Climate change is making trees grow larger in the Amazon rainforest

As tree sizes in the Amazon increase with rising carbon dioxide levels, these larger trees serve a pivotal role in whether the rainforest will effectively act as a carbon sink.

The effects of a changing climate on forest dynamics remain unpredictable. A prevailing hypothesis suggests that larger trees may become less prevalent as they are more vulnerable to climate-induced events such as droughts and strong winds. Understanding these dynamics is essential for predicting future climate scenarios, given that forests play a critical role in absorbing substantial amounts of CO₂ from the atmosphere, thus helping to mitigate global warming.

Researchers led by Adriane Esquivel-Muelbert from the University of Cambridge, in collaboration with the RAINFOR Amazon Forest Inventory Network, have been rigorously monitoring the diameter of trees across 188 plots in the Amazon basin, each averaging around 12,000 square meters. The monitoring durations spanned up to 30 years. Remarkably, during this period, atmospheric CO₂ concentrations have escalated by nearly 20%.

What we’re observing is an increase in tree size within certain areas of the forest, indicating that trees are now able to store more carbon than they did in the past, explains Esquivel-Muelbert. The data indicates an average increase in tree diameter of approximately 3.3% per decade.

The structure of the Amazon forest is undergoing noticeable changes throughout the entire basin, observes team member Rebecca Banbury Morgan from the University of Bristol, UK. We are witnessing a rise in larger trees while smaller ones are decreasing, thus shifting the average size distribution towards the larger specimens.

Typically, in untouched old-growth forests, the average diameter of trees remains relatively stable, thanks to the natural process of saplings replacing fallen giants. However, the current research suggests that Amazon trees are responding positively to the increased atmospheric CO₂ levels by growing larger and accumulating more biomass. The predominant successes are seen among the larger trees, which exhibit better competition for resources such as light and water, adds Esquivel-Muelbert.

This trend underscores the significance of larger trees in maintaining the forest’s carbon storage capacity. The potential loss of these giants could have severe repercussions for carbon sequestration efforts, asserts Esquivel-Muelbert.

A critical takeaway is that CO₂ has acted as a growth enhancer, facilitating tree growth, which is reassuring since timber is a vital global carbon reservoir, remarks Peter Etchells from Durham University, UK. The pressing question remains whether this positive trend will persist amid ongoing climate changes that may disrupt the delicate balance between growth, nutrients, temperature, and CO₂.