Consequences of pine colonization in dry oak woodlands: effects on water stress

The potential impacts of species colonization on the structure and functioning of ecosystems are poorly understood. We propose a novel approach for understanding the consequences of habitat colonization, highlighting the influence of colonists on the availability of limiting resources to resident species. We studied how colonization of dry oak woodlands by pines (Pinus halepensis) is affecting water stress of resident oaks (Quercus calliprinos). We monitored predawn leaf water potential (PLWP) of oaks monthly for 2 years. Using maximum likelihood and multi-model inference, we evaluated how the PLWP of oaks was affected by pine colonists. The influence of colonizing pines on PLWP of resident oaks varied in time and space from negative to positive depending on season, oak size, pine size, and proximity to pines presence. The water stress of oaks increased along the dry season (− 1.5 to − 4.5 MPa), with small oaks becoming more severely stressed than large ones (up to 60% difference). During the dry season, neighboring pine colonists increased the water stress of oaks (up to − 0.4 MPa difference), but during the wet season, they reduced the water stress mainly for large oaks. Our findings indicate that pine colonization differentially affects water limitation for resident oaks with implications for future development and regeneration. The influence of pine colonists shifted from positive to negative along an increasing water stress gradient, contrary to predictions by the stress gradient hypothesis. Our work demonstrates how colonization by non-resident species can influence key ecosystem processes through the redistribution of limiting resources. Identifying these processes is fundamental for understanding the consequences of colonization, mitigating these influences, and predicting future change in the structure and function of ecosystems.