Differences in xylem and leaf hydraulic traits explain differences in drought tolerance among mature Amazon rainforest trees

Abstract

Considerable uncertainty surrounds the impacts of anthropogenic climate change on the composition and structure of Amazon forests. Building upon results from two large-scale ecosystem drought experiments in the eastern Brazilian Amazon that observed increases in mortality rates among some tree species but not others, in this study we investigate the physiological traits underpinning these differential demographic responses. Xylem pressure at 50% conductivity (xylem-P 50 ), leaf turgor loss point (TLP), cellular osmotic potential (p o ), and cellular bulk modulus of elasticity (e), all traits mechanistically linked to drought tolerance, were measured on upper canopy branches and leaves of mature trees from selected species growing at the two drought experiment sites. Each species was placed a priori into one of four plant functional type (PFT) categories: drought-tolerant versus drought-intolerant based on observed mortality rates, and subdivided into early-versus late-successional based on wood density. We tested the hypotheses that the measured traits would be significantly different between the four PFTs and that they would be spatially conserved across the two experimental sites. Xylem-P 50 , TLP, and p o , but not e, occurred at significantly higher water potentials for the drought-intolerant PFT compared to the drought-tolerant PFT; however, there were no significant differences between the early-and late-successional PFTs. These results suggest that these three traits are important for determining drought tolerance, and are largely independent of wood density-a trait commonly associated with successional status. Differences in these physiological traits that occurred between the drought-tolerant and drought-intolerant PFTs were conserved between the two research sites, even though they had different soil types and dry-season lengths. This more detailed understanding of how xylem and leaf hydraulic traits vary between co-occuring drought-tolerant and drought-intolerant tropical tree species promises to facilitate a much-needed improvement in the representation of plant hydraulics within terrestrial ecosystem and biosphere models, which will enhance our ability to make robust predictions of how future changes in climate will affect tropical forests.

Dry‐season decline in tree sapflux is correlated with leaf turgor loss point in a tropical rainforest

“…Our results extend to the whole plant scale the previously recognized linkage between π tlp and stomatal closure Brodribb et al, 2003;Martin-StPaul et al, 2017). In addition, recent studies have reported that π tlp was correlated with lethal water potentials and could therefore be useful to predict the vulnerability of trees to major drought events Powell et al, 2017).…”

“…Our results extend to the whole plant scale the previously recognized linkage between π tlp and stomatal closure (Bartlett, Klein, Jansen, Choat, & Sack, ; Brodribb et al, ; Martin‐StPaul et al, ). In addition, recent studies have reported that π tlp was correlated with lethal water potentials and could therefore be useful to predict the vulnerability of trees to major drought events (Bartlett et al, ; Powell et al, ). π tlp has also been found to inform species distributions relative to water supply within and across biomes (Baltzer, Davies, Bunyavejchewin, & Noor, ; Bartlett, Scoffoni, & Sack, ) and community assemblages (Bartlett et al, ), both of which have proven challenging to model in the tropics.…”

Trait evolution in tropical rubber (Hevea brasiliensis) trees is related to dry season intensity

“…Wood VD and F were also weakly related to both measures of dry season intensity ( r 2 = 0.34–0.37), though the correlation with Pq may have been biased by tree diameter (VD and F were still significantly correlated with Pcv, Table ). In two large‐scale drought experiments in the Amazon rainforest, where water availability in the control and drought‐stressed variants was similar to the wet and dry extreme of the Hevea sample locations, mortality was related to wood and leaf traits including xylem vulnerability and turgor loss point (Powell et al., ), which are functionally related to the traits we studied (see introduction). It is therefore likely that traits that confer drought resistance have been selected variably across the range of Hevea .…”

Tree hydraulic traits are coordinated and strongly linked to climate‐of‐origin across a rainfall gradient

“…Embolism reduces the capacity of xylem to conduct water and, if unrepaired, may eventually lead to whole plant dehydration and mortality (Brodribb & Cochard, ; Urli et al, ). The ability to resist xylem embolism (i.e., P x ) has emerged as a key trait in assessing plant drought tolerance and is generally defined as the xylem water potential generating a 50% loss of conductance (P 50 ; Choat et al, ; Maherali, Pockman, & Jackson, ; Powell et al, ). The P 50 is often correlated with habitat water availability (Blackman et al, ; Bourne, Creek, Peters, Ellsworth, & Choat, ; Larter et al, ; Pockman & Sperry, ; Trueba et al, ) and has been shown to coincide with drought‐induced mortality in gymnosperms (Brodribb & Cochard, ), whereas in angiosperms, lethal water potentials are typically associated with greater losses of conductance, for example, P 88 or P 99 (Kursar et al, , Li, Feifel, et al, , Urli et al, ).…”