Sánchez-Costa E., Poyatos R., Sabaté S. (2015) Contrasting growth and water use strategies in four co-occurring Mediterranean tree species revealed by concurrent measurements of sap flow and stem diameter variations. Agricultural and Forest Meteorology. 207: 24-37.EnlaceDoi: 10.1016/j.agrformet.2015.03.012
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. © 2015 Elsevier B.V.
Moran-Lopez T., Poyatos R., Llorens P., Sabate S. (2014) Effects of past growth trends and current water use strategies on scots pine and pubescent oak drought sensitivity. European Journal of Forest Research. 133: 369-382.EnlaceDoi: 10.1007/s10342-013-0768-0
Drought-induced decline is affecting Pinus sylvestris populations in southern Europe, with very little impact on the more drought-tolerant Quercus pubescens. Although multiple studies have investigated interspecific differences in water use and growth strategies, the link between these two processes and how they vary within drought-exposed populations remains poorly understood. Here, we analysed tree ring and sap flow data from P. sylvestris and Q. pubescens stands in the Pyrenees in order to (1) evaluate differences in climate-growth responses among species, (2) disentangle the role of past growth trends and water use strategies in individual trees drought sensitivity and (3) assess whether such intraspecific patterns vary between species. Both species have suffered recent climatic constraints related to increased aridity. However, the effects of past growth trends and current water use traits on drought sensitivity varied among them. Initially, fast-growing 'drought-sensitive' pines displayed a higher gas exchange potential but were more sensitive to evaporative demand and soil moisture. They also showed lower water use efficiency for growth (WUEBAI) and current growth decline. In contrast, initially, slow-growing 'drought-tolerant' pines showed the opposite water use traits and currently maintain the highest growth rates. In comparison, neither current WUEBAI nor recent growth trends varied across Q. pubescens climate-growth groups. Nonetheless, 'drought-sensitive' oaks showed the lowest gas exchange potential and the highest growth rates under milder conditions. Our results show a strong effect of past growth trends and current water use strategies on tree resilience to increased aridity, which is more evident in P. sylvestris. © Springer-Verlag Berlin Heidelberg 2013.
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