Environmental Stress Shifts Growth Strategies of Oak and Heath Shrubs in the Florida Scrub

The ability of plant species to mitigate environmental stresses is crucial for maintaining their populations and communities. One potential mitigation approach is a growth strategy shift mediated by intraspecific trait variation. We evaluated potential stress-induced shifts in two growth strategies across six common Florida scrub shrub species in two major groups, oaks (Quercus spp., Fagaceae) and heaths (Ericaceae), along an elevational stress gradient. The acquisitive-conservative gradient explains how plants invest in aboveground tissues to photosynthesize more, while the collaboration gradient explains how plants attain belowground resources using a ‘do-it-yourself’ strategy or collaboratively ‘outsourcing’ with mutualistic mycorrhizal fungi. We expected high-stress environments to shift species towards conservative growth with increased symbiont collaboration compared to low-stress environments. For each species, we measured a suite of aboveground (all species) and belowground (oaks only) plant functional traits in two or more different habitats along an elevational stress gradient. Aboveground traits (leaf circularity and stomatal density) significantly decreased with stress across all species analyzed, suggesting a shift to a more conservative strategy. Belowground oak traits showed significant support for a shift to more collaboration with stress, with decreases in specific root length and increases in ectomycorrhizal tip density with stress. Overall, we found that oak and heath shrubs showed limited support for shifting to a conservative strategy with stress and relatively strong support for the oaks to shift to a more collaborative strategy. These findings advance our broader knowledge of how oak and heath species cope with environmental stress via shifting growth strategies.