Understanding how ferns vary their leaf traits across species and environments will improve models of ecosystem response to environmental change because the habitat specificity of ferns makes them excellent indicator species. Identifying drivers of variation in fern leaf traits will also support construction of predictive models for species ecological tolerances, distributions, and evolutionary trajectories. The goal of this dissertation was to execute five studies of diverse species across variable environments from three global regions, combining meta-analyses, field research, and experimental manipulations to investigate the causes and consequences of variation in fern leaf form and physiology.
Size-scaling of plant and leaf parts to maintain functional proportions was a significant factor explaining diversity in fern anatomy and morphology across species and environments, from differences in the density of veins to the size and shape of leaves. Adherence to global trends for the leaf economics spectrum - investment in higher leaf mass per area (LMA) at a cost of lower photosynthesis - also explained variation in leaf form and physiology due to different resource allocation patterns in leaves; however, investment into photosynthetic and non-photosynthetic leaf tissues was optimized at different scales (leaf or leaflet) depending upon the species.
Ferns ranged widely across resource gradients, and differences in moisture and irradiance were strongly associated with variation in leaf form and performance at growth and evolutionary timescales. I found evidence that ferns can adapt by optimizing rates of resource capture, metabolism and growth, and/or by optimizing stress tolerance and survival. Fast rates of water transport and gas exchange were not consistently associated with high irradiance, as seen for angiosperms, suggesting that optimization of photosynthesis under high irradiance is mediated by moisture availability. Species also showed a wider range of stomatal responses than previously reported for ferns and angiosperms, indicating a complexity of control that would modulate adaptation and acclimation at a wide range of timescales. Finally, trends of acclimation and adaptation of leaf traits tending in different directions emphasize the importance of specific compromises between long-term adaptation versus short-term acclimation to irradiance and moisture availability, and are expected to strongly influence fern abundances, distributions, and evolutionary trajectories.