Date of Award

Spring 5-2021

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Biology

First Advisor

Joshua Pope Adams

Abstract

The Southeastern U.S. is known as the “Wood Basket” for producing the vast majority of U.S. timber, but projected increases in the frequency of severe drought events could threaten timber plantations. Loblolly pine (Pinus taeda) is the principle timber species of the southeast and is distributed across the Atlantic Coastal Plains and west into Texas. Western Gulf states (LA, TX, AR, and OK) lie at the edge of the range for loblolly pine and are particularly vulnerable to any changes in precipitation or temperature. Current research suggests two main methods to increase plantation drought resistance: plant drought resistant seedlings or reduce tree density. This study seeks to evaluate whether genetic entry or plantation density management (thinning) has the capacity to ameliorate drought stress. Through a factorial design, the effect of drought on three genotypes at two density-levels was compared through examining leaf physiology. Leaf-level gas exchange parameters such as stomatal conductance (gs), maximum gross photosynthesis (Pgmax) and dark respiration (Rd) were measured seasonally, with an emphasis on summer months. For one summer, drought was simulated through trenching plot perimeters and constructing throughfall exclusion shelters to limit lateral water flow and precipitation, respectively. Our analysis found that each gas exchange parameter varied significantly between seasons. Significant genotypic differences were detected in Pgmax and Rd, but not gs. Following the initiation of drought simulation in June 2020, throughfall exclusion structures were successful at reducing the soil moisture of drought treatments by early August. Despite the reduced soil moisture, statistically significant effects of drought stress were not detected in gas exchange parameters. Even during the heavy precipitation from hurricane Laura, throughfall exclusion structures were able to maintain soil moisture differences between drought and non-drought treatments; however, the drought simulation was not able to push drought treatments past key ecological stress thresholds. We conclude that loblolly pine is indeed a very drought resistant species and that further evaluation is needed to understand genotypic differences in drought resistance and whether thinning can ameliorate plantation drought stress.

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