The recently (February 2024) published article by Flores et al. in Nature discusses potential critical transitions in the Amazon forest ecosystem. The authors present a comprehensive analysis of the factors that could lead to abrupt and irreversible changes in the Amazon rainforest, with significant implications for biodiversity and the global and regional climate.
The Amazon rainforest plays a crucial role in regulating the Earth's climate (as carbon storage, or through its cooling effects caused by evapotranspiration). It further holds 10% of the terrestrial biodiversity. However, human activities such as deforestation, effects of climate change, and fire have put the Amazon at risk of reaching critical tipping points that could lead to widespread forest degradation and loss, and therefore loss of biodiversity, socio-economic and cultural values as well as livelihoods for over 40 million people (of which 2.2 million indigenous people of more than 300 ethnicities).
For 65 million years, Amazonian forests remained relatively resilient to climatic variability. Now, the region is increasingly exposed to unprecedented stress from warming temperatures, extreme droughts, deforestation, and fires, even in central and remote parts of the system. Long existing feedbacks between the forest and environmental conditions are being replaced by novel feedbacks that modify ecosystem resilience, increasing the risk of critical transition.
By combining evidence from paleorecords, observational data and modelling studies, Flores et al. analysed existing evidence for five major drivers of water stress on Amazonian forests, as well as potential critical thresholds of those drivers that, if crossed, could trigger local, regional, or even biome-wide forest collapse. They highlight the interconnected nature of factors such as deforestation, climate change (temperature increase, changes in rainfall conditions), and extreme weather events (especially droughts), which could create feedback loops and push the forest towards a new, less biodiverse states.
Combining spatial information on warming and drying trends, repeated extreme drought events, road networks (as proxy for deforestation and forest degradation), resulted in an estimate that by 2050, 10% to 47% of Amazonian forests will be exposed to compounding disturbances that may trigger unexpected ecosystem transitions and potentially exacerbate regional climate change (10% with high transition potential (more than 2 disturbance types), 47% with moderate transition potential (more than 1 disturbance type)).
Flores et al. further discuss the question of CO2 fertilisation, which is often integrated into models as increasing photosynthetic rates and tree water-use efficiency. However, the authors also draw attention to findings about increased tree mortality rates and the importance of soil nutrient availability as a limiting factor to increased growth.
Using examples of disturbed forests across the Amazon, the authors identified the three most plausible ecosystem trajectories, involving different feedbacks and environmental conditions (see Figure below).
Fig.1: Alternative ecosystem trajectories for Amazonian forests that transition due to compounding disturbances (source: Flores et al. (2024))
The inherent complexity of the Amazon adds uncertainty about future dynamics, but also reveals opportunities for action (as not the entire Amazon forest will transition at once). Still, major efforts are needed for keeping the Amazon forest resilient. A combination of local/regional efforts to end deforestation and forest degradation and to expand restoration, and global efforts to stop greenhouse gas emissions are urgently needed!
Source and original article:
Flores, B. M., Montoya, E., Sakschewski, B., Nascimento, N., Staal, A., Betts, R. A., Levis, C., Lapola, D. M., Esquível-Muelbert, A., Jakovac, C., Nobre, C. A., Oliveira, R. S., Borma, L. S., Nian, D., Boers, N., Hecht, S. B., Ter Steege, H., Arieira, J., Lucas, I. L., … Hirota, M. (2024). Critical transitions in the Amazon forest system. Nature (London), 626(7999), 555–564. https://doi.org/10.1038/s41586-023-06970-0 Critical transitions in the Amazon forest system | Nature