Vegetation vulnerability is driven by either higher drought sensitivity or lower fog exposure in tropical cloud ecosystems.

Both reduced precipitation and reduced fog uplift increase drought-driven plant mortality. However, it is still unclear how plant vulnerability to drought in cloud ecosystems depends on the role of fog in relieving water stress via foliar water uptake (FWU). To investigate how plants in contrasting montane vegetation rely on fog to alleviate drought impacts, we measured 11 morpho-physiological traits in 10 phylogenetic pairs of plants in a montane grassland (~2000 m a.s.l.) and in a submontane forest (~700 m a.s.l.), both in southeast Brazil. Forest species are more sensitive to drought (i.e., lower conservative trait values, lower resistance to embolism, and lower FWU) than grassland species. Nonetheless, decreased frequency of fog events in the montane grassland may expose these species to a higher risk of dehydration, despite higher FWU capacity. Both forest and grassland vegetation are vulnerable to drought, but the vulnerability is attributable to different causes: higher sensitivity to drought in forests and lower fog exposure in grasslands. Therefore, for a more accurate description of plant responses to drought, we recommend introduction of theoretical-experimental models to assess drought vulnerability to changes in both atmospheric and soil water availability.