Walking through an undisturbed Amazonian forest is one of those unique experiences – the abundance of sounds, colors, and smells can be overwhelming. Underneath towering trees, smaller trees in line with our eye level, or saplings, seem to be waiting for a gap in the canopy to grow freely. Some of these smaller trees will only grow in the shade, but others are the offspring of larger trees and won’t grow much until they receive more sunlight.
This picture changes when we look at the understory of forests that have been affected by human disturbances, such as selective logging and wildfires. The saplings no longer seem to mirror the large trees. Despite belonging to different species, are these smaller trees functionally different from the larger ones? And are there factors affecting the recovery of the understory? These were the questions we started asking when we walked through a human-modified Amazonian forest. We wanted to understand what the saplings could tell us about the forests of the future.
We collected data for over 31,000 trees and saplings across 121 study plots and decided to focus on a key plant functional trait, wood density. We compared the plot-level wood density of human-modified forests with those of undisturbed forests, our control sites. We found that the wood density of saplings in human-modified forests was significantly lower than that in undisturbed forests, indicating that future human-modified forests may be functionally different from undisturbed ones.
We then investigated what could be the causes behind this difference, and whether there was any factor affecting the recovery of the understory following human disturbances. We found that the closer a forest was to a man-made edge, the more likely it was to have a low wood density sapling community. We also found that high densities of lianas could potentially be negatively affecting the recovery of the sapling community.
Our results highlight that human disturbances may have long-term effects on Amazonian forests. It also raises the question whether these human-modified forests will be able to, in the future, maintain the same processes and deliver the same ecosystem services as their undisturbed counterparts, given that their sapling communities are functionally different. It will take a long time until we are able to answer this, but in the meantime, we should use the precautionary principle and focus our conservation actions on avoiding disturbances in parts of the Amazon that are still largely undisturbed.
Erika Berenguer, Senior Research Associate, University of Oxford & Visiting Research Associate, Lancaster University, UK
This paper is published Open Access therefore available to read without a subscription. Read the full paper here: Seeing the woods through the saplings: Using wood density to assess the recovery of human‐modified Amazonian forests