- Recent research finds that Amazonian trees in fragmented forest landscapes remain especially vulnerable to windstorms for several years after being impacted by fire — and that, in particular, larger trees that store more carbon are most at risk.
- The research, the results of which were detailed in the Journal of Ecology last September, builds on the findings of a 2014 study that was based on data gathered during a decade-long field experiment involving three 50-hectare rainforest plots on the edge of agricultural fields in southeastern Amazonia — one plot was burned every year, another was burned every three years, and one control site was left unburned.
- The researchers found that trees in the burned plots were not only more likely to be uprooted or to have snapped off, usually at the same height as the fire damage the tree had sustained in the past, but that those fire-and-windstorm-damaged trees were much more likely to die in ensuing years.
Recent research finds that Amazonian trees in fragmented forest landscapes remain especially vulnerable to windstorms for several years after being impacted by fire — and that, in particular, larger trees that store more carbon are most at risk.
The research, the results of which were detailed in the Journal of Ecology last September, builds on the findings of a 2014 study led by Paulo Brando, an assistant scientist at the Woods Hole Research Center. Using data gathered during a decade-long field experiment involving three 50-hectare rainforest plots on the edge of agricultural fields in southeastern Amazonia — one plot was burned every year, another was burned every three years, and one control site was left unburned — Brando and team were able to show that forest fires and extreme weather events like droughts can accelerate tree mortality and could even trigger Amazon forest dieback “in the near-term.”
A 30-minute, high-intensity windstorm that subsequently swept through the experimental forest plots afforded the researchers the opportunity to follow up on their 2014 study with an assessment of how fires affect the ability of tropical forests to withstand windstorms, and thus to examine how they might fare in a future where global climate change has made droughts and other extreme climatic events even more common.
“Suddenly this storm sweeps through and there are chairs flying around our camp. But as we were cleaning up, we had a moment of realization that we’d just sampled the tree plots the week before with a three-dimensional laser scanning system called LiDAR. We had a unique opportunity to study, with high certainty and in great detail, how each of the three plots survived the storm,” Brando said in a statement about the new study published in the Journal of Ecology, which was led by Divino Silvério of Brazil’s Instituto de Pesquisa Ambiental da Amazônia and Universidade de Brasília.
The researchers found that trees in the burned plots were not only more likely to be uprooted or to have snapped off, usually at the same height as the fire damage the tree had sustained in the past, but that those fire-and-windstorm-damaged trees were much more likely to die in ensuing years.
The windstorm caused the greatest amount of tree damage in the two burned plots. Some 13 percent of trees were damaged in the plot burned every year and 17 percent were affected in the plot burned every three years, compared to just 8 percent in the unburned plot. The windstorm also caused substantial reductions in vegetation height — 14 percent reductions overall in the one-year-burn plot, 20 percent in the three-year-burn plot, and 12 percent in the control plot — while reducing above-ground biomass by 18 percent, 31 percent, and 15 percent, respectively.
Tree damage in all three plots was found to be most severe near the edges of agricultural fields; this was especially true for large trees and those in the three-year-burn plot. The most common types of tree damage and mortality were uprooting (20 percent of damaged trees) and trunk snapping (70 percent), “with the height of trunk failure on the burned plots often corresponding with the height of historical fire scars,” the authors write.
Tree mortality was also more pronounced in the burned plots. 80 percent of windstorm‐damaged trees in the one-year-burn plot and 90 percent of damaged trees in the three-year-burn plot were were dead 4 years after the windstorm occurred in 2012, whereas a relatively lower number of damaged trees in the control plot, just 57 percent, succumbed to their injuries.
The researchers say that their findings demonstrate the “synergistic effects” of three disturbance types in tropical forest: forest fires, fragmentation by human activities like clearance for agriculture, and windstorms. “Our results show that the effects of windstorms are exacerbated by prior degradation by fire and fragmentation. We highlight that understorey fires can produce long-lasting effects on tropical forests not only by directly killing trees but also by increasing tree vulnerability to wind damage due to fire scars and a more open canopy,” they write in the study.
“The lesson is that even areas that look like they’ve survived initial burning will continue degrading in ways that are unpredictable, non-linear, and worsened by global warming,” Brando said. “We can’t separate out the effects of burns, drought, and global warming — they can interact in unpredictable ways. It’s critically important that we learn all we can about how to protect its remaining canopy, especially for the oldest and largest trees that provide the most wildlife habitat, cycle the most moisture through the ecosystem, and store huge amounts of carbon.”
CITATION
• Brando, P. M., Balch, J. K., Nepstad, D. C., Morton, D. C., Putz, F. E., Coe, M. T., … & Alencar, A. (2014). Abrupt increases in Amazonian tree mortality due to drought–fire interactions. Proceedings of the National Academy of Sciences, 111(17), 6347-6352. doi:10.1073/pnas.1305499111
• Silvério, D. V., Brando, P. M., Bustamante, M. M., Putz, F. E., Marra, D. M., Levick, S. R., & Trumbore, S. E. Fire, fragmentation, and windstorms: A recipe for tropical forest degradation. Journal of Ecology. doi:10.1111/1365-2745.13076