Decoupling between growth rate and storage remobilization in broadleaf temperate tree species

Growth and non-structural carbohydrates response patterns of Eucommia ulmoides under salt and drought stress

Introduction

Salinity and droughts are severe abiotic stress factors that limit plant growth and development. However, the differences and similarities of non-structural carbohydrates (NSCs) responses patterns of trees under the two stress conditions remain unclear.

Methods

We determined and compared the growth, physiology, and NSCs response patterns and tested the relationships between growth and NSCs concentrations (or pool size) of Eucommia ulmoides seedlings planted in field under drought and salt stress with different intensities and durations.

Results and discussion

We found that drought and salt stress can inhibit the growth of E. ulmoides, and E. ulmoides tended to enhance its stress resistance by increasing proline concentration and leaf thickness or density but decreasing investment in belowground biomass in short-term stress. During short-term drought and salt stress, the aboveground organs showed different NSCs response characteristics, while belowground organs showed similar change characteristics: the starch (ST) and NSCs concentrations in the coarse roots decreased, while the ST and soluble sugar (SS) concentrations in the fine roots increased to enhance stress resistance and maintain water absorption function. As salt and drought stress prolonged, the belowground organs represented different NSCs response patterns: the concentrations of ST and SS in fine roots decreased as salt stress prolonged; while ST in fine roots could still be converted into SS to maintain water absorption as drought prolonged, resulting in an increase of SS and a decrease of ST. Significant positive relationships were found between growth and the SS and total NSCs concentrations in leaves and branches, however, no significant correlations were found between growth and below-ground organs. Moreover, relationships between growth and NSCs pool size across organs could be contrast.

Conclusion

Our results provide important insights into the mechanisms of carbon balance and carbon starvation and the relationship between tree growth and carbon storage under stress, which were of great significance in guiding for the management of artificial forest ecosystem under the context of global change.

Seasonal patterns of nonstructural carbohydrate storage and mobilization in two tree species with distinct life-history traits

Nonstructural carbohydrates (NSC) are essential for tree growth and adaptation, yet our understanding of the seasonal storage and mobilization dynamics of whole-tree NSC is still limited, especially when tree functional types are involved. Here, Quercus acutissima Carruth. and Pinus massoniana Lamb, with distinct life-history traits (i.e. a deciduous broadleaf species vs an evergreen coniferous species), were studied to assess the size and seasonal fluctuations of organ and whole-tree NSC pools with a focus on comparing differences in carbon resource mobilization patterns between the two species. We sampled the organs (leaf, branch, stem and root) of the target trees repeatedly over four seasons of the year. Then, NSC concentrations in each organ were paired with biomass estimates from the allometric model to generate whole-tree NSC pools. The seasonal dynamics of the whole-tree NSC of Q. acutissima and P. massoniana reached the peak in autumn and summer, respectively. The starch pools of the two species were supplemented in the growing season while the soluble sugar pools were the largest in the dormant season. Seasonal dynamics of organ-level NSC concentrations and pools were affected by organ type and tree species, with above-ground organs generally increasing during the growing season and P. massoniana roots decreasing during the growing season. In addition, the whole-tree NSC pools of P. massoniana were larger but Q. acutissima showed larger seasonal fluctuations, indicating that larger storage was not associated with more pronounced seasonal fluctuations. We also found that the branch and root were the most dynamic organs of Q. acutissima and P. massoniana, respectively, and were the major suppliers of NSC to support tree growth activities. These results provide fundamental insights into the dynamics and mobilization patterns of NSC at the whole-tree level, and have important implications for investigating environmental adaptions of different tree functional types.

Carbon stress causes earlier budbreak in shade-tolerant species and delays it in shade-intolerant species

PREMISE

Climate change may lead to C stress (negative C balance) in trees. Because nonstructural carbohydrates (NSC) are required during metabolic reactivation in the spring, C stress might delay budbreak timing. This effect is expected to be greater in shade-intolerant than in shade-tolerant species, owing to the faster C economy in the shade-intolerant.

METHODS

We experimentally induced C stress in saplings of six temperate tree species that differed in their light requirements by exposing them to either full light or shade from summer to spring, then recorded the date of first budbreak for the individuals. Because the levels of C reserves that represent effective C stress may differ among species, we estimated the degree of C stress by recording survival during the experiment and measuring whole-sapling NSC concentrations after budbreak.

RESULTS

Shade reduced NSC concentrations and increased the sugar fraction in the NSC in all species. In the shade, shade-intolerant species had higher mortality and generally lower NSC concentrations than the shade-tolerant species, indicating a trend for more severe C stress in species with faster C economy. In shade-intolerant species, budbreak started earlier and proceeded faster in full light than in shade, but in shade-tolerant species budbreak was delayed in full light. The effects of the light environments on budbreak were not greater in shade-intolerant than in shade-tolerant species.

CONCLUSIONS

Our study reveals a correspondence between budbreak responses to light and the light requirements of the species. This finding confirms that C metabolism has a significant role in triggering budbreak and demonstrates that whether C stress accelerates or delays budbreak depends on the species' light requirements.

Phosphorus addition regulates the growth of Chinese fir by changing needle nitrogen fractions in growing and dormant seasons

Forest productivity is generally limited by nutrient scarcity. This study aims to reveal seasonal interactions among leaf carbon (C), nitrogen (N) fractions and tree growth driven by nutrient addition in a subtropical forest. Here, a field nutrient addition experiment was conducted with six treatments, namely, +N₅ (5 g N m^-2 yr^-1), +N₁₀ (10 g N m^-2 yr^-1), +P₅ (5 g P m^-2 yr^-1), +N₅ + P₅, +N₁₀ + P₅, and control (N₀ + P₀). C fractions (structural and non-structural carbohydrates) and N fractions (soluble N, nucleic N and protein N) in needles as well as tree growth indicated by basal area increment (BAI) were measured in growing and dormant seasons. Total N and protein N in old needles were significantly increased by P addition, while no significant differences of non-structural carbohydrates in young (<1-year old) and old needles (>1-year old) were detected among the treatments in both seasons. N and P addition increased the structural carbohydrates of old needles in dormant season. P addition decreased and increased tree growth in growing and dormant seasons, respectively. The variation of BAI was explained 18.3 % by total N and 17.8 % by protein N in growing season, and was explained 33.9 % by total N and 34.2 % by protein N in dormant season. Our study suggested that the P addition effect on Chinese fir growth mostly depends on needle N fractions. This study highlights tree seasonal growth driven by nutrient alteration might be characterized by leaf N fractions rather than C fractions in subtropical forests.

Responses to defoliation of Robinia pseudoacacia L. and Sophora japonica L. are soil water condition dependent

Key message

Defoliation significantly affected biomass allocation of Robinia pseudoacacia L. and Sophora japonica L., but leaf physiology readjusted to control levels at the end of the experiment. Considering carbon or sink limitation and relative height growth rate, defoliated R. pseudoacacia grew faster than S. japonica under well-watered conditions, while defoliated S. japonica and R. pseudoacacia had similar performance under drought conditions.

Context

Climate change may result in increases of both drought intensity and insect survival, thereby affecting both exotic and native trees in warm temperate forests.

Aims

In this study, we examined the interaction effects of defoliation and drought on an exotic species Robinia pseudoacacia and a native species Sophora japonica in a warm temperate area, to provide a theoretical basis for predicting the distribution and dynamics of the two species under future climate change.

Methods

In a greenhouse, both species were exposed to three soil moisture (75%, 55%, and 35% of field capacity) and three defoliation treatments (no defoliation, 50% defoliation, and 100% defoliation). Leaf physiology, biomass, and non-structural carbohydrate were determined.

Results

Leaf physiology of defoliated trees did not differ from controls trees, but defoliated seedlings allocated relatively more resources to the leaves at the end of the experiment. In well-watered conditions, defoliated R. pseudoacacia was not carbon or sink limited and defoliated S. japonica was carbon limited, while defoliated individuals of the two species were sink limited under drought. Defoliated R. pseudoacacia grow more rapidly than S. japonica in well-watered conditions. Defoliated R. pseudoacacia had a similar growth rate to S. japonica in drought.

Conclusions

Defoliation clearly affects biomass allocation of the two species, but not leaf physiology. Considering the carbon or sink limitation, the growth of S. japonica and R. pseudoacacia may be limited by future global climate change scenarios.

Phenology explains different storage remobilization in two congeneric temperate tree species with contrasting leaf habit

The dependence of trees on carbon and nutrient storage is critical to predicting the forest vulnerability under climate change, but whether evergreen and deciduous species differ in their use and allocation of stored resources during spring phenology is unclear. Using a high temporal resolution, we evaluated the role of spring phenology and shoot growth as determinants of the carbon and nutrient storage dynamics in contrasting leaf habits. We recorded the phenology and shoot elongation and determined the concentrations of total non-structural carbohydrates (NSCs), starch, soluble carbohydrates, nitrogen (N) and phosphorus (P) in buds, expanding shoots and previously formed shoots of two sympatric Nothofagus species with contrasting leaf habit. Species reached similar shoot lengths, though shoot expansion started 35 days earlier and lasted c. 40 days more in the deciduous species. Thus, although the deciduous species had a relatively constant shoot growth rate, the evergreen species experienced a conspicuous growth peak for c. 20 days. In the evergreen species, the greatest decreases in NSC concentrations of previously formed shoots and leaves coincided with the maximum shoot expansion rate and fruit filling, with minimums of 63 and 65% relative to values at bud dormancy, respectively. In contrast, minimum NSC concentrations of the previously formed shoots of the deciduous species were only 73% and occurred prior to the initiation of shoot expansion. Bud N and P concentrations increased during budbreak, whereas previously formed shoots generally did not decrease their nutrient concentrations. Late spring phenology and overlapping of phenophases contributed to the greater dependence on storage of proximal tissues in the studied evergreen compared with deciduous species, suggesting that phenology is a key determinant of the contrasting patterns of storage use in evergreen and deciduous species.

Short-term forest resilience after drought-induced die-off in Southwestern European forests

Drought-induced die-off in forests is becoming a widespread phenomenon across biomes, but the factors determining potential shifts in taxonomic and structural characteristics following mortality are largely unknown. We report on short-term patterns of resilience after drought-induced episodes of tree mortality across 48 monospecific forests from Morocco to Slovenia. Field surveys recorded plants growing beneath a canopy of dead, defoliated and healthy trees. Site-level structural characteristics and management legacy were also recorded. Resilience was assessed with reference to forest composition (self-replacement), structure, and changes in the climatic suitability of the replacing community relative to the climatic suitability of the dominant pre-drought species. Species climatic suitability was estimated from species distribution models calculated for the baseline 1970-2000 period. Short-term resilience decreased under higher levels of drought-induced damage to the dominant species and with evidences of management legacy. Greater resilience of structural features (fewer gaps, greater canopy height) was observed overall in forests with a larger basal area. Less gaps were also associated with greater woody species richness after drought. Overall, Fagaceae-dominated forests exhibited greater structural resilience than conifer-dominated ones. On those sites that were more climatically suited to the dominant pre-drought species, replacing communities tended to exhibit lower climatic suitability than pre-drought dominant species. There was a greater loss of climatic suitability under a legacy of management and drought intensity, but less so in the replacing communities with higher woody species richness. Our study reveals that short-term forest resilience is determined by pre-drought stand characteristics, often reflecting previous management legacies, and by the impact of drought on both the dominant pre-drought species and post-drought replacing species in terms of their climatic suitability.

Trade-offs among transport, support, and storage in xylem from shrubs in a semiarid chaparral environment tested with structural equation modeling

Plant vascular systems play a central role in global water and carbon cycles and drought resistance. These vascular systems perform multiple functions that affect the fitness of plants, and trade-offs are present among these functions. Some trade-offs are well established, but studies have not examined the full suite of functions of these complex systems. Here, we used a powerful multivariate method, structural equation modeling, to test hypotheses about the trade-offs that govern this vital and globally important tissue. We show that xylem traits are broadly governed by trade-offs related to transport, mechanical support, and storage, which are rooted in cellular structure, and that the level of dehydration experienced by plants in the field exerts a strong influence over these relationships.

The xylem in plants is specialized to transport water, mechanically support the plant body, and store water and carbohydrates. Balancing these functions leads to trade-offs that are linked to xylem structure. We proposed a multivariate hypothesis regarding the main xylem functions and tested it using structural equation modeling. We sampled 29 native shrub species from field sites in semiarid Southern California. We quantified xylem water transport (embolism resistance and transport efficiency), mechanical strength, storage of water (capacitance) and starch, minimum hydrostatic pressures (P_min), and proportions of fibers, vessels, and parenchyma, which were treated as a latent variable representing “cellular trade-offs.” We found that xylem functions (transport, mechanical support, water storage, and starch storage) were independent, a result driven by P_min. P_min was strongly and directly or indirectly associated with all xylem functions as a hub trait. More negative P_min was associated with increased embolism resistance and tissue strength and reduced capacitance and starch storage. We found strong support for a trade-off between embolism resistance and transport efficiency. Tissue strength was not directly associated with embolism resistance or transport efficiency, and any associations were indirect involving P_min. With P_min removed from the model, cellular trade-offs were central and related to all other traits. We conclude that xylem traits are broadly governed by functional trade-offs and that the P_min experienced by plants in the field exerts a strong influence over these relationships. Angiosperm xylem contains different cell types that contribute to different functions and that underpin trade-offs.

Directly quantifying multiple interacting influences on plant competition

When plants compete what influences that interaction? To answer this we measured belowground competition directly, as the simultaneous capture of soil ammonium and nitrate by co-existing herbaceous perennials, Dactylis glomerata and Plantago lanceolata, under the influence of: species identity; N uptake and biomass of focal and neighbour plants; location (benign lowland versus harsher upland site); N availability (low or high N fertilizer); N ion, ammonium or nitrate production (mineralisation) rate, and competition type (intra- or interspecific), as direct effects or pairwise interactions in linear models. We also measured biomass as an indirect proxy for competition. Only three factors influenced both competitive N uptake and biomass production: focal species identity, N ion and the interaction between N ion and neighbour N uptake. Location had little effect on N uptake but a strong influence on biomass production. N uptake increased linearly with biomass only in isolated plants. Our results support the view that measuring resource capture or biomass production tells you different things about how competitors interact with one another and their environment, and that biomass is a longer-term integrative proxy for the outcomes of multiple separate interactions-such as competition for N-occurring between plants.

Precipitation Gradient Drives Divergent Relationship between Non-Structural Carbohydrates and Water Availability in Pinus tabulaeformis of Northern China

Seasonal non-structural carbohydrate (NSC) dynamics in different organs can indicate the strategies trees use to cope with water stress; however, these dynamics remain poorly understood along a large precipitation gradient. In this study, we hypothesized that the correlation between water availability and NSC concentrations in different organs might be strengthened by decreasing precipitation in Pinus tabulaeformis Carr. forests in temperate China. Our results show that the concentrations of soluble sugars were lower in stems and coarse roots, and starch was higher in branches in the early growing season at drier sites. Throughout the growing season, the concentrations of soluble sugars increased in drier sites, especially for leaves, and remained stable in wetter sites, while starch concentrations were relatively stable in branches and stems at all sites. The NSC concentrations, mainly starch, decreased in coarse roots along the growing season at drier sites. Trees have a faster growth rate with an earlier cessation in active stem growth at drier sites. Interestingly, we also found a divergent relationship between NSCs in different organs and mean growing season water availability, and a stronger correlation was observed in drier sites. These results show that pine forests in arid and semi-arid regions of northern China exhibit different physiological responses to water availability, improving our understanding of the adaptive mechanisms of trees to water limitations in a warmer and drier climate.

Starch storage capacity of sapwood is related to dehydration avoidance during drought

PREMISE

The xylem tissue of plants performs three principal functions: transport of water, support of the plant body, and nutrient storage. Tradeoffs may arise because different structural requirements are associated with different functions or because suites of traits are under selection that relate to resource acquisition, use, and turnover. The structural and functional basis of xylem storage is not well established. We hypothesized that greater starch storage would be associated with greater sapwood parenchyma and reduced fibers, which would compromise resistance to xylem tensions during dehydration.

METHODS

We measured cavitation resistance, minimum water potential, starch content, and sapwood parenchyma and fiber area in 30 species of southern California chaparral shrubs (evergreen and deciduous).

RESULTS

We found that species storing greater starch within their xylem tended to avoid dehydration and were less cavitation resistant, and this was supported by phylogenetic independent contrasts. Greater sapwood starch was associated with greater parenchyma area and reduced fiber area. For species without living fibers, the associations with parenchyma were stronger, suggesting that living fibers may expand starch storage capacity while also contributing to the support function of the vascular tissue. Drought-deciduous species were associated with greater dehydration avoidance than evergreens.

CONCLUSIONS

Evolutionary forces have led to an association between starch storage and dehydration resistance as part of an adaptive suite of traits. We found evidence for a tradeoff between tissue mechanical traits and starch storage; moreover, the evolution of novel strategies, such as starch-storing living fibers, may mitigate the strength of this tradeoff.

Inner bark as a crucial tissue for non-structural carbohydrate storage across three tropical woody plant communities

Non-structural carbohydrates (NSC) are crucial for forest resilience, but little is known regarding the role of bark in NSC storage. However, bark's abundance in woody stems and its large living fraction make it potentially key for NSC storage. We quantified total NSC, soluble sugar (SS) and starch concentrations in the most living region of bark (inner bark, IB), and sapwood of twigs, trunks and roots of 45 woody species from three contrasting tropical climates spanning global extremes of bark diversity and wide phylogenetic diversity. NSC concentrations were similar (total NSC, starch) or higher (SS) in IB than wood, with concentrations co-varying strongly. NSC concentrations varied widely across organs and species within communities and were not significantly affected by climate, leaf habit or the presence of photosynthetic bark. Starch concentration tended to increase with density, but only in wood. IB contributed substantially to NSC storage, accounting for 17-36% of total NSC, 23-47% of SS and 15-33% of starch pools. Further examination of the drivers of variation in IB NSC concentration, and taking into account the substantial contribution of IB to NSC pools, will be crucial to understand the role of storage in plant environmental adaptation.