Tropical rain forests provide ecosystem services well beyond their bounds. The Amazon, for example, acts as both a sink for carbon dioxide and a fountain of water vapor into the atmosphere that later falls as rain or snow, sometimes thousands of kilometers away. But human activities and climate change are major threats to these services.
Many studies have sought to understand how deforestation, which refers to the conversion of forested land for nonforest purposes like agriculture and which has soared again in many parts of the Amazon, affects carbon sequestration and evapotranspiration. Meanwhile, forest degradation, which includes logging, understory fires, and forest fragmentation, may affect as large an area as deforestation, yet its effects on water, energy, and carbon cycles in tropical forests are less well understood.
That’s at least in part because forest degradation is heterogeneous and because many degraded plots are in remote regions and on privately owned land, making field data difficult to gather. But in a new study, researchers used high-resolution lidar data collected by aircraft to overcome some of these accessibility challenges.
Here Longo et al. fed both lidar data and ground observations into an ecosystem demography model to compare water, energy, and carbon fluxes between the forest and the atmosphere in both degraded and intact regions of the Amazon forest spanning precipitation gradients. The lidar data revealed forest structure variability across five regions in the eastern Amazon (one in French Guiana and the rest in Brazil), each with different precipitation patterns and histories of land use change, to capture the diversity of degraded forests.
The model indicated that during a typical dry season, evapotranspiration and gross primary production were 34% and 35% lower, respectively, in degraded forests than in intact forests, whereas daytime surface temperatures were 6.5°C higher on average. However, during extreme droughts, the effect of degradation on these fluxes was much less apparent. In other words, intact and degraded forests behaved similarly when facing extreme water and heat stress.
The same pattern held true for fire risk: In a typical year, degraded forests were drier, warmer, and more susceptible to fire, but during droughts, intact forests were just as prone to fire, highlighting the critical role of climate variability in flammability.
Although the model had limitations—for example, it didn’t consider variations in soil depth and composition, which can markedly affect these fluxes—the study advances the use of remote sensing technology for tracking structural change in degraded forests as well as our understanding of how human disturbances beyond deforestation impact energy and carbon balances in the Amazon. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2020JG005677, 2020)
—Kate Wheeling, Science Writer