Drought in the Amazon: Small trees adapt better than large trees

Climate change is dramatically altering many of the world’s natural ecosystems. In the Amazon, severe and prolonged droughts are becoming more frequent and more extreme. These droughts have been shown to kill rainforest giants, releasing large amounts of carbon into the atmosphere, changing the forest’s water cycle and the structure of the forest. My latest paper published in Plant, Cell and Environment looks at how these ecosystem level changes affect carbon metabolism in small trees under prolonged drought conditions.

Small understory trees are key components of a fully functioning rainforest ecosystem. However, very little has been studied about these trees that range in diameter from 1 to 10 cm. Consequently, we still don’t know how they will respond to climate change. These trees represent the next generation of canopy trees, meaning their responses could be critical for the resilience of a forest under a changing climate. This is especially important with the loss of large canopy trees following drought conditions.

The understory of an intact rainforest is typically a dark and humid environment. The dense, multi-layered canopy above prevents most light from reaching the understory, meaning any trees that are found there must be tolerant of shaded conditions. Trees found in low-light conditions will typically downregulate their photosynthetic capacity to conserve resources as light is severely limiting. However, under prolonged drought conditions, large canopy trees are very vulnerable as they can no longer transport water to support their leaves, ultimately resulting in their death. The death of these giants can result in big changes to the canopy structure, causing a big increase in light availability to the forest understory. What does this mean for the small trees that are usually adapted to dark conditions? Can they upregulate their photosynthesis to make use of this extra light? Or is drought so severe that they cannot respond to changes in light availability?

In order to answer these questions, I travelled with a team of 10 researchers to the world’s longest running tropical drought experiment in the remote east Brazilian Amazon. Deep in the Caxiuanã forest reserve exists an experiment that has been excluding half of the rainfall from the soil since 2002 using clear plastic panels. It was at this experiment where I measured leaf traits of 66 small trees and compared them to traits of 61 surviving canopy trees during the peak dry season of 2017. By comparing measurements of photosynthesis, respiration and leaf morphology between trees found on the experimental drought plot and a neighbouring intact control plot, we can understand the impact of long-term drought on the understory trees.

Our results showed that small trees were resilient to the forest-level changes that had occurred because of drought. Despite reduced water availability, small trees were able to positively respond to increases in light availability in the understory by increasing their photosynthetic capacity, respiration rates and changing their leaf morphology. All of this occurred remarkably despite no changes in nutrient availability. Somehow, these trees had found a way to use their limited nitrogen and phosphorus reserves to make the photosynthesis reactions more efficient.

We found that the upregulation of photosynthesis and respiration was unique to understory trees, with large canopy trees showing no response to the drought treatment. This indicates that small trees are more plastic: they are more capable of changing their physiology in response to environmental cues than their larger neighbours. This finding potentially indicates that small trees may be more responsive to climate change than large trees, leading to a greater overall resilience in the forest than previously suspected.

Not all species responded in the same way however, with some hyperdominant genera such as Eschweilera showing positive responses and others, such as Protium showing almost no response to the drought treatment. What this means for the hyperdiverse Amazon community is not fully understood, but maybe species that can show greater responses have more potential to increase in abundance, whilst less responsive species may become less abundant. More work studying community dynamics will ultimately be needed to realise this.

Overall, we show that small Amazonian trees are capable of responding to increases in light availability following drought-induced mortality of canopy trees. This ability to respond to light whilst still experiencing drought conditions indicates a resilience in these trees. Ultimately, this may allow them to grow to be the next generation of canopy trees and recover some of the biomass lost following the death of larger canopy trees. Given sufficient time, we may expect that the small trees can grow to establish a more resilient forest and moderate some of the negative impacts of climate change on the Amazon forest.

Read the full article here:

Bartholomew, D. C., Bittencourt, P. R. L., da Costa, A. C. L., Banin, L. F., Costa, P. B., Coughlin, S. I., Domingues, T. F., Ferreira, L. V., Giles, A., Mencuccini M., Mercado, L., Miatto, R. C., Oliveira, A., Oliveira, R., Meir, P., Rowland, L. (2020) Small tropical forest trees have a greater capacity to adjust carbon metabolism to long-term drought than large canopy trees. Plant Cell & Environment. https://doi.org/10.1111/pce.13838

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