Linking reliance on deep soil water to resource economy strategies and abundance among coexisting understorey shrub species in subtropical pine plantations

Strategies for deep soil water acquisition (WA(deep)) are critical to a species' adaptation to drought. However, it is unknown how WA(deep) determines the abundance and resource economy strategies of understorey shrub species. With data from 13 understorey shrub species in subtropical coniferous plantations, we investigated associations between the magnitude of WA(deep), the seasonal plasticity of WA(deep), midday leaf water potential (psi(md)), species abundance and resource economic traits across organs. Higher capacity for WA(deep) was associated with higher intrinsic water use efficiency, but was not necessary for maintaining higher psi(md) in the dry season nor was it an ubiquitous trait possessed by the most common shrub species. Species with higher seasonal plasticity of WA(deep) had lower wood density, indicating that fast species had higher plasticity in deep soil resource acquisition. However, the magnitude and plasticity of WA(deep) were not related to shallow fine root economy traits, suggesting independent dimensions of soil resource acquisition between deep and shallow soil. Our results provide new insights into the mechanisms through which the magnitude and plasticity of WA(deep) interact with shallow soil and aboveground resource acquisition traits to integrate the whole-plant economic spectrum and, thus, community assembly processes.