Small trees that grow up in drought conditions could form the basis for more drought-resistant rainforests in the future, a new study suggests.
Using data from a 15-year experiment in Brazil, an international research team discovered that many small trees respond positively to the extra light they get when larger trees die, increasing their capacity for photosynthesis and their growth despite the lack of water.
“Having grown in up in drought conditions, these trees might develop traits that will help them cope with future droughts, even once they are fully grown,” says lead author David Bartholomew, from the University of Exeter, UK. “Ultimately, this may allow them to form the next generation of canopy trees, leading to greater overall resilience in the forest.”
The study, which is published in the journal Plant, Cell & Environment, brought together researchers from the UK, Brazil, Australia and Spain.
It was carried out at Caxiuanã National Forest Reserve at Para, in Brazil’s eastern Amazonia region. Here, as part of the world’s longest running tropical forest exclusion (TFE) experiment, 50% of rainfall is captured before it reaches the trees in one area, to allow comparison with a nearby control area.
Bartholomew and colleagues studied 66 small trees from 12 common genera (up to a 10-centimetre diameter and 1.3m from the ground) and 61 surviving canopy trees (more than 20 centimetres in diameter).
Small trees in the drought area showed increased capacity for photosynthesis, 32% more leaf respiration and 15% more leaf mass per area compared to those in the control area.
“Under prolonged drought, small trees experience changes to both their light and water availability, whereas larger canopy trees will predominantly only experience substantial shifts in water availability,” the researchers write in their paper.
“The different responses to prolonged drought suggest small trees are responding most strongly to changes in their light environment, while large trees are responding to reductions in soil moisture availability.
“These shifts may reflect changes in the most limiting resource from the understory to the top canopy, with understory trees strongly light limited and canopy trees strongly water or nutrient limited.”
The responses of different tree species did vary, the researchers say, with some showing a strong ability to adapt and others very little.