Testing hypothesized trade-offs between drought tolerance and growth rate in 40 oak species across an experimental hydrologic gradient
Life history trade-offs between competitive ability and stress tolerance are a long standing concern in ecology. Is there a cost to construction and maintenance of the physiological attributes that confer stress tolerance? Previous studies have found evidence for life history trade-offs in Quercus. We expand upon previous work to investigate trade-offs between seedling growth and drought tolerance. Specifically, we hypothesized that under low-water conditions, species adapted to drought have higher WUE and growth than species adapted to mesic environments. Under well-watered conditions, we expected species adapted to mesic environments to have higher RGR than species adapted to drought-prone environments. We characterized the growth responses of forty oak species to an experimental hydrologic gradient and measured their functional traits related to water use, including leaf osmotic potential, gas exchange and stomatal properties. This subset of species collectively represents ~10% of Quercusgeneric diversity, including representatives of all major phylogenetic lineages in the Americas. Irrigation levels spanned xeric, mesic and hydric soil moisture conditions crossed with two climatic treatments (tropical and temperate) in a split-plot design. Functional trait responses to the hydrologic gradient were compared to environmental parameters from species natural distributions.
Seedling performance, as measured by growth rates, were lowest across species under xeric conditions. As soil moisture declined, leaf and stem dieback increased, although growth rates and multiple functional attributes varied by species or clade. Species showed highest performance under conditions similar to their native range (e.g., xeric species perform better under drier conditions). Carbon fixation and stomatal conductance rates were reduced under xeric water treatments, but were higher in species from drier environments. WUE was elevated in plants growing in drier soils as well as in species native to drier environments. Growth rates were positively associated with stomatal pore area. Leaf osmotic potential (ΠA) varied with ΨPD, and reduced solute accumulation was associated with growth traits, including total leaf area and plant height. Differences between saturated osmotic potential (ΠO) in xeric-mesic soil treatments indicated xeric and hydric species upregulate solute accumulation in response to drought although mesic species did not. Although xeric species performed better under long-term drought, under well-watered conditions their growth increased less than mesic species. Combined, these data provide evidence of functional adaptation of oak species to their native hydrologic regime.