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

In 2015/16, a strong El Niño event caused anomalously high temperatures
and reduced precipitation resulting in Pantropical drought‐induced
diebacks and wildfires. Although many studies have documented the El Niño
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 Niño 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 Niño 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 Niño 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 Niño. 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 Niño and climate change.