Fog mitigates the consequences of a profligate water use strategy in a Macaronesian cloud forest tree species

How plants thrive in a cloud immersed environment where leaves are intermittently wet and the environmental conditions that drive transpiration and growth are limited, raises a relevant question in fog affected ecosystems. In order to provide insight into how cloud immersion and fog interception may affect Macaronesian laurisilva forests, micrometeorological variables, artificial fog water collection, throughfall, soil water content and the altitude of the trade wind inversion layer, together with the hourly sap flow, Q(t), of a dominant tree species, Myrica faya, were measured at an exposed site of the Anaga Massif Rural Park Biosphere Reserve (Tenerife, Canary Islands) over a period of one year. Foggy conditions led to a 45.1% reduction in global radiation and a more than a 10-fold decrease in sap flow, throughout all day hours. M. faya showed a weak control of the transpiration rate and a profligate water use strategy, such that a substantial night-time sap flow, Q(tn), was observed under high nocturnal atmospheric evaporative demand, representing 23.3% of the total daily Q(t), even though fog was more frequent at night. Fog water interception resulted in canopy wetting and dripping for at least 55.0% of the time, and an associated downward xylematic sap transport in the most apical branches, i.e. in foliar water uptake. This represented 4.0% of the upward sap flow and was observed in 26.7% of the hourly Q(t) records. Nocturnal transpiration was also enhanced by the entry of previous foliar moisture. This general plant and climatic phenomenology was related at the mesoscale with the trade wind inversion height in the subtropical Macaronesia area.