits, and mycorrhizal colonisation rates of eight coexisting woody species in an early successional plantation forest in subtropical China. By incorporating physiological uptake efficiency, we revealed a bi-dimensional root economics space comprising of an 'amount-efficiency' dimension represented by morphological and physiological traits, and a 'self-symbiosis' dimension dominated by architectural and mycorrhizal traits. The early pioneer species relied on root-fungal symbiosis, developing densely branched roots with high mycorrhizal colonisation rates for foraging mobile soil nitrate. The late pioneer species invested in roots themselves and allocated effort towards improving uptake efficiency of less-mobile ammonium. Within the root economics space, the covariation of axes with soil phosphorus availability also distinguished the strategy preference of the two successional groups. These results demonstrate the importance of incorporating physiological uptake efficiency into root economics space, and reveal a trade-off between expanding soil physical space exploration and improving physiological uptake efficiency for successional species coexistence in forests.

Above- and below-ground plant traits are not consistent in response to drought and competition treatments

BACKGROUND AND AIMS

Our understanding of plant responses to biotic and abiotic drivers is largely based on above-ground plant traits, with little focus on below-ground traits despite their key role in water and nutrient uptake. Here, we aimed to understand the extent to which above- and below-ground traits are co-ordinated, and how these traits respond to soil moisture gradients and plant intraspecific competition.

METHODS

We chose seedlings of five tropical tree species and grew them in a greenhouse for 16 weeks under a soil moisture gradient [low (drought), medium and high (well-watered) moisture levels] with and without intraspecific competition. At harvest, we measured nine above- and five below-ground traits of all seedlings based on standard protocols.

KEY RESULTS

In response to the soil moisture gradient, above-ground traits are found to be consistent with the leaf economics spectrum, whereas below-ground traits are inconsistent with the root economics spectrum. We found high specific leaf area and total leaf area in well-watered conditions, while high leaf dry matter content, leaf thickness and stem dry matter content were observed in drought conditions. However, below-ground traits showed contrasting patterns, with high specific root length but low root branching index in the low water treatment. The correlations between above- and below-ground traits across the soil moisture gradient were variable, i.e. specific leaf area was positively correlated with specific root length, while it was negatively correlated with root average diameter across moisture levels. However, leaf dry matter content was unexpectedly positively correlated with both specific root length and root branching index. Intraspecific competition has influenced both above- and below-ground traits, but interacted with soil moisture to affect only below-ground traits. Consistent with functional equilibrium theory, more biomass was allocated to roots under drought conditions, and to leaves under sufficient soil moisture conditions.

CONCLUSIONS

Our results indicate that the response of below-ground traits to plant intraspecific competition and soil moisture conditions may not be inferred using above-ground traits, suggesting that multiple resource use axes are needed to understand plant ecological strategies. Lack of consistent leaf-root trait correlations across the soil moisture gradient highlight the multidimensionality of plant trait relationships which needs more exploration.

Responses of root system architecture to water stress at multiple levels: A meta-analysis of trials under controlled conditions

Plants are exposed to increasingly severe drought events and roots play vital roles in maintaining plant survival, growth, and reproduction. A large body of literature has investigated the adaptive responses of root traits in various plants to water stress and these studies have been reviewed in certain groups of plant species at a certain scale. Nevertheless, these responses have not been synthesized at multiple levels. This paper screened over 2000 literatures for studies of typical root traits including root growth angle, root depth, root length, root diameter, root dry weight, root-to-shoot ratio, root hair length and density and integrates their drought responses at genetic and morphological scales. The genes, quantitative trait loci (QTLs) and hormones that are involved in the regulation of drought response of the root traits were summarized. We then statistically analyzed the drought responses of root traits and discussed the underlying mechanisms. Moreover, we highlighted the drought response of 1-D and 2-D root length density (RLD) distribution in the soil profile. This paper will provide a framework for an integrated understanding of root adaptive responses to water deficit at multiple scales and such insights may provide a basis for selection and breeding of drought tolerant crop lines.

Using Genomic Selection to Develop Performance-Based Restoration Plant Materials

Effective native plant materials are critical to restoring the structure and function of extensively modified ecosystems, such as the sagebrush steppe of North America’s Intermountain West. The reestablishment of native bunchgrasses, e.g., bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] À. Löve), is the first step for recovery from invasive species and frequent wildfire and towards greater ecosystem resiliency. Effective native plant material exhibits functional traits that confer ecological fitness, phenotypic plasticity that enables adaptation to the local environment, and genetic variation that facilitates rapid evolution to local conditions, i.e., local adaptation. Here we illustrate a multi-disciplinary approach based on genomic selection to develop plant materials that address environmental issues that constrain local populations in altered ecosystems. Based on DNA sequence, genomic selection allows rapid screening of large numbers of seedlings, even for traits expressed only in more mature plants. Plants are genotyped and phenotyped in a training population to develop a genome model for the desired phenotype. Populations with modified phenotypes can be used to identify plant syndromes and test basic hypotheses regarding relationships of traits to adaptation and to one another. The effectiveness of genomic selection in crop and livestock breeding suggests this approach has tremendous potential for improving restoration outcomes for species such as bluebunch wheatgrass.

Insights into the Interactions Between Root Phenotypic Traits and the Rhizosphere Bacterial Community

The root phenotypic traits have been considered as important factors in shaping the rhizosphere microbiome and regulating plant growth. However, the relationships between root phenotypic traits and the rhizosphere bacterial community remain unclear. We investigated two fields with different developing tobacco roots by a long-term positioning test in Hengshi. The well-developed root system (WDR) showed much more superiority in root phenotypic traits, including total root length, total projection area, surface area, and root tip number, than the underdeveloped root system. The specific root traits in WDR provided more ecological niches for the rhizosphere microorganisms, contributing to a more diverse microbial community and a more complex microbial network. The total root length and root tip number were the key factors shaping bacterial communities in the rhizosphere. In turn, the phyla Acidobacteria and Bacteroidetes might play vital roles in modifying root development and promoting plant growth according to their positive correlation with root phenotypic traits. Linking root phenotypic traits to the microbiome may enhance our understanding of rhizospheric interactions and their roles in developing rhizosphere ecosystems.

Effects of superhydrophobic sand mulching on evapotranspiration and phenotypic responses in tomato (Solanum lycopersicum) plants under normal and reduced irrigation

Irrigated agriculture in arid and semi-arid regions is a vital contributor to the global food supply. However, these regions endure massive evaporative losses that are compensated by exploiting limited freshwater resources. To increase water-use efficiency in these giga-scale operations, plastic mulches are utilized; however, their non-biodegradability and eventual land-filling renders them unsustainable. In response, we have developed superhydrophobic sand (SHS) mulching technology that is comprised of sand grains or sandy soils with a nanoscale coating of paraffin wax. Here, we investigate the effects of 1 cm-thick SHS mulching on the evapotranspiration and phenotypic responses of tomato (Solanum lycopersicum) plants as a model system under normal and reduced irrigation inside controlled growth chambers. Experimental results reveal that under either irrigation scenario, SHS mulching suppresses evaporation and enhances transpiration by 78% and 17%, respectively relative to the unmulched soil. Comprehensive phenotyping revealed that SHS mulching enhanced root xylem vessel diameter, stomatal aperture, stomatal conductance, and chlorophyll content index by 21%, 25%, 28%, and 23%, respectively, in comparison with the unmulched soil. Consequently, total fruit yields, total dry mass, and harvest index increased in SHS-mulched plants by 33%, 20%, and 16%, respectively compared with the unmulched soil. We also provide mechanistic insights into the effects of SHS mulching on plant physiological processes. These results underscore the potential of SHS for realizing food-water security and greening initiatives in arid regions.

Phenotypic differentiation and diversifying selection in populations of Eruca sativa along an aridity gradient

BACKGROUND

The aridity gradient in the eastern Mediterranean offers an opportunity to investigate intra-specific genetic differentiation and local adaptation in plant populations. Here we used genetic (FST) and quantitative trait (PST) differentiation to assess local adaptation among three natural populations of Eruca sativa (Brassicaceae) distributed along a climatic range representing desert, semi-arid and Mediterranean habitats.

RESULTS

Amplified fragment length polymorphism (AFLP) analysis revealed high genetic diversity in each population, but low genetic differentiation between populations and relatively high gene flow. Further phenotypic evaluation in a common garden experiment (conduced in a Mediterranean habitat) showed clear differences in phenological traits among populations (day of flowering and duration of the reproductive stage), shoot and root biomass, as well as fitness-related traits (total number of fruits and total seed weight). FST-PST comparison showed that PST values of the phenological traits, as well as below- and above-ground biomass and fitness-related traits, were higher than the FST values.

CONCLUSIONS

Overall, our results support the identification of genotypic and phenotypic differentiation among populations of E. sativa. Furthermore, the FST-PST comparison supports the hypothesis that these were subjected to past diversifying selection. Thus, the results clearly demonstrate adaptive divergence among populations along an aridity gradient, emphasize the ecological value of early flowering time in arid habitats, and contribute to our understanding of the possible impact of climate change on evolutionary processes in plant populations.

Cooperation between arbuscular mycorrhizal fungi and plant growth-promoting bacteria and their effects on plant growth and soil quality

The roles of arbuscular mycorrhizal fungi (AMF) and plant growth-promoting rhizobacteria (PGPR) in improving nutrition uptake and soil quality have been well documented. However, few studies have explored their effects on root morphology and soil properties. In this study, we inoculated Elymus nutans Griseb with AMF and/or PGPR in order to explore their effects on plant growth, soil physicochemical properties, and soil enzyme activities. The results showed that AMF and/or PGPR inoculation significantly enhanced aboveground and belowground vegetation biomass. Both single and dual inoculations were beneficial for plant root length, surface area, root branches, stem diameter, height, and the ratio of shoot to root, but decreased root volume and root average diameter. Soil total nitrogen, alkaline phosphatase, and urease activities showed significant growth, and soil electrical conductivity and pH significantly declined under the inoculation treatments. Specific root length showed a negative correlation with belowground biomass, but a positive correlation with root length and root branches. These results indicated that AMF and PGPR had synergetic effects on root morphology, soil nutrient availability, and plant growth.

Polyploidy but Not Range Size Is Associated With Seed and Seedling Traits That Affect Performance of Pomaderris Species

Ploidy and species range size or threat status have been linked to variation in phenotypic and phenological seed and seedling traits, including seed size, germination rate (speed) and seedling stature. There is surprisingly little known about the ecological outcomes of relationships between ploidy, key plant traits and the drivers of range size. Here we determined whether ploidy and range size in Pomaderris, a genus of shrubs that includes many threatened species, are associated with variation in seed and seedling traits that might limit the regeneration performance of obligate seeders in fire-prone systems. We experimentally quantified seed dormancy and germination processes using fire-related heat treatments and evaluated seedling performance under drought stress. We also examined the association of seed size with other seed and seedling traits. Polyploids had bigger seeds, a faster germination rate and larger and taller seedlings than diploids. There was a lack of any clear relationship between range size and seed or seedling traits. The ploidy effects observed for many traits are likely to be indirect and associated with the underlying seed size differences. These findings indicate that there is a higher potential competitive advantage in polyploid than diploid Pomaderris during regeneration, a critical stage in the post-fire environment. This insight to the regeneration phase may need to be considered when planning and prioritising management of threatened species.

Fast plants have water-use and drought strategies that balance rainfall retention and drought survival on green roofs

Green roofs can improve ecosystem services in cities; however, this depends on appropriate plant selection. For stormwater management, plants should have high water use to maximize retention and also survive dry periods. Plants adapted to wetter habitats develop "fast" traits for growth, whereas plants from drier habitats develop "slow" traits to conserve water use and survive drought. Therefore, we hypothesized that (1) plants with fast traits would have greater water use, (2) plants with slow traits would have greater drought tolerance, (3) fast-slow traits would be consistent across the plant, and (4) fast plants with greater water use could avoid drought stress. We evaluated 14 green roof species in a glasshouse experiment under well-watered (WW) and water-deficit (WD) conditions to determine relationships between fast-slow traits, water use, and drought resistance. Traits measured were shoot dry mass, specific leaf area (SLA), root mass fraction (RMF), and specific root length (SRL). Daily evapotranspiration per shoot dry mass was used to describe water use. Drought resistance was represented by (1) days to stomatal closure; (2) cumulative ET before stomatal closure; and (3) degree of iso-anisohydry (difference between midday leaf water potential (Ψ_MD ) of WW and WD plants; ΔΨ_MD ). Plants with greater water use had fast aboveground traits (greater shoot biomass and SLA). Plants with slow traits had greater drought tolerance as plants with lower shoot dry mass closed their stomata later under WD, and plants with greater root allocation were more anisohydric. Fast-slow traits were not consistent across the plant. Although SLA and SRL were positively related, SRL was not related to water use or drought resistance. Shoot dry mass was inversely related to SLA and had a stronger influence on stomatal closure. Though plants with greater water use under well-watered conditions closed their stomates earlier to avoid drought stress, they were not more isohydric (smaller ∆Ψ_MD ) and did not necessarily use more water under WD. Fast aboveground traits can be used to select green roof plants with high water use that avoid drought stress to optimize rainfall retention without jeopardizing drought survival. This will facilitate rapid plant selection using trait information from online databases.

Root traits as drivers of plant and ecosystem functioning: current understanding, pitfalls and future research needs

The effects of plants on the biosphere, atmosphere and geosphere are key determinants of terrestrial ecosystem functioning. However, despite substantial progress made regarding plant belowground components, we are still only beginning to explore the complex relationships between root traits and functions. Drawing on the literature in plant physiology, ecophysiology, ecology, agronomy and soil science, we reviewed 24 aspects of plant and ecosystem functioning and their relationships with a number of root system traits, including aspects of architecture, physiology, morphology, anatomy, chemistry, biomechanics and biotic interactions. Based on this assessment, we critically evaluated the current strengths and gaps in our knowledge, and identify future research challenges in the field of root ecology. Most importantly, we found that belowground traits with the broadest importance in plant and ecosystem functioning are not those most commonly measured. Also, the estimation of trait relative importance for functioning requires us to consider a more comprehensive range of functionally relevant traits from a diverse range of species, across environments and over time series. We also advocate that establishing causal hierarchical links among root traits will provide a hypothesis-based framework to identify the most parsimonious sets of traits with the strongest links on functions, and to link genotypes to plant and ecosystem functioning.

Drought stress triggers differential survival and functional trait responses in the establishment of Arnica montana seedlings

The establishment and survival of seedlings are critical stages in the life cycle of plants and therefore usually well timed to humid and favourable conditions. Climate projections suggest that the threatened mountain grassland species Arnica montana may be increasingly exposed to drought stress. However, studies that focus on the species' early development are missing. We evaluated impacts of drought-induced stress on A. montana seedlings in their early establishment phase and identified traits that could cause the species' fitness to decline. In a greenhouse experiment, we tested the response of A. montana seedlings to different drought levels (moderate, strong, extreme). To assess their fitness under increasing drought, we evaluated survival of the seedlings based on four senescence stages and measured the performance of above- and belowground morphological and physiological functional traits. Arnica montana seedlings showed high resistance to drought. Senescence accelerated and survival declined only under strong and extreme drought conditions. However, the seedlings' vegetative performance decreased even with moderate drought, as indicated by smaller values of most leaf traits and some root traits. Physiological trait response was less sensitive. Drought stress hinders the establishment and survival of A. montana seedlings. Following the functional trait responses to drought and their association with survival, we suggest declining leaf length, leaf width, and leaf number are sensitive traits that can lead to a decline in performance.

Leaf traits and performance vary with plant age and water availability in Artemisia californica

BACKGROUND AND AIMS

Leaf functional traits are strongly tied to growth strategies and ecological processes across species, but few efforts have linked intraspecific trait variation to performance across ontogenetic and environmental gradients. Plants are believed to shift towards more resource-conservative traits in stressful environments and as they age. However, uncertainty as to how intraspecific trait variation aligns with plant age and performance in the context of environmental variation may limit our ability to use traits to infer ecological processes at larger scales.

METHODS

We measured leaf physiological and morphological traits, canopy volume and flowering effort for Artemisia californica (California sagebrush), a dominant shrub species in the coastal sage scrub community, under conditions of 50, 100 and 150 % ambient precipitation for 3 years.

KEY RESULTS

Plant age was a stronger driver of variation in traits and performance than water availability. Older plants demonstrated trait values consistent with a more conservative resource-use strategy, and trait values were less sensitive to drought. Several trait correlations were consistent across years and treatments; for example, plants with high photosynthetic rates tended to have high stomatal conductance, leaf nitrogen concentration and light-use efficiency. However, the trade-off between leaf construction and leaf nitrogen evident in older plants was absent for first-year plants. While few traits correlated with plant growth and flowering effort, we observed a positive correlation between leaf mass per area and performance in some groups of older plants.

CONCLUSIONS

Overall, our results suggest that trait sensitivity to the environment is most visible during earlier stages of development, after which intraspecific trait variation and relationships may stabilize. While plant age plays a major role in intraspecific trait variation and sensitivity (and thus trait-based inferences), the direct influence of environment on growth and fecundity is just as critical to predicting plant performance in a changing environment.

Patterns of intraspecific trait variation along an aridity gradient suggest both drought escape and drought tolerance strategies in an invasive herb

BACKGROUND AND AIMS

In water-limited landscapes, some plants build structures that enable them to survive with minimal water (drought resistance). Instead of making structures that allow survival through times of water limitation, annual plants may invoke a drought escape strategy where they complete growth and reproduction when water is available. Drought escape and resistance each require a unique combination of traits and therefore plants are likely to have a suite of trait values that are consistent with a single drought response strategy. In environments where conditions are variable, plants may additionally evolve phenotypically plastic trait responses to water availability. Invasive annual species commonly occur in arid and semi-arid environments and many will be subject to reduced water availability associated with climate change. Assessing intraspecific trait variation across environmental gradients is a valuable tool for understanding how invasive plants establish and persist in arid environments.

METHODS

In this study, we used a common garden experiment with two levels of water availability to determine how traits related to carbon assimilation, water use, biomass allocation and flowering phenology vary in California wild radish populations across an aridity gradient.

KEY RESULTS

We found that populations from arid environments have rapid flowering and increased allocation to root biomass, traits associated with both drought escape and tolerance. Early flowering was associated with higher leaf nitrogen concentration and lower leaf mass per area, traits associated with high resource acquisition. While trait values varied across low- and high-water treatments, these shifts were consistent across populations, indicating no differential plasticity across the aridity gradient.

CONCLUSIONS

While previous studies have suggested that drought escape and drought resistance are mutually exclusive drought response strategies, our findings suggest that invasive annuals may employ both strategies to succeed in novel semi-arid environments. As many regions are expected to become more arid in the future, investigations of intraspecific trait variation within low water environments help to inform our understanding of potential evolutionary responses to increased aridity in invasive species.

Plant traits are differentially linked to performance in a semiarid ecosystem

A central principle in trait-based ecology is that trait variation has an adaptive value. However, uncertainty over which plant traits influence individual performance across environmental gradients may limit our ability to use traits to infer ecological processes at larger scales. To better understand which traits are linked to performance under different precipitation regimes, we measured above- and belowground traits, growth, and reproductive allocation for four annual and four perennial species from a coastal sage scrub community in California under conditions of 50%, 100%, and 150% ambient precipitation. Across water treatments, annual species displayed morphological trait values consistent with high rates of resource acquisition (e.g., low leaf mass per area, low root tissue density, high specific root length), and aboveground measures of resource acquisition (including photosynthetic rate and leaf N concentration) were positively associated with plant performance (reproductive allocation). Results from a structural equation model demonstrated that leaf traits explained 38% of the variation in reproductive allocation across the water gradient in annual species, while root traits accounted for only 6%. Although roots play a critical role in water uptake, more work is needed to understand the mechanisms by which root trait variation can influence performance in water-limited environments. Perennial species showed lower trait plasticity than annuals across the water gradient and were more variable as a group in terms of trait-performance relationships, indicating that species rely on different functional strategies to respond to drought. Our finding that species identity drives much of the variation in trait values and trait-performance relationships across a water gradient may simplify efforts to model ecological processes, such as productivity, that are potentially influenced by environmentally induced shifts in trait values.

Effects of Slope Aspect and Rainfall on Belowground Deep Fine Root Traits and Aboveground Tree Height

The vertical root distribution and rooting depth are the main belowground plant functional traits used to indicate drought resistance in arid and semiarid regions. The effects of the slope aspect on the aboveground traits are visible but not the belowground deep root traits. We aimed to investigate the fine root traits of the locust tree (Robinia pseudoacacia L.) planted on southerly and northerly aspects, and the variations in the rooting depth in regions with different rainfall, as well as assessing how deep rooting, might affect the response to drought in a loess region. We selected three study sites with different rainfall amounts, with six sampling plots at each site (three each with southerly and northerly aspects). Soil core samples were collected down to the depth where no roots were present. The locust trees tended to develop deeper fine roots rather than greater heights. The tree height and diameter were greater for locust trees on northerly aspects, whereas trees on southerly aspects had significantly deeper rooting depths. Fine root traits (root length, root area, and root dry weight density) were higher in the southerly aspect for both Changwu and Ansai, but lower in Suide. The ratio of the root front depth tree height ranged from 1.04 to 3.17, which was higher on southerly than northerly aspects, and it increased as the rainfall decreased. Locust tree growth traits (belowground fine root and aboveground tree height) were positively correlated with the mean annual rainfall. The soil moisture content of the topsoil decreased as the rainfall decreased, but the pattern varied in the deep layer. Our results suggest that the variations in the belowground rooting depth under different slope aspects may be related to plant survival strategies. The vertical extension of the rooting depth and tree height may be key functional traits that determine plant growth in drought-prone regions.

Trait integration and functional differentiation among co-existing plant species

PREMISE

Determining which traits characterize strategies of coexisting species is important to developing trait-based models of plant communities. First, global dimensions may not exist locally. Second, the degree to which traits and trait spectra constitute independent dimensions of functional variation at various scales continues to be refined. Finally, traits may be associated with existing categorical groupings.

METHODS

We assessed trait integration and differentiation across 57 forest understory plant species in Douglas-fir forests of western Oregon, United States. We combined measurements for a range of traits with literature-based estimates of seed mass and species groupings. We used network analysis and nonmetric multidimensional scaling ordination (NMS) to determine the degree of integration.

RESULTS

We observed a strong leaf economics spectrum (LES) integrated with stem but not root traits. However, stem traits and intrinsic water-use efficiency integrated LES and root traits. Network analyses indicated a modest grouping of a priori trait dimensions. NMS indicated that multivariate differences among species were related primarily to (1) rooting depth and plant height vs. specific root length, (2) the LES, and (3) leaf size vs. seed mass. These differences were related to species groupings associated with growth and life form, leaf lifespan and seed dispersal mechanisms.

CONCLUSIONS

The strategies of coexisting understory plant species could not be reduced to a single dimension. Yet, species can be characterized efficiently and effectively for trait-based studies of plant communities by measuring four common traits: plant height, specific leaf area, leaf size, and seed mass.

Trait-environment interactions affect plant establishment success during restoration

Establishment and persistence are central to community assembly and are determined by how traits interact with the environment to determine performance (trait-environment interactions). Community assembly studies have rarely considered such trait-environment interactions, however, which can lead to incorrect inferences about how traits affect assembly. We evaluated how functional traits, environmental conditions, and trait-environment interactions structure plant establishment, as a measure of performance. Within 12 prairie restorations created by sowing 70 species, we quantified environmental conditions and counted individuals of each seeded species to quantify first-year establishment. Three trait-environment interactions structured establishment. Leaf nitrogen interacted with herbivore pressure, as low leaf nitrogen species established relatively better under higher herbivory than species with high leaf nitrogen. Soil moisture interacted with root mass fraction (RMF), with low-RMF species establishing better with low soil moisture and higher-RMF species better on wetter soils. Specific leaf area (SLA) interacted with light availability, as low-SLA species established better under high light conditions and high-SLA species under low light conditions. Our work illustrates how community assembly can be better described by trait-environment interactions than correlating traits or environment with performance. This knowledge can assist species selection to maximize restoration success.

Invasive annuals respond more negatively to drought than native species

In his foundational list of 'ideal weed' characteristics, Baker (1965) proposed that weedy plants maximize reproductive output under high resource availability. Since then, the idea that invasive plant species are more responsive to fluctuating resources compared with native or noninvasive species has gained considerable traction, although few studies extend this hypothesis to include reproductive output. We revisit Baker's hypothesis in the context of invasion and drought in California grasslands, exploring whether invasives show greater growth and reproductive responses to water availability compared with the native wildflowers they displace. In an outdoor potted study, we grew eight native and eight invasive species of annuals commonly found in southern California grasslands to reproductive maturity under both well-watered and drought conditions. While drought negatively impacted plant performance overall, invasives showed more negative responses for growth and reproductive traits. Invasives also grew larger than native species, especially under well-watered conditions, and produced seed with higher rates of germination. Invasives may be more negatively impacted by drought compared with natives, but they are also able to capitalize on high resource conditions and greatly increase reproductive output. Such opportunistic responses exhibited by invasives might explain previously observed fluctuations in their abundance under variable precipitation.

The Effects of Temporal Variation on Fitness, Functional Traits, and Species Distribution Patterns

Temporal variation is a powerful source of selection on life history strategies and functional traits in natural populations. Theory predicts that the rate and predictability of fluctuations should favor distinct strategies, ranging from phenotypic plasticity to bet-hedging, which are likely to have important consequences for species distribution patterns and their responses to environmental change. To date, we have few empirical studies that test those predictions in natural systems, and little is known about how genetic, environmental, and developmental factors interact to define the “fluctuation niche” of species in temporally variable environments. In this study, we evaluated the effects of hydrological variability on fitness and functional trait variation in three closely related plant species in the genus Lasthenia that occupy different microhabitats within vernal pool landscapes. Using a controlled greenhouse experiment, we manipulated the mean and variability in hydrological conditions by growing plants at different depths with respect to a shared water table and manipulating the magnitude of stochastic fluctuations in the water table over time. We found that all species had similarly high relative fitness above the water table, but differed in their sensitivities to water table fluctuations. Specifically, the two species from vernal pools basins, where soil moisture is controlled by a perched water table, were negatively affected by the stochasticity treatments. In contrast, a species from the upland habitat surrounding vernal pools, where stochastic precipitation events control soil moisture variation, was insensitive to experimental fluctuations in the water table. We found strong signatures of genetic, environmental (plastic), and developmental variation in four traits that can influence plant hydrological responses. Three of these traits varied across plant development and among experimental treatments in directions that aligned with constitutive differences among species, suggesting that multiple sources of variation align to facilitate phenotypic matching with the hydrological environment in Lasthenia. We found little evidence for predicted patterns of phenotypic plasticity and bet-hedging in species and traits from predictable and stochastic environments, respectively. We propose that selection for developmental shifts in the hydrological traits of Lasthenia species has reduced or modified selection for plasticity at any given stage of development. Collectively, these results suggest that variation in species’ sensitivities to hydrological stochasticity may explain why vernal pool Lasthenia species do not occur in upland habitat, and that all three species integrate genetic, environmental, and developmental information to manage the unique patterns of temporal hydrological variation in their respective microhabitats.

Plasticity of Fine-Root Traits Under Long-Term Irrigation of a Water-Limited Scots Pine Forest

Trait-based approaches are increasingly used to investigate plant strategies for resource acquisition, growth, or competition between individual organisms or across species. However, the characterization of responses to environmental stimuli by fine-root systems of trees at the trait level is rather limited, particularly regarding the timing and degree of plasticity of the traits involved. These aspects become especially relevant under current climate-driven shifts in environmental conditions. In the present study, we examined the responses of the fine roots of Scots pines to increased soil water availability from long-term irrigation starting in the year 2003. The Scots pine forest is situated in a water-limited region in the central European Alps where increased tree mortality has been observed over the last two decades. The fine-root traits investigated include root system traits, root dynamic traits, architectural traits, and morphological traits. A first survey of fine-root traits in 2005 using ingrowth cores did not reveal any trait-based responses resulting from the irrigation treatment over a three-year period. Fine-root biomass, as periodically recorded by coring the topsoil from 2003 to 2016, showed a significant increase compared to the non-irrigated controls between three and nine years after the start of treatment. Overall, a maximum biomass increase due to the irrigation treatment was recorded in 2016 with about 80% higher biomass compared to controls. The analysis of fine-root traits revealed that irrigation significantly increased biomass, length, and production, but did not alter morphological and architectural traits, such as diameter, frequency of tips, specific root length (SRL), and root tissue density (RTD). In contrast, clear significant differences were found for all traits except for length when comparing the two root sampling methods, namely, ingrowth cores and soil coring. However, there were no interactions between the irrigation treatment and the sampling methods used and, therefore, the methods used did not affect the documented patterns, just the actual measured trait values.

Seedling traits predict drought-induced mortality linked to diversity loss

Trait-based approaches are increasingly used to predict ecological consequences of climate change, yet seldom have solid links been established between plant traits and observed climate-driven community changes. Most analyses have focused on aboveground adult plant traits, but in warming and drying climates, root traits may be critical, and seedlings may be the vulnerable stage. Relationships of seedling and root traits to more commonly measured traits and ecological outcomes are poorly known. In an annual grassland where winter drought-induced seedling mortality is driving a long-term decline in native diversity, using a field experiment during the exceptionally dry winter of 2017-2018, we found that seedling mortality was higher and growth of seedlings and adults were lower in unwatered than watered sites. Mortality of unwatered seedlings was higher in species with shorter seedling roots, and also in species with the correlated traits of small seeds, high seedling specific leaf area (SLA), and tall seedlings. Adult traits varied along an axis from short-stature, high SLA and foliar N, and early flowering to the opposite values, and were only weakly correlated with seedling traits and seedling mortality. No evidence was found for adaptive plasticity, such as longer roots or lower SLA in unwatered plants. Among these species, constitutive variation in seedling root length explained most of the variation in survival of a highly vulnerable life stage under winter drought. Selective loss of species with high adult SLA, observed in this community and others under drought stress, may be the byproduct of other correlated traits.

Testing the trait-based community framework: Do functional traits predict competitive outcomes?

Plant traits can be used to understand a range of ecological processes, including competition with invasive species. The extent to which native and invasive species are competing via limiting similarity or trait hierarchies has important implications for the management of invaded communities. We screened 47 native species that co-occur with Festuca perennis, a dominant invader in California serpentine grassland, for traits pertaining to resource use and acquisition. We then grew F. perennis with 10 species spanning a range of functional similarity in pairwise competition trials. Functionally similar species did not have a strong adverse effect on F. perennis performance as would be expected by limiting similarity theory. Phylogenetic relatedness, which may integrate a number of functional traits, was also a poor predictor of competitive outcome. Instead, species with high specific root length, low root-to-shoot biomass ratio, and low leaf nitrogen concentration were more effective at suppressing the growth of F. perennis. Our results suggest that fitness differences (i.e., trait hierarchies) may be more important than niche differences (i.e., limiting similarity) in structuring competitive outcomes in this system and may be a promising approach for the restoration of invaded systems.

Experimentally altered rainfall regimes and host root traits affect grassland arbuscular mycorrhizal fungal communities

Future climate scenarios predict changes in rainfall regimes. These changes are expected to affect plants via effects on the expression of root traits associated with water and nutrient uptake. Associated microorganisms may also respond to these new precipitation regimes, either directly in response to changes in the soil environment or indirectly in response to altered root trait expression. We characterized arbuscular mycorrhizal (AM) fungal communities in an Australian grassland exposed to experimentally altered rainfall regimes. We used Illumina sequencing to assess the responses of AM fungal communities associated with four plant species sampled in different watering treatments and evaluated the extent to which shifts were associated with changes in root traits. We observed that altered rainfall regimes affected the composition but not the richness of the AM fungal communities, and we found distinctive communities in the increased rainfall treatment. We found no evidence of altered rainfall regime effects via changes in host physiology because none of the studied traits were affected by changes in rainfall. However, specific root length was observed to correlate with AM fungal richness, while concentrations of phosphorus and calcium in root tissue and the proportion of root length allocated to fine roots were correlated to community composition. Our study provides evidence that climate change and its effects on rainfall may influence AM fungal community assembly, as do plant traits related to plant nutrition and water uptake. We did not find evidence that host responses to altered rainfall drive AM fungal community assembly in this grassland ecosystem.

Transcriptomic signature of drought response in pearl millet (Pennisetum glaucum (L.) and development of web-genomic resources

Pearl millet, (Pennisetum glaucum L.), an efficient (C4) crop of arid/semi-arid regions is known for hardiness. Crop is valuable for bio-fortification combating malnutrition and diabetes, higher caloric value and wider climatic resilience. Limited studies are done in pot-based experiments for drought response at gene-expression level, but field-based experiment mimicking drought by withdrawal of irrigation is still warranted. We report de novo assembly-based transcriptomic signature of drought response induced by irrigation withdrawal in pearl millet. We found 19983 differentially expressed genes, 7595 transcription factors, gene regulatory network having 45 hub genes controlling drought response. We report 34652 putative markers (4192 simple sequence repeats, 12111 SNPs and 6249 InDels). Study reveals role of purine and tryptophan metabolism in ABA accumulation mediating abiotic response in which MAPK acts as major intracellular signal sensing drought. Results were validated by qPCR of 13 randomly selected genes. We report the first web-based genomic resource ( http://webtom.cabgrid.res.in/pmdtdb/ ) which can be used for candidate genes-based SNP discovery programs and trait-based association studies. Looking at climatic change, nutritional and pharmaceutical importance of this crop, present investigation has immense value in understanding drought response in field condition. This is important in germplasm management and improvement in endeavour of pearl millet productivity.

Rapid root responses of seedlings exposed to a postdrought water pulse

PREMISE OF THE STUDY

Mediterranean-type climate ecosystems experience significant variability in precipitation within and across years and may be characterized by periods of extreme drought followed by a brief, high-intensity precipitation pulse. Rapid root growth could be a key factor in effective utilization of precipitation pulses, leading to higher rates of seedling establishment. Changes in root growth rate are rarely studied, however, and patterns in seedling root traits are not well explored. We investigated the influence of an extreme postdrought precipitation event on seedlings that occur in southern California coastal sage scrub.

METHODS

We measured root elongation rate, root tip appearance rate, new leaf appearance rate, and canopy growth rate on 18 mediterranean species from three growth forms.

KEY RESULTS

Root elongation rate responded more strongly to the precipitation pulse than did root tip appearance rate and either metric of aboveground growth. The majority of species exhibited a significant change in root growth rate within 1 week of the pulse. Responses varied in rapidity and magnitude across species, however, and were not generally predictable based on growth form.

CONCLUSIONS

While the majority of species exhibited shifts in belowground growth following the pulse, the direction and magnitude of these morphological responses were highly variable within growth form. Understanding the implications of these different response strategies for plant fitness is a crucial next step to forecasting community dynamics within ecosystems characterized by resource pulses.

Edaphic history over seedling characters predicts integration and plasticity of integration across geologically variable populations of Arabidopsis thaliana

PREMISE OF THE STUDY

Studies on phenotypic plasticity and plasticity of integration have uncovered functionally linked modules of aboveground traits and seedlings of Arabidopsis thaliana, but we lack details about belowground variation in adult plants. Functional modules can be comprised of additional suites of traits that respond to environmental variation. We assessed whether shoot and root responses to nutrient environments in adult A. thaliana were predictable from seedling traits or population-specific geologic soil characteristics at the site of origin.

METHODS

We compared 17 natural accessions from across the native range of A. thaliana using 14-day-old seedlings grown on agar or sand and plants grown to maturity across nutrient treatments in sand. We measured aboveground size, reproduction, timing traits, root length, and root diameter. Edaphic characteristics were obtained from a global-scale dataset and related to field data.

KEY RESULTS

We detected significant among-population variation in root traits of seedlings and adults and in plasticity in aboveground and belowground traits of adult plants. Phenotypic integration of roots and shoots varied by population and environment. Relative integration was greater in roots than in shoots, and integration was predicted by edaphic soil history, particularly organic carbon content, whereas seedling traits did not predict later ontogenetic stages.

CONCLUSIONS

Soil environment of origin has significant effects on phenotypic plasticity in response to nutrients, and on phenotypic integration of root modules and shoot modules. Root traits varied among populations in reproductively mature individuals, indicating potential for adaptive and integrated functional responses of root systems in annuals.

Intraspecific Trait Variation and Coordination: Root and Leaf Economics Spectra in Coffee across Environmental Gradients

Hypotheses on the existence of a universal "Root Economics Spectrum" (RES) have received arguably the least attention of all trait spectra, despite the key role root trait variation plays in resource acquisition potential. There is growing interest in quantifying intraspecific trait variation (ITV) in plants, but there are few studies evaluating (i) the existence of an intraspecific RES within a plant species, or (ii) how a RES may be coordinated with other trait spectra within species, such as a leaf economics spectrum (LES). Using Coffea arabica (Rubiaceae) as a model species, we measured seven morphological and chemical traits of intact lateral roots, which were paired with information on four key LES traits. Field collections were completed across four nested levels of biological organization. The intraspecific trait coefficient of variation (cv) ranged from 25 to 87% with root diameter and specific root tip density showing the lowest and highest cv, respectively. Between 27 and 68% of root ITV was explained by site identity alone for five of the seven traits measured. A single principal component explained 56.2% of root trait covariation, with plants falling along a RES from resource acquiring to conserving traits. Multiple factor analysis revealed significant orthogonal relationships between root and leaf spectra. RES traits were strongly orthogonal with respect to LES traits, suggesting these traits vary independently from one another in response to environmental cues. This study provides among the first evidence that plants from the same species differentiate from one another along an intraspecific RES. We find that in one of the world's most widely cultivated crops, an intraspecific RES is orthogonal to an intraspecific LES, indicating that above and belowground responses of plants to managed (or natural) environmental gradients are likely to occur independently from one another.

A global Fine-Root Ecology Database to address below-ground challenges in plant ecology

Variation and tradeoffs within and among plant traits are increasingly being harnessed by empiricists and modelers to understand and predict ecosystem processes under changing environmental conditions. While fine roots play an important role in ecosystem functioning, fine-root traits are underrepresented in global trait databases. This has hindered efforts to analyze fine-root trait variation and link it with plant function and environmental conditions at a global scale. This Viewpoint addresses the need for a centralized fine-root trait database, and introduces the Fine-Root Ecology Database (FRED, http://roots.ornl.gov) which so far includes > 70 000 observations encompassing a broad range of root traits and also includes associated environmental data. FRED represents a critical step toward improving our understanding of below-ground plant ecology. For example, FRED facilitates the quantification of variation in fine-root traits across root orders, species, biomes, and environmental gradients while also providing a platform for assessments of covariation among root, leaf, and wood traits, the role of fine roots in ecosystem functioning, and the representation of fine roots in terrestrial biosphere models. Continued input of observations into FRED to fill gaps in trait coverage will improve our understanding of changes in fine-root traits across space and time.

Growth and biomass allocation in seedlings of rain-forest trees in New Caledonia: monodominants vs. subordinates and episodic vs. continuous regenerators

Some species-rich secondary forests in New Caledonia have a monodominant canopy. Here we investigate growth and biomass allocation traits that might explain single-species’ dominance of these post-disturbance stands, and their later decline in the absence of large-scale disturbance. Seedlings of 20 rain-forest trees were grown in two light treatments in a nursery house. In the sun treatment, monodominants grew faster (56.7 ± 1.4 mg g−1 wk−1) than subordinates (40.2 ± 2.6 mg g−1 wk−1). However, some episodically regenerating (ER) subordinates had high growth rates similar to those of monodominants. In the shade treatment, monodominants and subordinates had similar growth rates (33.7 ± 2.6 and 34.0 ± 1.9 mg g−1 wk−1 respectively). Notably, monodominants in both sun and shade treatments had lower root mass fraction (0.29 ± 0.02 and 0.27 ± 0.02 g g−1 respectively) than subordinates (0.39 ± 0.02 and 0.37 ± 0.02 g g−1). Fast growth in sunny conditions is probably imperative for these relatively shade-intolerant ER monodominants. In field conditions, high shoot mass fraction combined with efficient root performance may facilitate faster growth in monodominants competing with other ER species in sunlit sites. Slower growth in shade may contribute to loss of dominance over time in undisturbed forests.