Contrasting growth and water use strategies in four co-occurring Mediterranean tree species revealed by concurrent measurements of sap flow and stem diameter variations

Abstract

Drought limits tree water use and growth of Mediterranean trees. However, growth and water use strate-gies are rarely addressed simultaneously across species and drought conditions. Here, we investigatethe link between stem diameter variations and sap flow in four co-existing Mediterranean trees (Pinushalepensis Mill., Quercus pubescens Willd., Quercus ilex L. and Arbutus unedo L.), under relatively wet (2011)and dry (2012) conditions. Continuous stem diameter variations were converted to basal area increment(BAI) and de-trended to estimate tree water deficit ( W), an indicator of stem hydration. P. halepensis andQ. pubescens showed the most and the least conservative sap flow density (JS) regulation under drought,respectively, with Q. ilex and A.unedo showing intermediate drought responses. All species, except A.unedo, showed some between-year variability in the environmental control of JS. Seasonal stem shrink-age in response to drought (i.e., increasing W) and subsequent trunk rehydration after rainfall (i.e.,decreasing W) occurred in all species. Vapor pressure deficit (VPD) and soil moisture ( ) interacted todetermine seasonal variation in W. Interestingly, in the dry year, 2012, more species-specific differ-ences were found in the responses of W to and VPD. Across species, JSand W began to decline atsimilar soil moisture thresholds, underpinning the tight link between JSand W under varying droughtconditions. Annual BAI decreased proportionally more than tree-level transpiration (JT) between the wet(2011) and the dry (2012) year, hence growth-based WUE (WUEBAI= BAI/JT) decreased for all species,albeit less acutely for P. halepensis. Overall, despite their contrasting leaf habit and wood type, the stud-ied Mediterranean tree species show coordinated responses of transpiration, water storage dynamicsand growth-based WUE which allow them to cope with seasonal and interannual drought.