• Energy Research

Shifts in rainfall patterns and increasing temperatures associated with climate change are likely to cause widespread forest decline in regions where droughts are predicted to increase in duration and severity. One primary cause of productivity loss and plant mortality during drought is hydraulic failure. Drought stress creates trapped gas emboli in the water transport system, which reduces the ability of plants to supply water to leaves for photosynthetic gas exchange and can ultimately result in desiccation and mortality. At present we lack a clear picture of how thresholds to hydraulic failure vary across a broad range of species and environments, despite many individual experiments. Here we draw together published and unpublished data on the vulnerability of the transport system to drought-induced embolism for a large number of woody species, with a view to examining the likely consequences of climate change for forest biomes. We show that 70% of 226 forest species from 81 sites worldwide operate with narrow (<1 megapascal) hydraulic safety margins against injurious levels of drought stress and therefore potentially face long-term reductions in productivity and survival if temperature and aridity increase as predicted for many regions across the globe. Safety margins are largely independent of mean annual precipitation, showing that there is global convergence in the vulnerability of forests to drought, with all forest biomes equally vulnerable to hydraulic failure regardless of their current rainfall environment. These findings provide insight into why drought-induced forest decline is occurring not only in arid regions but also in wet forests not normally considered at drought risk.

I conducted a literature survey to assess the available information on relationships between size--expressed in terms of diameter and dry biomass--and hydraulic efficiency of woody structures at different scales, from stem segments to whole trees. Three data sets were constructed: the first described the relationship between segment diameter and hydraulic conductivity (k(h); kg m s(-1) MPa(-1)) in four species; the second, for the same four species, described the intraspecific trajectories of change in total hydraulic conductance (G; kg s(-1) MPa(-1)) during ontogeny, i.e., from saplings to mature trees, thereby providing a comparison between allometric scaling laws at the scales of segments and whole trees; the third comprised pooled means for nine species that described the interspecific trajectory of change in G with tree size. The scaling coefficients obtained were compared with predictions made with an architectural fractal-like model incorporating tissue-specific hydraulic architecture parameters (West et al. 1999). When data on segment k(h) were examined, the fractal-like model closely predicted the scaling of k(h) with segment diameter in four species. However, the model failed to predict accurately in all species the intraspecific scaling at the branch and whole-tree levels, and consistently overestimated the scaling coefficients. The results suggest that ontogenetic changes in tree size during the life cycle of one tree may result in tradeoffs between optimal hydraulic supply to the existing leaf area and maintenance costs of the supporting xylem tissue. The model of West et al. (1999) may be useful for understanding broad interspecific patterns, but not for understanding more subtle ontogenetic changes.

Abstract Tree transpiration depends on biotic and abiotic factors that might change in the future, including precipitation and soil moisture status. Although short-term sap flux responses to soil moisture and evaporative demand have been the subject of attention before, the relative sensitivity of sap flux to these two factors under long-term changes in soil moisture conditions has rarely been determined experimentally. We tested how long-term artificial change in soil moisture affects the sensitivity of tree-level sap flux to daily atmospheric vapor pressure deficit ( VPD) and soil moisture variations, and the generality of these effects across forest types and environments using four manipulative sites in mature forests. Exposure to relatively long-term (two to six years) soil moisture reduction decreases tree sap flux sensitivity to daily VPD and relative extractable water ( REW ) variations, leading to lower sap flux even under high soil moisture and optimal VPD . Inversely, trees subjected to long-term irrigation showed a significant increase in their sensitivity to daily VPD and REW , but only at the most water-limited site. The ratio between the relative change in soil moisture manipulation and the relative change in sap flux sensitivity to VPD and REW variations was similar across sites suggesting common adjustment mechanisms to long-term soil moisture status across environments for evergreen tree species. Overall, our results show that long-term changes in soil water availability, and subsequent adjustments to these novel conditions, could play a critical and increasingly important role in controlling forest water use in the future.

AbstractTranspiration from the Amazon rainforest generates an essential water source at a global and local scale. However, changes in rainforest function with climate change can disrupt this process, causing significant reductions in precipitation across Amazonia, and potentially at a global scale. We report the only study of forest transpiration following a long‐term (>10 year) experimental drought treatment in Amazonian forest. After 15 years of receiving half the normal rainfall, drought‐related tree mortality caused total forest transpiration to decrease by 30%. However, the surviving droughted trees maintained or increased transpiration because of reduced competition for water and increased light availability, which is consistent with increased growth rates. Consequently, the amount of water supplied as rainfall reaching the soil and directly recycled as transpiration increased to 100%. This value was 25% greater than for adjacent nondroughted forest. If these drought conditions were accompanied by a modest increase in temperature (e.g., 1.5°C), water demand would exceed supply, making the forest more prone to increased tree mortality.

Relations concernant la biomasse aérienne pour le hêtre (Fagus moesiaca Cz.), dans le massif du Vermio au nord de la Grèce, et équations génériques pour Fagus sp. Dans l'article suivant sont présentées des équations allométriques décrivant les relations taille-forme pour une espèce particulière de hêtre, Fagus moesiaca (Cz.), dans le massif du Vermio, au nord de la Grèce. Le diamètre à hauteur de poitrine explique en grande partie la variabilité des variables indépendantes (biomasse aérienne totale, biomasse des tiges, biomasse des branches), tandis que la hauteur de l'arbre est la deuxième variable indépendante la plus importante lors de l'estimation de la masse du feuillage. Les équations développées aux états-Unis et en Europe pour les différentes espèces de hêtre (Fagus spp.) sont également présentées. Leur validation, obtenue avec les données recueillies sur le terrain, indique que les régressions américaines prédisent de manière précise la biomasse totale de l'arbre dans la forêt étudiée. En outre, les données de terrain ont été utilisées pour tester un modèle théorique récent, ce qui a permis de mettre en évidence de larges variations entre les valeurs théoriques et empiriques. Finalement, des équations génériques concernant Fagus spp. ont été développées à partir de ces données et de plusieurs autres équations publiées. La validation de ces équations génériques indique que des prédictions précises peuvent être obtenues quand les régressions sont appliquées à une large échelle géographique. Allometric equations describing tree size-shape relationships for beech (Fagus moesiaca Cz.) in the Vermio Mountains of Northern Greece are presented. Diameter at breast height explained most of the variability in the dependent variables (total aboveground, stem, and branch biomass), while tree height was the second most important regressor in estimating foliage mass. Equations developed in USA and Europe for Fagus spp. were also reported and validation with the field data indicated that the American regressions closely predicted total tree biomass for the study forest. In addition, the raw data were used to test a recent theoretical model and large deviations were found between theoretical and empirical values. Finally, generalised equations for Fagus spp. were developed based on these data and several other published equations. Validation of the generalised equations indicated that accurate predictions may be obtained when these regressions are applied over a broad geographical area.

Abstract. Forest ecosystem models based on heuristic water stress functions poorly predict tropical forest response to drought because they do not capture the diversity of hydraulic traits (including variation in tree size) observed in tropical forests. We developed a Richards’ equation-based model of plant hydraulics in which all parameters of its constitutive equations are biologically-interpretable and measureable plant hydraulic traits (e.g., turgor loss point πtlp, bulk elastic modulus ε, hydraulic capacitance Cft, xylem hydraulic conductivity ks,max, water potential at 50 % loss of conductivity for both xylem (P50,x) and stomata (P50,gs), and the leaf:sapwood area ratio Al:As). We embedded this plant hydraulics model within a forest simulator (TFS) that modeled individual tree light environments and their upper boundary condition (transpiration) as well as provided a means for parameterizing individual variation in hydraulic traits. We synthesized literature and existing databases to parameterize all hydraulic traits as a function of stem and leaf traits wood density (WD), leaf mass per area (LMA) and photosynthetic capacity (Amax) and evaluated the coupled model’s (TFS-Hydro) predictions against diurnal and seasonal variability in stem and leaf water potential as well as stand-scaled sap flux. Our hydraulic trait synthesis revealed coordination among leaf and xylem hydraulic traits and statistically significant relationships of most hydraulic traits with more easily measured plant traits. Using the most informative empirical trait-trait relationships derived from this synthesis, the TFS-Hydro model parameterization is capable of representing patterns of coordination and trade-offs in hydraulic traits. TFS-Hydro successfully captured individual variation in leaf and stem water potential due to increasing tree size and light environment, with model representation of hydraulic architecture and plant traits exerting primary and secondary controls, respectively, on the fidelity of model predictions. The plant hydraulics model made substantial improvements to simulations of total ecosystem transpiration under control conditions, but the absence of a vertically stratified soil hydrology model precluded improvements to the simulation of drought response. Remaining uncertainties and limitations of the trait paradigm for plant hydraulics modeling are highlighted.