Seasonal cycles of sap flow and stem radius variation of Spartocytisus supranubius in the alpine zone of Tenerife, Canary Islands

We analyzed the seasonal patterns of sap flow density ( Q s ) and stem radius variation (SRV) of Spartocytisus supranubius , a dominant, endemic tree-like shrub of the mountain vegetation at high elevation in Tenerife, Canary Islands. We tested the hypotheses that drought and its effect on water status and on radial growth (RG) is primarily related to cool–wet-season precipitation preceding the current year rather than to the dry summer per se . During 2013 and 2014 Q s and SRV were monitored with Granier-type sap flow sensors and automatic band dendrometers, respectively. Tree water deficit (Δ W ) was extracted from SRV, and standard meteorological factors were used to calculate daily reference evapotranspiration ( ET r ) and soil moisture deficit. In both years investigated Q s was highest during the dry summer coinciding with periods of high ET r , indicating that plants were able to tap water from deep soil layers originating from precipitation prior the current year’s growth. The high RG and low ΔW rates observed throughout both dry summers are consistent with a direct access of tap roots to deep soil water reserves. Seasonal variations in RG of S . supranubius were mainly determined by the course of Q s and thus dependent on temperature and tree water status.