Phenotypic plasticity and performance of seven oak species seedlings across a hydrologic gradient: Support for the specialization hypothesis
Seedling establishment and growth is dependent on water availability. A developing seedling’s biomass and form allows for limited maintenance of homeostasis and photosynthesis under water stress conditions. Under mesic (non-stressful) conditions, seedling fitness and survivorship may be similar between species, but contrasting tolerances among species at the extremes of a hydrologic gradient may be critical in habitat partitioning and species coexistence. Using a common garden with three watering levels, we tested the extent to which broadly co-occurring species of Quercus physiologically differentiate at extremes of a hydrologic gradient. These species have contrasting habitat preferences as described by their wetland indicator status. Irrigation levels spanned xeric, mesic and hydric soil moisture conditions. We collected and grew the oaks from the Smithsonian Environmental Research Center (SERC), in Edgewater, Maryland for two growing seasons and measured their growth, survival and a suite of physiological traits. Phenotypic traits and plasticity patterns were compared to environmental parameters determined to be important in species’ environmental niche models (ENM).
Seedling performance, as measured by growth rate, was highest for all species under mesic conditions. At the extremes of the hydrological gradient, growth rates and multiple functional attributes varied by species. Whole plant hydraulic conductance covaried with growth and was highest in treatment conditions that resembled their habitat preferences. Likewise, species performance (e.g. RGR, AGR) across the experimental hydrologic gradient corresponded with habitat preferences. For all species, seedling quantum yield determined from chlorophyll fluorescence was depressed (Φ<0.8) when grown under hydric conditions. NPQ was elevated in xeric and hydric conditions, indicating upregulation of photoprotective mechanisms in response to reduced photosynthesis. Seedling δ13C (a measure of integrated water use efficiency, WUE) varied between upland and wetland species such that species typically occupying uplands showed lower isotope discrimination. This work builds upon previous studies showing trade-offs in physiological responses of co-occurring Quercus species across an environmental gradient, providing support for the specialization hypothesis.