Rainforest trees respond to drought by modifying their hydraulic architecture

Abstract

Increased drought is forecasted for tropical regions, with severe implications for the health and function of forest ecosystems. How mature forest trees will respond to water deficit is poorly known. We investigated wood anatomy and leaf traits in lowland tropical forest trees after 24 months of experimental rainfall exclusion. Sampling sun‐exposed young canopy branches from target species, we found species‐specific systematic variation in hydraulic‐related wood anatomy and leaf traits in response to drought stress. Relative to controls, drought‐affected individuals of different tree species variously exhibited trait measures consistent with increasing hydraulic safety. These included narrower or less vessels, reduced vessel groupings, lower theoretical water conductivities, less water storage tissue and more abundant fiber in their wood, and more occluded vessels. Drought‐affected individuals also had thinner leaves, and more negative pre‐dawn or mid‐day leaf water potentials. Future studies examining both wood and leaf hydraulic traits should improve the representation of plant hydraulics within terrestrial ecosystem and biosphere models, and help fine‐tune predictions of how future climate changes will affect tropical forests globally.

Amazonia trees have limited capacity to acclimate plant hydraulic properties in response to long‐term drought

“…Tropical forest drought experiments have reported decreases in hydraulic efficiency (Schuldt et al, ), or no change in embolism resistance (Rowland, Costa, et al, ) in comparison to non‐droughted, control forest trees. However, shifts in anatomical traits related to hydraulic safety were observed in a TFE experiment located in tropical Australian rainforest (Tng et al, ). Our data indicate that in tropical trees exposed to prolonged soil moisture stress, neither the traits associated with hydraulic safety nor those associated with hydraulic efficiency adjust to enable acclimation.…”

“…Currently, however, there is limited knowledge about the plasticity of traits in Amazonian trees and therefore their capacity to acclimate functionally to new environmental conditions, particularly drought. The few drought experiments constructed till date in tropical rainforests show some tree traits are plastic and respond to drought (Binks et al, ; Schuldt et al, ; Tng et al, ). However, to our knowledge no studies exist which test the potential of tropical trees to adjust their hydraulic system to long‐term drought, including traits indicating both hydraulic safety and efficiency (Meir et al, ).…”

Isotopic and morphologic proxies for reconstructing light environment and leaf function of fossil leaves: a modern calibration in the Daintree Rainforest, Australia

“…At each sampling location, leaf area index (LAI) was determined using a LAI‐2200C Plant Canopy Analyzer (LI‐COR). In addition, and to examine the impact of environmental stress upon the fidelity of leaf traits, parallel upper canopy crown and canopy interior samples were also collected from individuals impacted by a long‐term (3‐yr) ongoing throughfall exclusion (i.e., “drought”) experiment within the DRO crane plot (Tng et al, 2018). The throughfall exclusion plot (0.4 ha), wherein rainfall/throughfall is intercepted and diverted off the plot has resulted in an area of elevated water stress within the crane‐accessible arc that includes replicated individuals of species in natural conditions.…”

Conceptualising the Global Forest Response to Liana Proliferation

“…These drivers will need to represent the major biotic and abiotic influences on liana abundance, size, and vigour. Besides established measures of water availability (e.g., precipitation, annual moisture index, potential evapotranspiration, and seasonality), water deficit is emerging as an important predictor of forest functioning (Tng et al, 2018) and structure (Pfeifer et al, 2018). Moreover, within the moist forests suitable for lianas, disturbance is the primary driver of liana proliferation and competitive advantage over trees (see section above, Variation Between Forest Recovery Response Classes; Figure 5), hence the most crucial driver measurements will be rates and/or quantities of tree-cutting/wind damage, and/or directly proportional changes in canopy openness or light levels.…”