Drought and Freezing Compromise Woody-Plant Functioning in High Mountain Ecosystems Under Mediterranean Climate: The...

Bencomia exstipulata is a broad-leaved evergreen shrub endemic to the high-altitude harsh landscapes of the Canary Islands. Whether the extremely reduced distribution of its wild specimens (at around 2000 m a.s.l. in the National Parks of 'El Teide' and 'Caldera de Taburiente') responds to specific microclimatic needs remains undetermined. None of its ecophysiological aspects has been evaluated to date. To fill this gap of knowledge, we have (1) characterised its leaf phenology and physiology, (2) evaluated its tolerance to drought at leaf and xylem levels, and (3) investigated its response to freezing stress at biophysical and photochemical levels. Our results revealed that B. exstipulata has a fast leaf turnover with high rates of photosynthesis and stomatal conductance and high xanthophylls per chlorophyll ratios. Leaves had thin cuticle, high minimum leaf conductance, and encrypted and abaxial stomata. Adult leaves lost 50% of their rehydration capacity at around 45% relative water content, and stem xylem was relatively vulnerable to embolism (with -3 MPa of water potential at 50% loss of conductivity), but no native embolism was found. Under freezing temperatures, leaves showed a supercooling strategy (ice nucleation at -14°C) and low photoprotective responsiveness. We conclude that B. exstipulata lacks clear adaptations to drought, has high constitutive photoprotection, low photoprotective responsivity, and a supercooling strategy to face freezing. In its native montane ecosystem, with a Mediterranean-type climate, severe drought could induce significant xylem-embolism, while severe freezing could lead to irreparable leaf damage, being both risks potentially lethal in the mid- to long term.