Divergence in strategies for coping with winter embolism among co‐occurring temperate tree species: the role of positive xylem pressure, wood type and tree stature

Coupled hydraulics and carbon economy underlie age-related growth decline and revitalisation of sand-fixing shrubs after crown removal

Crown removal revitalises sand-fixing shrubs that show declining vigour with age in drought-prone environments; however, the underlying mechanisms are poorly understood. Here, we addressed this knowledge gap by comparing the growth performance, xylem hydraulics and plant carbon economy across different plant ages (10, 21 and 33 years) and treatments (control and crown removal) using a representative sand-fixing shrub (Caragana microphylla Lam.) in northern China. We found that growth decline with plant age was accompanied by simultaneous decreases in soil moisture, plant hydraulic efficiency and photosynthetic capacity, suggesting that these interconnected changes in plant water relations and carbon economy were responsible for this decline. Following crown removal, quick resprouting, involving remobilisation of root nonstructural carbohydrate reserves, contributed to the reconstruction of an efficient hydraulic system and improved plant carbon status, but this became less effective in older shrubs. These age-dependent effects of carbon economy and hydraulics on plant growth vigour provide a mechanistic explanation for the age-related decline and revitalisation of sand-fixing shrubs. This understanding is crucial for the development of suitable management strategies for shrub plantations constructed with species having the resprouting ability and contributes to the sustainability of ecological restoration projects in water-limited sandy lands.

Maintenance of xylem hydraulic function during winter in the woody bamboo Phyllostachys propinqua McClure

Background

Frost is a common environmental stress for temperate plants. Xylem embolism occurs in many overwintering plants due to freeze-thaw cycles, so coping with freeze-thaw-induced embolisms is essential for the survival of temperate plants.

Methods

This study was conducted on Phyllostachys propinqua McClure, a woody bamboo species that was grown under natural frost conditions to explore its responses to winter embolisms. From autumn to the following spring, the following measurements were recorded: predawn branch and leaf embolism, branch and leaf relative water content (RWC), root pressure and soil temperature, xylem sap osmotic potential, branch and leaf electrolyte leakage (EL), branch nonstructural carbohydrate (NSC) content and leaf net photosynthetic rate.

Results

P. propinqua had a mean vessel diameter of 68.95 ±1.27 µm but did not suffer severe winter embolism, peaking around  60% in winter (January), with a distinct reduction in March when root pressure returned. Leaves had a more severe winter embolism, up to 90%. Leaf RWC was much lower in winter, and leaf EL was significantly higher than branch EL in all seasons. Root pressure remained until November when soil temperature reached 9 °C, then appeared again in March when soil temperatures increased from -6 °C (January) to 11 °C. Xylem sap osmotic potential decreased from autumn to winter, reaching a minimum in March, and then increasing again. Soluble sugar (SS) concentration increased throughout the winter, peaked in March, and then decreased.

Conclusions

These results suggest that (1) there is a hydraulic segmentation between the stem and leaf, which could prevent stem water loss and further embolization in winter; (2) maintenance of root pressure in early winter played an important role in reducing the effect of freeze-thaw cycles on the winter embolism; (3) the physiological process that resulted in a decrease in xylem sap osmotic potential and tissue water content, and an accumulation of SS associated with cold acclimation also aided in reducing the extent of freeze-thaw-induced embolism. All these strategies could be helpful for the maintenance of xylem hydraulic function of this bamboo species during winter.

Ring- and diffuse-porous tree species from a cold temperate forest diverge in stem hydraulic traits, leaf photosynthetic traits, growth rate and altitudinal distribution

In cold and humid temperate forests, low temperature, late frost and frequent freeze-thaw cycles are the main factors limiting tree growth and survival. Ring- and diffuse-porous tree species differing in xylem anatomy coexist in these forests, but their divergent adaptations to these factors have been poorly explored. To fill this knowledge gap, we compared four ring-porous and four diffuse-porous tree species from the same temperate forest in Northeast China by quantifying their leaf and stem functional traits, their stem growth rates using tree ring analysis and their resistance to cold represented by upper altitude species distribution borders from survey data. We found that the ring-porous trees were characterized by traits related to more rapid water transport, carbon gain and stem growth rates than those of the diffuse-porous species. Compared with the diffuse-porous species, the ring-porous species had a significantly higher shoot hydraulic conductance (Ks-shoot, 0.52 vs 1.03 kg m-1 s-1 MPa-1), leaf photosynthetic rate (An, 11.28 vs 15.83 μmol m-2 s-1), relative basal area increment (BAIr, 2.28 vs 0.72 cm year-1) and stem biomass increment (M, 0.34 vs 0.09 kg year-1 m-1). However, the observed upper elevational distribution limit of the diffuse-porous species was higher than that of the ring-porous species and was associated with higher values of conservative traits, such as longer leaf life span (R2 = 0.52). Correspondingly, BAIr and M showed significant positive correlations with acquisitive traits such as Ks-shoot (R2 = 0.77) and leaf photosynthetic rate (R2 = 0.73) across the eight species, with the ring-porous species occurring at the fast-acquisitive side of the spectrum and the diffuse-porous species located on the opposite side. The observed contrasts in functional traits between the two species groups improved our understanding of their differences in terms of growth strategies and adaptive capabilities in the cold, humid temperate forests.

Climate-driven sapwood-specific hydraulic conductivity and the Huber value but not leaf-specific hydraulic conductivity on a global scale

Efficient water transport is crucial for plant growth and survival. Plant hydraulic conductivity varies between functional groups and biomes and is strongly influenced by changing environmental conditions. However, correlations of conductivity-related hydraulic traits with climatic variables are not fully understood, preventing clarification of plant form and function under climate change scenarios. By compiling leaf-specific hydraulic conductivity (K_L), sapwood-specific hydraulic conductivity (K_s), and Huber values (H_v, sapwood area to leaf area ratio) along with climatic variables including mean annual temperature (MAT), mean annual precipitation (MAP) and aridity index (AI) for 428 species across a wide range of plant functional types (PFTs) and biomes at a global scale, we found greater variability of K_L within PFTs and biomes than across PFTs and biomes. Interaction effects between PFTs and biomes on K_L and K_s were found. The interaction between MAT and MAP played a significant role in K_s and H_v (t = 3.89, P < 0.001 for K_s and t = -5.77, P < 0.001 for H_v). With increasing AI, K_s increased and H_v decreased. K_L was not influenced by the investigated climatic variables. Our study provides a better understanding of the dynamics of hydraulic structure and function across functional groups and biomes and of the abiotic drivers of their large-scale variations.

Xylem hydraulics strongly influence the niche differentiation of tree species along the slope of a river valley in a water-limited area

Xylem hydraulic characteristics govern plant water transport, affecting both drought resistance and photosynthetic gas exchange. Therefore, they play critical roles in determining the adaptation of different species to environments with various water regimes. Here, we tested the hypothesis that variation in xylem traits associated with a trade-off between hydraulic efficiency and safety against drought-induced embolism contributes to niche differentiation of tree species along a sharp water availability gradient on the slope of a unique river valley located in a semi-humid area. We found that tree species showed clear niche differentiation with decreasing water availability from the bottom towards the top of the valley. Tree species occupying different positions, in terms of vertical distribution distance from the bottom of the valley, showed a strong trade-off between xylem water transport efficiency and safety, as evidenced by variations in xylem structural traits at both the tissue and pit levels. This optimized their xylem hydraulics in their respective water regimes. Thus, the trade-off between hydraulic efficiency and safety contributes to clear niche differentiation and, thereby, to the coexistence of tree species in the valley with heterogeneous water availability.

Stomatal responses of Eucalyptus spp. under drought can be predicted with a gain-risk optimization model

The frequency and severity of drought events are expected to increase due to climate change, with optimal environmental conditions for forestry likely to shift. Modeling plant responses to a changing climate is therefore vital. We tested the process-based gain-risk model to predict stomatal responses to drought of two Eucalyptus hybrids. The process-based gain-risk model has the advantage that all the parameters used within the model are based on measurable plant traits. The gain-risk model proposes that plants optimize photosynthetic gain while minimizing a hydraulic cost. Previous versions of the model used hydraulic risk as a cost function; however, they did not account for delayed or reduced hydraulic recovery rates from embolism post-drought. Hydraulic recovery has been seen in many species, however it is still unclear how this inclusion of a partial or delayed hydraulic recovery would affect the predictive power of the gain-risk model. Many hydraulic parameters required by the model are also difficult to measure and are not freely available. We therefore tested a simplified gain-risk model that includes a delayed or reduced hydraulic recovery component post-drought. The simplified gain-risk model performed well at predicting stomatal responses in both Eucalyptus grandis × camaldulensis (GC) and Eucalyptus urophylla × grandis (UG). In this study two distinct strategies were seen between GC and UG, with GC being more resistant to embolism formation, however it could not recover hydraulic conductance compared with UG. The inclusion of a delayed or reduced hydraulic recovery component slightly improved model predictions for GC, however not for UG, which can be related to UG being able to recover lost hydraulic conductance and therefore can maintain stomatal conductance regardless of hydraulic risk. Even though the gain-risk model shows promise in predicting plant responses, more information is needed regarding hydraulic recovery after drought.

Applying Plant Hydraulic Physiology Methods to Investigate Desiccation During Prolonged Cold Storage of Horticultural Trees

Plant nursery production systems are a multi-billion-dollar, international, and horticultural industry that depends on storing and shipping live plants. The storage environment represents potentially desiccating and even fatal conditions for dormant, bareroot, and deciduous horticulture crops, like orchard trees, forestry trees, ornamental trees, and grapevines. When tree mortality is considered within a plant hydraulic framework, plants experiencing water stress are thought to ultimately die from hydraulic failure or carbon starvation. We hypothesized that the hydraulic framework can be applied to stored crops to determine if hydraulic failure or carbon starvation could be attributed to mortality. We used deciduous trees as model species because they are important horticultural crops and provide a diversity of hydraulic strategies. We selected cultivars from six genera: Acer, Amelanchier, Gleditsia, Gymnocladus, Malus, and Quercus. For each cultivar, we measured stem hydraulic conductance and vulnerability to embolism. On a weekly basis for 14 weeks (March-June), we removed trees of each cultivar from cold storage (1-2°C). Each week and for each cultivar, we measured stem water potential and water content (n = 7) and planted trees to track survival and growth (n = 10). At three times during this period, we also measured non-structural carbohydrates. Our results showed that for four cultivars (Acer, Amelanchier, Malus, and Quercus), the stem water potentials measured in trees removed from storage did not exceed stem P ₅₀, the water potential at which 50% of stem hydraulic conductivity is lost. This suggests that the water transport system remains intact during storage. For two cultivars (Gleditsia and Gymnocladus), the water potential measured on trees out of storage exceeded stem P ₅₀, yet planted trees from all weeks survived and grew. In the 14 weeks, there were no significant changes or directional trends in stem water potential, water content, or NSC for most cultivars, with a few exceptions. Overall, the results show that the trees did not experience detrimental water relations or carbon starvation thresholds. Our results suggest that many young deciduous trees are resilient to conditions caused by prolonged dormancy and validate the current storage methods. This experiment provides an example of how a mechanistically based understanding of physiological responses can inform cold storage regimes in nursery tree production.

Contrasting Carbon Allocation Strategies of Ring-Porous and Diffuse-Porous Species Converge Toward Similar Growth Responses to Drought

Extreme climatic events that are expected under global warming expose forest ecosystems to drought stress, which may affect the growth and productivity. We assessed intra-annual growth responses of trees to soil water content in species belonging to different functional groups of tree-ring porosity. We pose the hypothesis that species with contrasting carbon allocation strategies, which emerge from different relationships between wood traits and canopy architecture, display divergent growth responses to drought. We selected two diffuse-porous species (Acer saccharum and Betula alleghaniensis) and two ring-porous species (Quercus rubra and Fraxinus americana) from the mixed forest of Quebec (Canada). We measured anatomical wood traits and canopy architecture in eight individuals per species and assessed tree growth sensitivity to water balance during 2008-2017 using the standardized precipitation evapotranspiration index (SPEI). Stem elongation in diffuse-porous species mainly depended upon the total number of ramifications and hydraulic diameter of the tree-ring vessels. In ring-porous species, stem elongation mainly depended upon the productivity of the current year, i.e., number of vessels and basal area increment. Diffuse-porous and ring-porous species had similar responses to soil water balance. The effect of soil water balance on tree growth changed during the growing season. In April, decreasing soil temperature linked to wet conditions could explain the negative relationship between SPEI and tree growth. In late spring, greater water availability affected carbon partitioning, by promoting the formation of larger xylem vessels in both functional groups. Results suggest that timings and duration of drought events affect meristem growth and carbon allocation in both functional groups. Drought induces the formation of fewer xylem vessels in ring-porous species, and smaller xylem vessels in diffuse-porous species, the latter being also prone to a decline in stem elongation due to a reduced number of ramifications. Indeed, stem elongation of diffuse-porous species is influenced by environmental conditions of the previous year, which determine the total number of ramifications during the current year. Drought responses in different functional groups are thus characterized by different drivers, express contrasting levels of resistance or resilience, but finally result in an overall similar loss of productivity.

Responses of functional traits to seven-year nitrogen addition in two tree species: coordination of hydraulics, gas exchange and carbon reserves

Atmospheric nitrogen (N) deposition has been observed to impact plant structure and functional traits in terrestrial ecosystems. Although the effect of N deposition on plant water use has been well-evaluated in laboratories and in experimental forests, the linkages between water and carbon relations under N deposition are unclear. Here, we report on hydraulics, gas exchange and carbon reserves of two broad-leaved tree species (Quercus mongolica and Fraxinus mandshurica) in mature temperate forests after a seven-year experiment with different levels of N addition (control (CK), low (23 kg N ha-1 yr-1), medium (46 kg N ha-1 yr-1) and high (69 kg N ha-1 yr-1)). We investigated variation in hydraulic traits (xylem-specific hydraulic conductivity (Ks), native percentage loss of conductivity (PLC) and leaf water potential), xylem anatomy (vessel diameter and density), gas exchange (maximum net photosynthesis rate and stomatal conductance) and carbon reserves (soluble sugars, starch and total nonstructural carbohydrates (NSC)) with different N addition levels. We found that medium N addition significantly increased Ks and vessel diameter compared to control, but accompanied increasing PLC and decreasing leaf water potential, suggesting that N addition results in a greater hydraulic efficiency and higher risk of embolism. N addition promoted photosynthetic capacity via increasing foliar N concentration but did not change stomatal conductance. In addition, we found increase in foliar soluble sugar concentration and decrease in starch concentration with N addition, and positive correlations between hydraulic traits (vessel diameter and PLC) and soluble sugars. These coupled responses of tree hydraulics and carbon metabolism are consistent with a regulatory role of carbohydrates in maintaining hydraulic integrity. Our study provides an important insight into the relationship of plant water transport and carbon dynamics under increasing N deposition.

Die hard: timberline conifers survive annual winter embolism

During winter, timberline trees are exposed to drought and frost, factors known to induce embolism. Studies indicated that conifers cope with winter embolism by xylem refilling. We analysed the loss of hydraulic conductivity (LC) in Picea abies branch xylem over 10 years, and correlated winter embolism to climate parameters. LC was investigated by direct X-ray micro-computer tomography (micro-CT) observations and potential cavitation fatigue by Cavitron measurements. Trees showed up to 100% winter embolism, whereby LC was highest, when climate variables indicated frost drought and likely freeze-thaw stress further increased LC. During summer, LC never exceeded 16%, due to hydraulic recovery. Micro-CT revealed homogenous embolism during winter and that recovery was based on xylem refilling. Summer samples exhibited lower LC in outermost compared to older tree rings, although no cavitation fatigue was detected. Long-term data and micro-CT observations demonstrate that timberline trees can survive annual cycles of pronounced winter-embolism followed by xylem refilling. Only a small portion of the xylem conductivity cannot be restored during the first year, while remaining conduits are refilled without fatigue during consecutive years. We identify important research topics to better understand the complex induction and repair of embolism at the timberline and its relevance to general plant hydraulics.

Variation in Stem Xylem Traits is Related to Differentiation of Upper Limits of Tree Species along an Elevational Gradient

The distribution limits of many plants are dictated by environmental conditions and species’ functional traits. While many studies have evaluated how plant distribution is driven by environmental conditions, there are not many studies investigating xylem vessel properties with altitude, and whether these traits correlate with altitudinal distribution of tree. Here, we investigated the upper limits of distribution for ten deciduous broadleaf tree species from three temperate montane forest communities along a large elevational gradient on the north-facing slope of Changbai Mountain in Northeast China. We measured stem xylem traits associated with a species’ ability to transport water and resist freezing-induced cavitation that theoretically represent important adaptations to changes in climatic conditions along the elevational gradient. Hydraulically weighted vessel diameter (Dh) was negatively correlated with with the upper limit across the ten studied tree species; however, the correlation seems to be driven by the large differences between ring- and diffuse-porous tree species groups. The ring-porous tree species (e.g., Fraxinus mandshurica Rupr., Maackia amurensis Rupr. et Maxim., and Phellodendron amurense Rupr.) had considerably wider vessels than the diffuse-porous species and were all limited to low-elevation communities. The coefficient of variation (CV) for Dh was 0.53 among the 10 studied species, while the intraspecific analysis showed that the highest CV was only 0.22 among the 10 species. We found no evidence of a relationship between Dh and the upper limits across the seven diffuse-porous species. In contrast to elevation, hydraulic-related xylem traits had no clear patterns of change with precipitation, indicating that hydraulic functionality was largely decoupled from the influences of precipitation in the study area. This finding suggests that xylem traits are associated with altitudinal limits of species distribution, which is mostly evidenced by the contrasts between ring- and diffuse-porous species in xylem anatomy and their altitudinal distributions.

Growing-season temperature and precipitation are independent drivers of global variation in xylem hydraulic conductivity

Stem xylem-specific hydraulic conductivity (K_S ) represents the potential for plant water transport normalized by xylem cross section, length, and driving force. Variation in K_S has implications for plant transpiration and photosynthesis, growth and survival, and also the geographic distribution of species. Clarifying the global-scale patterns of K_S and its major drivers is needed to achieve a better understanding of how plants adapt to different environmental conditions, particularly under climate change scenarios. Here, we compiled a xylem hydraulics dataset with 1,186 species-at-site combinations (975 woody species representing 146 families, from 199 sites worldwide), and investigated how K_S varied with climatic variables, plant functional types, and biomes. Growing-season temperature and growing-season precipitation drove global variation in K_S independently. Both the mean and the variation in K_S were highest in the warm and wet tropical regions, and lower in cold and dry regions, such as tundra and desert biomes. Our results suggest that future warming and redistribution of seasonal precipitation may have a significant impact on species functional diversity, and is likely to be particularly important in regions becoming warmer or drier, such as high latitudes. This highlights an important role for K_S in predicting shifts in community composition in the face of climate change.

Weak Tradeoff and Strong Segmentation Among Plant Hydraulic Traits During Seasonal Variation in Four Woody Species

Plants may maintain long-term xylem function via efficiency-safety tradeoff and segmentation. Most studies focus on the growing season and community level. We studied species with different efficiency-safety tradeoff strategies, Quercus acutissima, Robinia pseudoacacia, Vitex negundo var. heterophylla, and Rhus typhina, to determine the seasonality of this mechanism. We separated their branches into perennial shoots and terminal twigs and monitored their midday water potential (Ψ_md), relative water content (RWC), stem-specific hydraulic conductivity (K_s), loss of 12, 50, and 88% of maximum efficiency (i.e., P₁₂, P₅₀, P₈₈) for 2 years. There were no correlations between water relations (Ψ_md, RWC, K_s) and embolism resistance traits (P₁₂, P₅₀, P₈₈) but they significantly differed between the perennial shoots and terminal twigs. All species had weak annual hydraulic efficiency-safety tradeoff but strong segmentation between the perennial shoots and the terminal twigs. R. pseudoacacia used a high-efficiency, low-safety strategy, whereas R. typhina used a high-safety, low-efficiency strategy. Q. acutissima and V. negundo var. heterophylla alternated these strategies. This mechanism provides a potential basis for habitat partitioning and niche divergence in the changing warm temperate zone environment.

Frost fatigue and its spring recovery of xylem conduits in ring-porous, diffuse-porous, and coniferous species in situ

Frost-induced embolism and frost fatigue are two major aspects of frost damage to xylem water transport in trees. In this study, three species of each ring-porous, diffuse-porous, and coniferous trees growing in situ were used to explore their differences in winter embolism and frost fatigue. Changes in predawn water potential, predawn native embolism, maximal specific conductivity (K_max), and cavitation resistance (P₅₀, xylem water potential at 50% loss of conductivity) of current-year branches were measured from autumn to spring. Maximum native embolism of late winter was near 100% for ring-porous species, approximately 80% for diffuse-porous species, and below 50% for conifers. In early spring, there was no significant reduction of native embolism until formation of new vessels in ring-porous trees, while diffuse-porous trees and conifers exhibited a reduction in native embolism before development of new xylem. There was a significant decrease in P₅₀ of ring- and diffuse-porous species over winter; however, in May P₅₀ was markedly reduced along with formation of new vessels. K_max of ring- and diffuse-porous species significantly decreased from autumn to late winter. The results revealed that vulnerability to cavitation and frost fatigue was related to conduit diameter. The strategies for coping with winter embolism differed among the three wood types: in ring-porous species there was no active embolism refilling; in diffuse-porous species there was refilling associated with positive xylem pressure; and in conifers there was refilling without positive xylem pressure. New vessels could completely restore stem hydraulic conductivity but only partially restore xylem cavitation resistance in spring.

Leaf economics and plant hydraulics drive leaf : wood area ratios

Biomass and area ratios between leaves, stems and roots regulate many physiological and ecological processes. The Huber value H_v (sapwood area/leaf area ratio) is central to plant water balance and drought responses. However, its coordination with key plant functional traits is poorly understood, and prevents developing trait-based prediction models. Based on theoretical arguments, we hypothesise that global patterns in H_v of terminal woody branches can be predicted from variables related to plant trait spectra, that is plant hydraulics and size and leaf economics. Using a global compilation of 1135 species-averaged H_v , we show that H_v varies over three orders of magnitude. Higher H_v are seen in short small-leaved low-specific leaf area (SLA) shrubs with low K_s in arid relative to tall large-leaved high-SLA trees with high K_s in moist environments. All traits depend on climate but climatic correlations are stronger for explanatory traits than H_v . Negative isometry is found between H_v and K_s , suggesting a compensation to maintain hydraulic supply to leaves across species. This work identifies the major global drivers of branch sapwood/leaf area ratios. Our approach based on widely available traits facilitates the development of accurate models of above-ground biomass allocation and helps predict vegetation responses to drought.

Compound leaves are associated with high hydraulic conductance and photosynthetic capacity: evidence from trees in Northeast China

Characterizing differences in key functional traits between simple-leaved (SL) and compound-leaved (CL) tree species can contribute to a better understanding of the adaptive significance of compound leaf form. In particular, this information may provide a mechanistic explanation to the long-proposed fast-growth hypothesis of CL tree species. Here, using five SL and five CL tree species co-occurring in a typical temperate forest of Northeast China, we tested whether higher hydraulic efficiency underlies potentially high photosynthetic capacity in CL species. We found that the CL species had significantly higher hydraulic conductance at the whole-branch level than the SL species (0.52 ± 0.13 vs 0.15 ± 0.04 × 10-4 kg m-2 s-1 Pa-1, P = 0.029). No significant difference in net photosynthetic rate (14.7 ± 2.43 vs 12.5 ± 2.05 μmol m-2 s-1, P = 0.511) was detected between these two groups, but this was largely due to the existence of one outlier species in each of the two functional groups. Scrutinization of the intragroup variations in functional traits revealed that distinctions of the two outlier species in wood type (ring- vs diffuse-porous) from their respective functional groups have likely contributed to their aberrant physiological performances. The potentially high photosynthetic capacity of CL species seems to require ring-porous wood to achieve high hydraulic efficiency. Due to its limitation on leaf photosynthetic capacity, diffuse-porous wood with lower hydraulic conductivity largely precludes its combination with the 'throw-away' strategy (i.e., annually replacing the stem-like rachises) of compound-leaved tree species, which intrinsically requires high carbon assimilation rate to compensate for their extra carbon losses. Our results for the first time show clear differentiation in hydraulic architecture and CO2 assimilation between sympatric SL and CL species, which contributes to the probing of the underlying mechanism responsible for the potential fast growth of trees with compound leaves.

Embolism recovery strategies and nocturnal water loss across species influenced by biogeographic origin

Drought-induced tree mortality is expected to increase in future climates with the potential for significant consequences to global carbon, water, and energy cycles. Xylem embolism can accumulate to lethal levels during drought, but species that can refill embolized xylem and recover hydraulic function may be able to avoid mortality. Yet the potential controls of embolism recovery, including cross-biome patterns and plant traits such as nonstructural carbohydrates (NSCs), hydraulic traits, and nocturnal stomatal conductance, are unknown. We exposed eight plant species, originating from mesic (tropical and temperate) and semi-arid environments, to drought under ambient and elevated CO2 levels, and assessed recovery from embolism following rewatering. We found a positive association between xylem recovery and NSCs, and, surprisingly, a positive relationship between xylem recovery and nocturnal stomatal conductance. Arid-zone species exhibited greater embolism recovery than mesic zone species. Our results indicate that nighttime stomatal conductance often assumed to be a wasteful use of water, may in fact be a key part of plant drought responses, and contribute to drought survival. Findings suggested distinct biome-specific responses that partially depended on species climate-of-origin precipitation or aridity index, which allowed some species to recover from xylem embolism. These findings provide improved understanding required to predict the response of diverse plant communities to drought. Our results provide a framework for predicting future vegetation shifts in response to climate change.

The interaction between nonstructural carbohydrate reserves and xylem hydraulics in Korean pine trees across an altitudinal gradient

Nonstructural carbohydrates (NSC) have been proposed to play an important role in maintaining the hydraulic integrity of trees, particularly in environments with high risks of embolism formation, but knowledge about the interaction between NSC reserves and xylem hydraulics is still very limited. We studied the variation of NSC reserves and hydraulic traits in Pinus koraiensis Sieb. et Zucc. (Korean pine) in March and June across a relatively large altitudinal gradient in Changbai Mountain of Northeast China. One of the major aims was to investigate the potential role NSC plays in maintaining hydraulic integrity of overwintering stems in facing freezing-induced embolism. Consistent with our hypotheses, substantial variations in both NSC contents and hydraulic traits were observed across altitudes and between the two seasons. In March, when relatively high degrees of winter embolism exist, the percentage loss of conductivity (PLC) showed an exponential increase with altitude. Most notably, positive correlations between branch and trunk soluble sugar content and PLC (P = 0.053 and 0.006) were observed across altitudes during this period. These correlations could indicate that more soluble sugars are required for maintaining stem hydraulic integrity over the winter by resisting or refilling freezing-induced embolism in harsher environments, although more work is needed to establish a direct causal relationship between NSC dynamics and xylem hydraulics. If the correlation is indeed directly associated with varying demands for maintaining hydraulic integrity across environmental gradients, greater carbon demands may compromise tree growth under conditions of higher risk of winter embolism leading to a trade-off between competitiveness and stress resistance, which may be at least partially responsible for the lower dominance of Korean pine trees at higher altitudes.

Embolism and mechanical resistances play a key role in dehydration tolerance of a perennial grass Dactylis glomerata L

Background and Aims

More intense droughts under climate change threaten species resilience. Hydraulic strategies determine drought survival in woody plants but have been hardly studied in herbaceous species. We explored the intraspecific variability of hydraulic and morphological traits as indicators of dehydration tolerance in a perennial grass, cocksfoot (Dactylis glomerata), which has a large biogeographical distribution in Europe.

Methods

Twelve populations of cocksfoot originating from Mediterranean, Temperate and Northern European areas were grown in a controlled environment in pots. Dehydration tolerance, leaf and stem anatomical traits and xylem pressure associated with 88 or 50 % loss of xylem conductance (P88, P50) were measured.

Key Results

Across the 12 populations of cocksfoot, P50 ranged from -3.06 to - 6.36 MPa, while P88 ranged from -5.06 to -11.6 MPa. This large intraspecific variability of embolism thresholds corresponded with the biogeographical distribution and some key traits of the populations. In particular, P88 was correlated with dehydration tolerance (r = -0.79). The dehydration-sensitive Temperate populations exhibited the highest P88 (-6.1 MPa). The most dehydration-tolerant Mediterranean populations had the greatest leaf dry matter content and leaf fracture toughness, and the lowest P88 (-10.4 MPa). The Northern populations displayed intermediate trait values, potentially attributable to frost resistance. The thickness of metaxylem vessel walls in stems was highly correlated with P50 (r = -0.92), but no trade-off with stem lignification was observed. The relevance of the linkage between hydraulic and stomatal traits is discussed for drought survival in perennial grasses.

Conclusions

Compared with woody species, the large intraspecific variability in dehydration tolerance and embolism resistance within cocksfoot has consequences for its sensitivity to climate change. To better understand adaptive strategies of herbaceous species to increasing drought and frost requires further exploration of the role of hydraulic and mechanical traits using a larger inter- and intraspecific range of species.

Divergent hydraulic strategies to cope with freezing in co-occurring temperate tree species with special reference to root and stem pressure generation

Some temperate tree species mitigate the negative impacts of frost-induced xylem cavitation by restoring impaired hydraulic function via positive pressures, and may therefore be more resistant to frost fatigue (the phenomenon that post-freezing xylem becomes more susceptible to hydraulic dysfunction) than nonpressure-generating species. We test this hypothesis and investigate underlying anatomical/physiological mechanisms. Using a common garden experiment, we studied key hydraulic traits and detailed xylem anatomical characteristics of 18 sympatric tree species. These species belong to three functional groups, that is, one generating both root and stem pressures (RSP), one generating only root pressure (RP), and one unable to generate such pressures (NP). The three functional groups diverged substantially in hydraulic efficiency, resistance to drought-induced cavitation, and frost fatigue resistance. Most notably, RSP and RP were more resistant to frost fatigue than NP, but this was at the cost of reduced hydraulic conductivity for RSP and reduced resistance to drought-induced cavitation for RP. Our results show that, in environments with strong frost stress: these groups diverge in hydraulic functioning following multiple trade-offs between hydraulic efficiency, resistance to drought and resistance to frost fatigue; and how differences in anatomical characteristics drive such divergence across species.

Evaluation of Morpho-Physiological Traits Adjustment of Prosopis tamarugo Under Long-Term Groundwater Depletion in the Hyper-Arid Atacama Desert

Water extraction from the underground aquifers of the Pampa del Tamarugal (Atacama Desert, Chile) reduced the growing area of Prosopis tamarugo, a strict phreatic species endemic to northern Chile. The objective of this work was to evaluate the effect of various architectural and morpho-physiological traits adjustment of P. tamarugo subjected to three groundwater depletion intervals (GWDr): <1 m (control), 1-4 m and 6-9 m. The traits were evaluated at three levels, plant [height, trunk cross-section area, leaf fraction (fGCC), and crown size], organ [length of internodes, leaf mass per unit area (LMA), leaflet mass and area], and tissue level [wood density (WD), leaf ¹³C, ¹⁸O isotope composition (δ), and intrinsic water use efficiency (iWUE)]. In addition, soil water content (VWC) to 1.3 m soil depth, pre-dawn and midday water potential difference (ΔΨ), and stomatal conductance (g_s) were evaluated. At the deeper GWDr, P. tamarugo experienced significant growth restriction and reduced fGCC, the remaining canopy had a significantly higher LMA associated with smaller leaflets. No differences in internode length and WD were observed. Values for δ¹³C and δ¹⁸O indicated that as GWDr increased, iWUE increased as a result of partial stomata closure with no significant effect on net assimilation over time. The morpho-physiological changes experienced by P. tamarugo allowed it to acclimate and survive in a condition of groundwater depletion, keeping a functional but diminished canopy. These adjustments allowed maintenance of a relatively high g_s; ΔΨ was not different among GWDrs despite smaller VWC at greater GWDr. Although current conservation initiatives of this species are promising, forest deterioration is expected continue as groundwater depth increases.