Trees harvesting the clouds: fog nets threatened by climate change.

The major direct drivers of environmental change in mountains are relief, hydroclimate and land use. Mountain forests affected by persistent seasonal or annual fog are called montane cloud forests (Bubb et al. 2004). On the slopes of temperate mountains, the elevation of the cloud base marks the transition from mon-tane (hardwood-deciduous, lower-altitude) to cloud (conifer-evergreen, higher-altitude) forests (Braun 1964). Montane cloud forests intercept fog and cloud droplets (horizontal precipitation), which can add substantial amounts of water to the hydrologic system (Bruijnzeel et al. 2011). Suppressed incoming radiation and transpiration rates may also be favourable for low water use (Bruijnzeel and Veneklaas 1998). Fog drip and cloud shading regulate ecosystem water balance and atmospheric vapour pressure deficit. In an intensified drought scenario (McDowell et al. 2011), in which water is the primary limiting factor to vegetation growth, horizontal precipitation and low transpiration might have a significant impact on water availability and, thus, shape mountain vegetation significantly. A potential increase in cloud base height and associated effects, on energy and water balance, underlie concerns regarding how cloud regime and fog frequency may be altered in coming decades (Williams et al. 2015). The combination of shallow root systems, reduced photosynthetic rates and lower air temperatures of temperate montane cloud forests may lead to increased sensitivity of vegetation damage by drought and warrants revisiting the sensitivity of stomatal con-ductance to varying vapour pressure deficit and regulation of water potential in these foggy ecosystems (Jane and Green 1985). Discontinuities in vegetation composition and community structure may offer insights into the factors controlling ecological processes along environmental gradients, such as those associated with elevational changes in temperature and moisture (Martin et al. 2007). Discrete montane forest ecotones are associated with pronounced environmental (and micro-environmental) disconti-nuities or with contrasting disturbance history (Palombo et al. 2014). However, the ecophysiological consequences of water uptake and transport mechanisms in temperate montane cloud forest tree species remain largely unexplored, as do the feedbacks on vegetation boundaries and interactions with atmospheric depositions. In this issue, Berry et al. (2015) compare plant water status during cloud-immersed and non-immersed conditions and hydraulic vulnerability in branches and roots of species across a temperate , mountain fog ecotone. Cloud forest species were expected to have less drought-resistance characteristics (e.g., lower water potential at 50% loss of hydraulic conductivity) and greater improvements in hydraulic functioning during fog relative to non-cloud forest species. In both conifer cloud forest …