Climatic niches link southern oak (genus Quercus) evolution and physiology

Claudia J. Garnica-Díaz, Raiza Castillo-Argaez, Nicholas Smith and Grace P. John, Biology, University of Florida, Gainesville, FL, Susannah Dorrance, Biology, University of North Florida, Jacksonville, FL, Jeff Chieppa and Michael Aspinwall, School of Forestry and Wildlife Sciences, Auburn University, Auburn, AL, Kathryn Fuller, Hawkesbury Institute for the Environment, Western Sydney University, Richmond, NSW, Australia

Background/Question/Methods
Oaks (genus Quercus) has been described as an evolutionary success, due to their broad distribution across the northern hemisphere. Alternative niches occupation via structural and physiological diversity may explain the overlapping geographic distributions of oak species. We used southern oak seedlings in a common garden as a model system to test the influence of species anatomy, physiology, and ancestry on climatic niches, to establish (1) whether climatic niches reflect phylogenetic relatedness and (2) if the evolutionary patterns in climatic niche reflect species differences in leaf physiology. We further tested (3) if species leaf physiology and climatic niches are linked to underlying anatomical and morphological drivers. We measured stomatal conductance (gs) and saturated photosynthetic rate (As), stomatal size (Ss) and density (Sd), leaf vein diameter (Vs) and density (VLA), wood conduit diameter (Ws), and density (Wd), root conduit diameter (Rs) and density (Rd), and leaf mass per area (LMA) for potted seedlings of nine Oak species in the Virentes, Lobatae, and Quercus sections. We used distribution ranges based on GBIF, BISON and FIA to characterize climatic niches (mean annual temp (T), precipitation (PPT), and aridity index (AI)) from WorldClim. Evolutionary patterns were tested using trimmed oak megaphylogeny (Hipp et al 2020).
Results/Conclusions
We found that climatic niche aligned with phylogenetic relatedness resulting in the divergence between sections such that the Quercus section utilizes moist, temperate habitats, the section Lobatae utilizes relatively dry, temperate habitats, and section Virentes utilizes hot habitats with variable moisture Both stomatal gs and As differed by phylogenetic section (p-value: 0.027 and 0.001, respectively), however, As showed stronger ancestral alignment with T, while gs only aligned with the niche divergence of Virentes. As also scaled with LMA (R2= 0.447, P= 0.001), contrary to expectations of the global leaf economic spectrum, and LMA variance aligned well with the phylogenetic patterns for thermal niche differentiation, suggesting structural and physiological traits divergence may have been driven by selection for phenology and life history. Notably, anatomical traits scaled across organs and were highly indicative of gs independent of species likely due to a strong developmental or mechanical constraint on hydraulic anatomy. Our study provides evidence for climatic niche adaptations in southern Oaks, the linkage between climatic niche and physiological adaptation, and presents novel coordination of hydraulic anatomy across plant organs.