Canopy transpiration of a semi arid Pinus canariensis forest at a treeline ecotone in two hydrologically contrasting years

Given its importance, transpiration of forest canopies has been measured in a wide range of environments. However, measurements in treeline ecosystems with Mediterranean climate are rare more so in semi arid climates. Therefore in the present study we investigate inter-annual variation in canopy transpiration (Ec) of a Pinus canariensis forest located on a treeline ecotone in Tenerife, Canary Islands, Spain, where the climate is typically semi arid Mediterranean. Meteorological conditions and sap flow were continuously monitored during two consecutive years. In both years, atmospheric conditions were typical for the study site. Nevertheless large differences were detected in precipitation (P) with a high inter-annual variability. Calendar year (January-December) P was higher in 2008 than in 2009, while in the corresponding hydrological year (October-September), P was considerably lower in 2008 than in 2009. Despite this difference in P between both years top soil water availability was almost exhausted during both summers. However, the effect of top soil drought, showing a visible seasonality on Ec was only clearly detected in 2008 with values down to 0.02 mm day−1 during summer. During the summer 2009, in contrast, Ec rates were up to 1.12 mm day−1 showing that P. canariensis is able to tap water from deep soil layers when P of the previous wet period (October-April) recharged these reserves. This points out the importance of evaluating sap flow studies in semi arid treelines with respect to hydrological year P. Climate change models predict a reduction of more than 30% in precipitation. In our study the effect of only one hydrological dry year resulted in severe water stress. Thus, when winter rainfall is low for a number of years, chronic drought may have negative implications for tree and stand transpiration in semi arid treelines.