Growth, resource storage, and adaptation to drought in California and eastern Mediterranean oak seedlings

Low recruitment of new plants as currently observed in several California oak species might partly result from insufficient storage of vital resources (reduced ability to regrow after disturbance) or from low adaptation to environmental stress, such as drought. We conducted two studies under divergent environmental conditions to compare seedlings of California oaks with ecologically equivalent species from the eastern Mediterranean region, where recruitment has been substantial over the past 25-30 years. Resource concentrations (nonstructural carbohydrates and nitrogen) in taproots and the ratio of root resource content to shoot biomass did not provide evidence that Californian oak seedlings have smaller reserves for resprouting than eastern Mediterranean species in these studies. However, we observed lower seedling survival among California species, when compared with Mediterranean species, under conditions of higher drought stress. In the California deciduous Quercus douglasii Hook. & Arn., water-use efficiency as measured by leaf gas exchange and leaf δ¹³C was lower compared with its Mediterranean equivalent Quercus ithaburensis Decne. In addition, stomatal conductance in the California evergreen Quercus agrifolia Née appeared to be less sensitive to vapor pressure deficit than in its Mediterranean equivalent Quercus calliprinos Webb. Our results suggest lower adaptation to drought in the California species tested here as compared with Mediterranean species, which will negatively impact recruitment of oaks from California under the currently drier soil conditions.