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Transpiration in humid tropical forests modulates the global water cycle and is a key driver of climate regulation. Yet, our understanding of how tropical trees regulate sap flux in response to climate variability remains elusive. With a progressively warming climate, atmospheric evaporative demand [i.e., vapor pressure deficit (VPD)] will be increasingly important for plant functioning, becoming the major control of plant water use in the twenty-first century. Using measurements in 34 tree species at seven sites across a precipitation gradient in the neotropics, we determined how the maximum sap flux velocity (vmax) and the VPD threshold at which vmax is reached (VPDmax) vary with precipitation regime [mean annual precipitation (MAP); seasonal drought intensity (PDRY)] and two functional traits related to foliar and wood economics spectra [leaf mass per area (LMA); wood specific gravity (WSG)]. We show that, even though vmax is highly variable within sites, it follows a negative trend in response to increasing MAP and PDRY across sites. LMA and WSG exerted little effect on vmax and VPDmax, suggesting that these widely used functional traits provide limited explanatory power of dynamic plant responses to environmental variation within hyper-diverse forests. This study demonstrates that long-term precipitation plays an important role in the sap flux response of humid tropical forests to VPD. Our findings suggest that under higher evaporative demand, trees growing in wetter environments in humid tropical regions may be subjected to reduced water exchange with the atmosphere relative to trees growing in drier climates.

Abstract: Although much is known about drivers of productivity in Douglas-fir and red alder stands, less is known about how productivity may relate to stand transpiration and water use efficiency. We took advantage of a 15-year-old experiment involving Douglas-fir (Pseudotsuga menziesii) and red alder (Alnus rubra) in the western Cascade Range of western Oregon to test the following hypotheses: (a) more productive stands transpire more water, (b) the relationship between productivity and transpiration differs between species, and (c) the relationship between productivity and transpiration differs between sites varying in soil moisture and fertility. Furthermore, the experimental design included alder, a facultative nitrogen-fixing species, which could also affect fertility. Fixed area plots (20×20m) were planted as monocultures of each species or in mixtures at a common density (1100treesha−1) in a randomized-block design. Transpiration of Douglas-fir and red alder was measured using heat dissipation sensors installed in eight trees per plot and scaled to the plot level based on sapwood basal area for each species. Although up to 53% of the variability in tree transpiration was explained by basal area, irrespective of species or site conditions, the two stands with the highest biomass and sapwood basal area did not transpire the most. Instead of more productive stands transpiring more water, the greatest variability in both productivity and transpiration was determined by site conditions and to a lesser degree, species composition. For example, 70% of the variation in tree biomass increment (TBI) was determined by leaf area index, which was much higher at the site with higher fertility and soil moisture (p <0.05). Despite marked phenological and physiological differences, Douglas-fir and red alder performed similarly. Only 19% of annual water use of Douglas-fir occurred between October and March when alder was leafless. Also, there was no evidence of a fertilization effect of the nitrogen-fixing red alder on the Douglas-fir: the nitrogen concentration and N-isotopic ratio of Douglas-fir needles did not differ whether trees were grown in monoculture or in mixtures with red alder. We conclude that lower soil fertility and contrasting microclimate at one site relative to the other suppressed NPP while maintaining higher transpiration, thus reducing water use efficiency. [Copyright &y& Elsevier]

Depending on severity, wildfire alters stand biomass, tree species distribution, and age, which may modify stand transpiration (Et) and the amount of water available to other parts of the hydrologic cycle. Our objective was to determine how wildfire severity affected Et in mixed pine/oak (Pinus taeda L./Quercus stellata Wangehn., Quercus marilandica Muenchh.) stands in the Lost Pines eco‐region (Bastrop, TX, USA). Transpiration was estimated for mature pines and oaks at unburned and moderately burned sites and oak resprouts and pine saplings at a severely burned plot. On average, mature pines had 36% greater sap flux rates (Js) than mature oaks in the unburned and moderately burned stands. Under low moisture stress, regenerating pines had greater Js than resprouting oaks, but Js quickly decreased as soil moisture declined. By contrast, mature pines were unaffected by dry periods. Pines contributed most to Et at the unburned and moderate stands. Conversely, oak Et dominated the severely burned stand, contributing over 95%. Transpiration was greatest at the moderately burned stand (2.02 mm day−1), followed by the unburned (1.44 mm day−1), and the severely burned stands (0.46 mm day−1). Despite greater Js in resprouts and saplings, reductions in total sapwood area resulted in lower stand‐level daily Et at the severe site. Although severe fire decreased stand transpiration through reductions in vegetation density, individual oak resprouts appear to thrive, undeterred by high vapour pressure deficit. Without pine planting, oaks will likely dominate severely burned stands that could result in shifts to local hydrology and microclimate. [ABSTRACT FROM AUTHOR]