Fragmentation reduces severe drought impacts on tree functioning in holm oak forests

Fragmentation and increased summer drought are two main threats to Mediterranean forests. Forest fragmentation has many negative impacts on forests but could attenuate water stress on Mediterranean species by reducing intraspecific competition or improving soil properties at forest edges. However, little is known about the combined effects of drought and fragmentation on tree functioning. We evaluated the effect of forest fragmentation on tree functioning under severe drought periods in continental holm oak forests (Quercus ilex). We monitored the functional response of focal trees before and during summer drought in two regions of the Iberian Peninsula with contrasting climates. Forest interiors, edges and small forest fragments were compared. Predawn leaf water potential (Psi(pd)), leaf stomatal conductance to water vapor (g(s)), maximum leaf photochemical efficiency (F-v/F-m), ground (F-o) and maximum (F-m) fluorescence in dark adapted, fully-developed leaves were assessed in the field. Trees located at forest interiors and edges showed strong stomatal closure and clear symptoms of photochemical inhibition, revealing that the trees were operating at predawn water potential beyond tolerance thresholds. In contrast, trees located in small forest fragments were able to maintain high levels of functionality for all ecophysiological parameters during the drought. These differences in tree functioning among fragments were more noticiable in the driest region. Our results prove that forest fragmentation attenuates drought impacts on tree functioning, and that these positive effects become more important during extremely dry periods. These novel findings are essential for realistic predictions of the functionality of fragmented forests under increasing and more severe droughts.