Pascale Maillard

Forest trees adopt effective strategies to optimize nitrogen (N) use through internal N recycling. In the context of more recurrent environmental stresses due to climate change, the question remains whether increased frequency of drought or defoliation threatens this internal nitrogen recycling strategy. We submitted 8-year-old beech trees to two years of either severe drought (Dro) or manual defoliation (Def) to create a state of N starvation. At the end of the 2nd year before leaf senescence, we labeled the foliage of the Dro and Def trees, as well as that of control (Co) trees, with ¹⁵N-urea. Leaf N resorption, winter tree N storage (total N, ¹⁵N, amino acids, soluble proteins) and N remobilization in spring were evaluated for the three treatments. Defoliation and drought did not significantly impact foliar N resorption or N concentrations in organs in winter. Total N amounts in Def tree remained close to those in Co tree, but winter N was stored more in the branches than in the trunk and roots. Total N amount in Dro trees was drastically reduced (−55%), especially at the trunk level, but soluble protein concentrations increased in the trunk and fine roots compared to Co trees. During spring, ¹⁵N was mobilized from the trunk, branches and twigs of both Co and Def trees to support leaf growth. It was only provided through twig ¹⁵N remobilization in the Dro trees, thus resulting in extremely reduced Dro leaf N amounts. Our results suggest that stress-induced changes occur in N metabolism but with varying severity depending on the constraints: within-tree ¹⁵N transport and storage strategy changed in response to defoliation whereas a soil water deficit induced a drastic reduction of the N amounts in all the tree organs. Consequently, N dysfunction could be involved in drought-induced beech tree mortality under the future climate.

Knowledge of the physiological mechanisms underlying species vulnerability to drought is critical to better understand patterns of tree mortality. Investigating plant adaptive strategies to drought should thus help to fill this knowledge gap, especially in tropical rainforests exhibiting high functional diversity. In a semi-controlled drought experiment on 12 rainforest tree species, we investigated the diversity in hydraulic strategies and whether they determined the ability of saplings to use stored non-structural carbohydrates during an extreme imposed drought. We further explored the importance of water- and carbon-use strategies in relation to drought-survival through a modelling approach. Hydraulic strategies varied considerably across species with a continuum between dehydration- tolerance and -avoidance. During dehydration leading to hydraulic failure and irrespective of hydraulic strategies, species showed strong declines in whole-plant starch concentrations and a maintenance or even an increase in soluble sugar concentrations potentially favouring osmotic adjustments. Residual water losses mediated the trade-off between time to hydraulic failure and growth, suggesting that it is linked to the ‘fast-slow’ continuum of plant performances and that dehydration avoidance is an effective drought-survival strategy at the sapling stage. Further investigations on residual water losses may be key to understanding the response of tropical rainforest tree communities to climate change.

Non‐structural carbohydrates (NSC) are considered as indicators of the balance between tree carbon sources and sinks and reflect functional strategies throughout different biomes. However, little is known about the contribution of NSC to tree economics, and in particular, whether leaf, stem and coarse root traits co‐ordinate together into a whole‐tree economics spectrum. Twenty‐four functional traits (including NSC content) were measured in leaves, stem and coarse root xylem of up to 90 angiosperm tree species in temperate, Mediterranean and tropical climates. By performing principal component analysis and standardized major axis regression, we explored the relationships between NSC and other functional traits, as well as the effects of climate and phylogeny on these relationships. Our results revealed a covariation between leaf and coarse root NSC content and leaf economic traits, whereas stem NSC content was largely decoupled from the leaf economics spectrum that was mostly driven by leaf nitrogen content and leaf mass per area. Coarse root xylem traits were closely correlated with leaf traits, while most stem xylem traits were independent from the leaf economics spectrum but covaried with coarse root traits. Trade‐offs among traits in tropical species opposed those from other climates. Evolutionary history affected relationships among certain traits but did not change overall patterns. We conclude that due to leaf habit, an extended growing season and heightened defences, tropical species form distinct conservative resource acquisition strategies. Across all climates, as the stem must provide an efficient transport route between roots and leaves, while maintaining the display of branch and leaf organs, stem xylem design and NSC storage capacity lead to a stem economics spectrum that is largely independent from the leaf spectrum, but is coupled with that of coarse roots due to anatomical continuity. Read the free Plain Language Summary for this article on the Journal blog.

Summary: Wood performs several functions to ensure tree survival and carbon allocation to a finite stem volume leads to trade‐offs among cell types. It is not known to what extent these trade‐offs modify functional trade‐offs and if they are consistent across climates and evolutionary lineages. Twelve wood traits were measured in stems and coarse roots across 60 adult angiosperm tree species from temperate, Mediterranean and tropical climates. Regardless of climate, clear trade‐offs occurred among cellular fractions, but did not translate into specific functional trade‐offs. Wood density was negatively related to hydraulic conductivity (K_th) in stems and roots, but was not linked to nonstructural carbohydrates (NSC), implying a functional trade‐off between mechanical integrity and transport but not with storage. NSC storage capacity was positively associated with K_th in stems and negatively in roots, reflecting a potential role for NSC in the maintenance of hydraulic integrity in stems but not in roots. Results of phylogenetic analyses suggest that evolutionary histories cannot explain covariations among traits. Trade‐offs occur among cellular fractions, without necessarily modifying trade‐offs in function. However, functional trade‐offs are driven by coordinated changes among xylem cell types depending on the dominant role of each cell type in stems and roots.