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

Drought limits tree water use and growth of Mediterranean trees. However, growth and water use strategies are rarely addressed simultaneously across species and drought conditions. Here, we investigate the link between stem diameter variations and sap flow in four co-existing Mediterranean trees (Pinus halepensis 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 (ΔW), an indicator of stem hydration. P. halepensis and Q. 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 shrinkage in response to drought (i.e., increasing ΔW) and subsequent trunk rehydration after rainfall (i.e., decreasing ΔW) occurred in all species. Vapor pressure deficit (VPD) and soil moisture (θ) interacted to determine seasonal variation in ΔW. Interestingly, in the dry year, 2012, more species-specific differences were found in the responses of ΔW to θ and VPD. Across species, JS and ΔW began to decline at similar soil moisture thresholds, underpinning the tight link between JS and ΔW under varying drought conditions. 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 studied Mediterranean tree species show coordinated responses of transpiration, water storage dynamics and growth-based WUE which allow them to cope with seasonal and interannual drought.