Water availability drives stem growth and stem water deficit of <Emphasis Type="Italic">Pinus canariensis</Emphasis> in a drought-induced treeline in Tenerife

Tree growth of Pinus canariensis at treeline in Tenerife, Canary Islands, is thought to be primarily controlled by wet season precipitation (P) prior to the current year´s growth. Therefore, we investigated the inter-annual variations in stem water deficit (ΔW) and radial growth (RG) during two consecutive years differing in wet season P. ΔW was extracted from stem circumference variations, and the influence of environmental variables was evaluated by Pearson correlation statistics. Wet season P was considerably lower in 2008 than in 2009; despite this difference in P between both years, shallow soil water availability was almost exhausted during both summers. However, the effect of shallow soil drought showing a clear seasonality of ΔW and RG was only detected in 2008. In summer 2009, RG rates were highest during the summer indicating that P. canariensis was able to tap water from deep soil layers originating from P prior to the current year´s growth. The ability to use deep soil water during extended periods of shallow soil water deficit was also reflected in a close positive correlation between RG and whole-tree water use. In our study, the effect of only one hydrological dry year resulted in a severe reduction in annual RG. Thus, when wet season P is low for a number of years, chronic drought may have negative implications for tree growth at treeline in Tenerife.