Wide variation in hydraulic safety margins across and within southern Amazon forests

Abstract Trees of southern Amazonian forests face an increase in the duration and intensity of soil and atmospheric drought, which impose a higher risk of hydraulic failure in the water transport system, decreasing the water supply to foliage and increasing the risk of dehydration and death. However, it is not well understood how key hydraulic‐related traits vary among southern Amazonian forests and whether Amazonian trees adjust them to be more drought tolerant along a gradient of increasing aridity. We measured leaf water potential (Ψ leaf ) in the dry season, xylem water potential associated with 50% (Ψ 50 ) and 88% embolism formation, hydraulic safety margin (HSM 50 ) and wood density (WD) in four forests at the southern edge of the Amazon. We compared our results with measurements from the literature for 10 additional forests distributed across the Amazon and subjected to different maximum cumulative water deficit (MCWD), a climate index for drought severity. For some analyses, species sampled within each forest were weighted by basal area to obtain community‐weighted means (CWMs). Southern edge forests had high resistance to hydraulic failure, with more negative Ψ 50 than other Amazonian forests. HSM 50 across species correlated positively with predawn Ψ leaf , suggesting that shallower rooting species in these forests face greater risk of hydraulic failure. Across all sites considered (southern and wider Amazon), HSM 50_CWM did not correlate with MCWD, despite a more negative Ψ 50_CWM with increasing MCWD. However, forests at the driest range of MCWD showed greater variability in hydraulic failure risk and contained more species potentially operating beyond their hydraulic limits. Synthesis . Although southern Amazonian forest species have undergone selection for high hydraulic resistance, they are not equally safe to hydraulic failure, highlighting the importance of site‐ and species‐dependent factors. On a broader scale across the Amazon, tree communities in drier sites maintain HSM 50_CWM similar to those in wetter sites. This may be the result of species turnover towards taxa with more negative Ψ 50 or shifts in water use strategies, such as tight stomatal regulation, deeper rooting or seasonal leaf shedding.