Variation in hydraulic vulnerability among tree species in a bottomland hardwood forest

Tree hydraulic traits offer useful insights into species- and ecosystem-level responses to droughts, which threaten forests globally. However, hydraulic traits are poorly described in floodplain ecosystems like bottomland hardwood (BLH) forests in the southeastern U.S.A. A tradeoff between flood tolerance and drought tolerance suggests that BLH tree species may have hydraulic traits that make them vulnerable to droughts. To assess this, we measured 20 tree species in a BLH forest in Louisiana for their water potential at 50% loss of maximum stem hydraulic conductivity (P50), minimum leaf water potential (Ѱmin), and hydraulic safety margin based on Ѱmin (HSMѰmin). We tested whether BLH species exhibit: (1) less-negative P50 values than upland-temperate species; (2) HSMѰmin values similar to those in upland-forests globally; and (3) associations between P50, HSMѰmin, drought-tolerance scores, and growth rates. BLH species had wide interspecific trait variation (P50: -4.21 to -1.09 MPa, Ѱmin: -3.60 to -1.23 MPa, HSMѰmin: -1.4 to 1.89 MPa). Contrary to expectations, BLH species had similar P50 values to upland-temperate species. While the species-level average HSMѰmin was low (0.03 MPa), it was consistent with other ecosystems, indicating potential vulnerability to drought. However, high community-level hydraulic variability (i.e., the community-weighted standard deviation of HSMѰmin = 0.78 MPa) suggests that the forest is potentially buffered against drought. We found no significant relationships between hydraulic traits and drought tolerance scores or growth rates. Overall, BLH forest trees exhibit hydraulic traits comparable to those of upland systems, which may confer similar capacity to buffer against droughts.