Drought reduces the growth and health of tropical rainforest understory plants

Tree saplings and shrubs are frequently overlooked components of tropical rainforest biodiversity, and it may be hypothesized that their small stature and shallow root systems predisposes them to be vulnerable to drought. However, these purported influences of drought on growth, physiological performance and plant traits on tree saplings and shrubs have yet to be studied in simulated drought conditions in the field. We simulated drought using a rainfall exclusion experiment in 0.4 ha of lowland tropical rainforest in northeast Australia in 2015. After six months, we compared the average change in aboveground biomass and plant health of drought-affected tree saplings and understory shrubs with control individuals. We also assessed photosynthetic function, plant health and leaf traits in eight target species. Both tree saplings and shrubs had significantly lower aboveground biomass in the drought treatment compared to the control. Drought-affected individuals of target species exhibited a significantly higher incidence of disease and insect attack, and reduced photosynthesis, leaf fresh mass and leaf toughness compared to control individuals. We conclude that reduced growth and photosynthetic capability, an increased susceptibility to insect attack, and leaf trait changes constitute a near immediate drought response in tropical rainforest tree saplings and shrubs. Our results show that these often-overlooked lifeforms are likely to be the most rapidly and negatively impacted component of tropical rainforest biodiversity under drought conditions.