Canopy transpiration of a Pinus canariensis forest at the tree line: implications for its distribution under predicted climate warming

摘要

Canopy transpiration (E_c) of a 50-year-old Pinus canariensis Chr. Sm. Ex DC. stand at tree line in Tenerife, Canary Islands, was estimated continuously throughout a year from March 1, 2008, to February 28, 2009, by means of xylem sap flow measurements. E_c varied markedly throughout the entire year generally following the seasonal trends in soil water availability and varied between 0.89 mm day~(-1) under the conditions of non-limiting soil water availability and close to zero under soil drought. This is because canopy conductance declined significantly with increasing evaporative demand and thus significantly reduced tree water loss, and this decrease was more pronounced during the soil drought. Total annual E_c was 79.6 mm, which is significantlybelow the values estimated for other Mediterranean forest ecosystems and even 70 % lower than the value estimated for a P. canariensis forest at 1,650 m a.s.l. where the soil water content was higher than at the tree line site. Therefore, these resultshighlighted the importance of drought stress in tree line ecotone and should be taken more into account in semiarid tree lines.