PS 4-51 - Elevational shifts in coastal forest tree species distributions: What can we infer about environmental drivers?

Abstract: Maritime Live Oak (MLO) forests of the Southeastern U.S are globally rare and highly valued for their foundational ecological role and iconic status in coastal landscapes. In coastal Georgia, these forests are dominated by Quercus virginiana (live oak), and often interspersed with other canopy species such as Q. hemisphaerica (laurel oak), Q. nigra (water oak), Pinus taeda and P. elliottii (loblolly and slash pine). Forest stewards across several of Georgia’s barrier islands are concerned about very low rates of observed live oak regeneration, noting also that these forests face several anthropogenically linked stressors, including decreasing freshwater availability due to aquifer depletion and climate change effects on precipitation and storms. In this study, we used elevation as a proxy for groundwater availability to examine tree distribution patterns that would be consistent with hydrological impacts on forest dynamics. Previous studies in mixed live oak forests found that live oaks tend to dominate at higher soil moisture, while laurel oaks are more abundant in drier locations. We established two 100 x 200m plots in mature MLO forests on Jekyll Island, GA, aligned to span elevation gradients. In the plots, we mapped and measured the diameter of all trees. In 200 50x50cm subplots at stratified random locations, we counted seedlings of each species and measured canopy density with hemispherical photography. For each plot, we binned elevations into five zones, and compared the density of mature trees, saplings, and seedlings across elevation zones. We found that mature live oak density was greatest at lower elevations but well-represented across the gradient, while mature laurel oaks increased from virtually absent at lower elevations to codominant at higher elevations. Saplings and seedlings showed very different patterns. In particular, live oak seedlings were more strongly restricted to lower elevations than adults, whereas laurel oak seedlings were more abundant at low elevation than mature trees. We evaluate how the observed patterns align with expected distributions under the hydrological change hypothesis as well as hypotheses of multi-stage environmental filters. The observed elevational shifts in size classes for both live oaks and laurel oaks are consistent with the expected effects of reduced water availability over time, and we infer that other ecological filters are also likely influencing laurel oak distribution. We discuss how the findings are guiding a suite of additional research questions regarding the resilience and future of these cherished forests.