Semester of Graduation

Summer 2025

Degree

Master of Science (MS)

Department

School of Renewable Natural Resources

Document Type

Thesis

Abstract

Drought is a threat to forests globally, with severe impacts on ecosystem structure and function predicted this century. Tree hydraulic traits are useful predictors of drought responses. However, hydraulic-trait data remain limited in floodplain ecosystems like bottomland hardwood (BLH) forests of the Southeastern U.S.A. These systems may be especially vulnerable to drought due to trade-offs between flood- and drought tolerance, yet hydraulic diversity within BLH tree species remains poorly understood. Empirical assessments of hydraulic traits, trade-offs, and alignment with drought tolerance indices in this ecosystem can help predict BLH forests response to future drought.

To address this, I measured hydraulic traits for 20 species in a Louisiana BLH forest, including the water potential at 50% loss of maximum stem hydraulic conductivity (P50), minimum leaf water potential (Ѱmin), Ѱmin hydraulic safety margin (HSMѰmin), and turgor loss point hydraulic safety margin (HSMTLP). I tested the following hypotheses: (1) BLH species exhibit less negative P50 values than upland-temperate species; (2) HSMѰmin values are like those observed in upland-forests globally; (3) species with more negative P50 and wider HSMѰmin have higher drought tolerance scores and slower growth-rates; and (4) HSMѰmin is correlated with HSMTLP.

Results revealed wide interspecific variation in hydraulic traits (P50: -4.6 to -1.09 MPa, Ѱmin: -3.60 to -1.23 MPa, HSMѰmin: -1.4 to 1.9 MPa, HSMTLP: -0.88 to 2.6 MPa). Contrary to expectations, BLH species showed similar P50 values to upland-temperate species. While the species-level average HSMѰmin was narrow (-0.11 MPa), it was consistent with other forest biomes, indicating potential drought vulnerability. However, high community-level variability (0.61 MPa, i.e., the standard deviation of community-weighted HSMѰmin) suggests that species diversity may improve the forest potential to buffer against drought. No significant relationship between hydraulic traits and drought tolerance scores, likely reflecting the influence of other traits such as deep root or deciduousness. No fast-slow trade-off was detected possibly due to the limitations in growth-rate classification. Lastly, HSMѰmin and HSMTLP were strongly correlated, primarily driven by P50. Despite frequent water availability, the BLH forest exhibits hydraulic traits comparable to upland systems, and high hydraulic diversity may enhance the forest's ability to withstand water stress.

Date

7-9-2025

Committee Chair

Brett Wolfe

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