Stomatal closure is a pervasive response among trees exposed to flooded soil. We tested whether this response is caused by reduced hydraulic conductance in the soil-to-leaf hydraulic continuum (ktotal), and particularly by reduced root hydraulic conductance (kroot), which has been widely hypothesized. We tracked stomatal conductance at the leaf level (gs) and canopy scale (Gs) along with physiological conditions in two temperate tree species, Magnolia grandiflora and Quercus virginiana, that were subjected to flood and control conditions in a greenhouse experiment. Flooding reduced gs, Gs, kroot and ktotal. Path analysis showed strong support for direct effects of ktotal on gs and for flood duration on ktotal, but not kroot on ktotal. A process-based model that accounted for the ktotal reduction predicted the timeseries of Gs in flood and control treatment trees reasonably well (predicted versus observed Gs R2 = 0.80 and 0.51 for M. grandiflora and Q. virginiana, respectively). However, accounting only for kroot reduction in flooded trees was insufficient for predicting observed Gs reduction. Together, these results suggest that hydraulic constraints were not limited to roots and highlight the need to account for flooding effects on ktotal when projecting forest ecosystem function using process-based models.

Water movement from soil through trees to the atmosphere in the transpiration stream is a major component of Earth’s water cycle. Barro Colorado Island (BCI), Panama, has hosted research at the forefront of plant hydraulics, a field that aims to discover the mechanisms that drive variation in transpiration among plants and in response to environmental conditions. Work on BCI has shown that tropical tree species vary in the soil depths at which they extract water, the water-moving efficiency of their vascular systems, and the vulnerability of their vascular systems to water deficit during seasonal and extreme droughts. Combined, these factors lead to diverse patterns of transpiration among tropical trees, which affect dynamics of tree growth and mortality. The understanding gained on BCI has helped to form the basis for predicting how transpiration in individual trees and in tropical forests will respond to climate change.

When municipal effluent is discharged into swamps, baldcypress trees (Taxodium distichum) generally respond with increased growth. This growth increase may be associated with increased transpiration (E), but if functional traits are also affected, they may modulate the response of E to environmental conditions such as atmospheric vapor pressure deficit (VPD). We measured tree growth and sap flux to assess water-use traits and daily E in mature baldcypress trees that received effluent discharge and compared them to nearby trees that did not receive effluent. Basal area increment and foliar N:P were higher in trees that received effluent. For any given tree size, E was 5.8 L day−1 higher in trees receiving effluent. Sap-flux-based whole-tree canopy conductance at the reference VPD of 1 kPa and its sensitivity to VPD were both positively related to foliar N:P, suggesting that effluent discharge shifted trees to higher water use when VPD was relatively low, but that water use was more sensitive to VPD. Our results suggest that when effluent discharge increases forested wetland productivity it can also increase E, but these effects may be diminished by high VPD.

Bark water vapor conductance (gbark) modulates forest transpiration during droughts, when leaf transpiration is highly reduced. If disturbances such as windstorms and floods impact gbark, they could affect tree performance during subsequent droughts. Bark traits, particularly lenticel traits, likely drive variation in gbark and may influence the effects of disturbances on gbark. We assessed variation in gbark and bark traits in tree branches of 15 tree species in a temperate forest in Louisiana, USA, and performed experiments to test whether gbark in tree branches was affected by bending (simulating wind) and whether gbark of tree saplings was affected by soil flooding. Among tree species, mean branch gbark ranged from 2.22 to 12.02 mmol m-2 s-1. Stem bending increased gbark by 23% compared to unbent controls. Although gbark was unaffected by 38 days of flooding, after 69 days post-flood it was reduced by 41% compared to unflooded controls. The relationships between gbark and bark traits, including bark thickness, lenticel density, and lenticel size, were inconsistent across the survey and experiments. Together, these results show that gbark is variable among species and mutable. Uncovering the drivers of gbark variation within and among trees, including exposure to extreme weather events, will inform projections of forest dynamics under climate change.

Key message Tree species in a temperate floodplain forest had leaf turgor loss point values similar to those of upland forest trees, suggesting physiological drought tolerance in this generally non-water-limited system. Abstract Leaf turgor loss point (TLP) is a key plant trait associated with drought tolerance. In the bottomland hardwood (BLH) forests that grow in floodplains of the southeastern USA, drought stress is generally low but may increase with climate change. To address drought tolerance among BLH trees, we measured TLP among 20 species in a BLH forest in Louisiana, USA. We tested whether (1) TLP is higher in BLH tree species than in upland temperate-zone trees; (2) lower TLP is associated with higher drought tolerance among BLH species; (3) TLP drops during the growing season within BLH trees; and (4) within species, TLP is lower in more water limited, non-flooded BLH habitats than in seasonally flooded habitats. Among BLH tree species, TLP was −2.23 ± 0.28 (mean ± SD) and, contrary to our hypothesis, weakly positively correlated with drought tolerance. Within BLH species, TLP was lower in non-flooded habitats than seasonally flooded habitats and TLP decreased between the early and late growing season, more so in the non-flooded habitat. Overall, our results show that TLP among BLH trees is relatively low and plastic for a system that is generally not water limited, which may contribute to drought tolerance in future scenarios.

The representation of stomatal regulation of transpiration and CO2 assimilation is key to forecasting terrestrial ecosystem responses to global change. Given its importance in determining the relationship between forest productivity and climate, accurate and mechanistic model representation of the relationship between stomatal conductance (gs ) and assimilation is crucial. We assess possible physiological and mechanistic controls on the estimation of the g1 (stomatal slope, inversely proportional to water use efficiency) and g0 (stomatal intercept) parameters, using diurnal gas exchange surveys and leaf-level response curves of six tropical broadleaf evergreen tree species. g1 estimated from ex situ response curves averaged 50% less than g1 estimated from survey data. While g0 and g1 varied between leaves of different phenological stages, the trend was not consistent among species. We identified a diurnal trend associated with g1 and g0 that significantly improved model projections of diurnal trends in transpiration. The accuracy of modeled gs can be improved by accounting for variation in stomatal behavior across diurnal periods, and between measurement approaches, rather than focusing on phenological variation in stomatal behavior. Additional investigation into the primary mechanisms responsible for diurnal variation in g1 will be required to account for this phenomenon in land-surface models.

Intensified droughts are affecting tropical forests across the globe. However, the underlying mechanisms of tree drought response and mortality are poorly understood. Hydraulic traits and especially hydraulic safety margins (HSMs), that is, the extent to which plants buffer themselves from thresholds of water stress, provide insights into species-specific drought vulnerability. We investigated hydraulic traits during an intense drought triggered by the 2015-2016 El Nino on 27 canopy tree species across three tropical forest sites with differing precipitation. We capitalized on the drought event as a time when plant water status might approach or exceed thresholds of water stress. We investigated the degree to which these traits varied across the rainfall gradient, as well as relationships among hydraulic traits and species-specific optimal moisture and mortality rates. There were no differences among sites for any measured trait. There was strong coordination among traits, with a network analysis revealing two major groups of coordinated traits. In one group, there were water potentials, turgor loss point, sapwood capacitance and density, HSMs, and mortality rate. In the second group, there was leaf mass per area, leaf dry matter content, hydraulic architecture (leaf area to sapwood area ratio), and species-specific optimal moisture. These results demonstrated that while species with greater safety from turgor loss had lower mortality rates, hydraulic architecture was the only trait that explained species' moisture dependency. Species with a greater leaf area to sapwood area ratio were associated with drier sites and reduced their transpirational demand during the dry season via deciduousness.

Within vascular plants, the partitioning of hydraulic resistance along the soil-to-leaf continuum affects transpiration and its response to environmental conditions. In trees, the fractional contribution of leaf hydraulic resistance (Rleaf ) to total soil-to-leaf hydraulic resistance (Rtotal ), or fRleaf (=Rleaf /Rtotal ), is thought to be large, but this has not been tested comprehensively. We compiled a multibiome data set of fRleaf using new and previously published measurements of pressure differences within trees in situ. Across 80 samples, fRleaf averaged 0.51 (95% confidence interval [CI] = 0.46-0.57) and it declined with tree height. We also used the allometric relationship between field-based measurements of soil-to-leaf hydraulic conductance and laboratory-based measurements of leaf hydraulic conductance to compute the average fRleaf for 19 tree samples, which was 0.40 (95% CI = 0.29-0.56). The in situ technique produces a more accurate descriptor of fRleaf because it accounts for dynamic leaf hydraulic conductance. Both approaches demonstrate the outsized role of leaves in controlling tree hydrodynamics. A larger fRleaf may help stems from loss of hydraulic conductance. Thus, the decline in fRleaf with tree height would contribute to greater drought vulnerability in taller trees and potentially to their observed disproportionate drought mortality.

Considering the global intensification of aridity in tropical biomes due to climate change, we need to understand what shapes the distribution of drought sensitivity in tropical plants. We conducted a pantropical data synthesis representing 1117 species to test whether xylem-specific hydraulic conductivity (KS ), water potential at leaf turgor loss (ΨTLP ) and water potential at 50% loss of KS (ΨP50 ) varied along climate gradients. The ΨTLP and ΨP50 increased with climatic moisture only for evergreen species, but KS did not. Species with high ΨTLP and ΨP50 values were associated with both dry and wet environments. However, drought-deciduous species showed high ΨTLP and ΨP50 values regardless of water availability, whereas evergreen species only in wet environments. All three traits showed a weak phylogenetic signal and a short half-life. These results suggest strong environmental controls on trait variance, which in turn is modulated by leaf habit along climatic moisture gradients in the tropics.