Evaluating the effect of drier and warmer conditions on water use by Quercus pyrenaica

Under climate change, severe and recurrent droughts can reduce forest production and cause widespread tree dieback. The response of different vegetation types to climate change can vary greatly and, therefore, must be individually assessed. This study was carried out in a Mediterranean oak forest (Quercus pyrenaica) subject to seasonal summer drought. To examine the response of the forest to the climate conditions predicted under climate change, a Soil-Vegetation-Atmosphere Transfer model [SPA, Williams, M., Rastetter, E.B., Fernandes, D.N., Goulden, M.L, Wofsy, S.C., Shaver, G.R., Melillo, J.M., Munger, J.W., Fan, S.M., Nadelhoffer, K.J., 1996. Modelling the soil-plant-atmosphere continuum in a Quercus-Acer stand at Harvard Forest: the regulation of stomatal conductance by light, nitrogen and soil/plant hydraulic properties. Plant, Cell, Environment 19, 911-927] was used. The model was parameterized using mostly local measurements (independent of the verification data) and tested against in situ sap flow measurements obtained during year 2007. The predictions of the model were broadly consistent with the observed dynamics of sap flow (the model explained 71% of the variance in daily transpiration and 75% of half-hourly sap flow), leaf water potentials and soil water content. Once the model had been validated, simulations were carried out under warmer and dryer conditions. Predicted warmer conditions (4 degrees C) caused a moderate increase in total simulated transpiration. Less frequent precipitation (40% longer dry periods between rainfall events) had very little effect on transpiration. In contrast, transpiration was reduced by 17% when the soil water reserves at the beginning of the summer were lower than in 2007, corresponding to those measured in a very dry year (2005). The reduction was exacerbated when changes in temperature and rainfall were also considered (up to 28% decline in transpiration). The higher atmospheric CO2 concentrations (712 ppm) simulated together with climate change, did not prevent the decline in tree water use or soil water storage at the end of the summer. All scenarios caused the soil water storage to reach extremely low values at the end of the dry season (a minimum of 25 mm). It is concluded that climate change is likely to have a negative impact on tree water use and soil water resources in the study area, increasing the water deficit by as much as 30%. (C) 2009 Elsevier B.V. All rights reserved.