2015/16 El Nino increased water demand and pushed plants from a Mesic tropical montane grassland beyond their hydraulic safety limits

In 2015/16, a strong El Nino event caused anomalously high temperatures and reduced precipitation resulting in Pantropical drought-induced diebacks and wildfires. Although many studies have documented the El Nino impacts on tropical forests, little we know about its effects on tropical grasslands. Here, we investigated plant drought responses during and after the 2015/16 El Nino event (Jun 2016 to Aug 2017) in 12 species with contrasting drought strategies (tolerance, avoidance and escape) in a Brazilian tropical montane grassland. We tested if (1) the El Nino event induced meteorological drought anomalies, (2) the atmospheric and/or soil drought led to plant water stress and (3) plants showed signs of drought recovery. In contrast to other tropical areas, we found that the 2015/16 El Nino event did not strongly affect precipitation in our study site. However, it increased air temperature and vapour pressure deficit, thus pushing all grassland species, even the most drought-tolerant ones, beyond their hydraulic safety margins during the dry season. Most species showed signs of drought recovery, returning to positive hydraulic margins in the wet season after the El Nino. However, the finding that all evaluated species, regardless of their drought-response strategy, are already operating close to their hydraulic safe thresholds for stomatal closure and turgor loss suggests that this cool-humid tropical montane grassland is especially vulnerable to meteorological extremes exacerbated by the additive effects of El Nino and climate change.