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photo by Chris Linder.
Working at a field site like Tanguro Ranch, with its vast area and variety of landscape types, allows us to pursue research that seeks to better understand all aspects of the landscape, including both human-dominated and natural ecosystems, how they are related, and how they are changing. Ongoing research has been studying watersheds, nutrient cycling, forest resilience, biodiversity, and sustainable agriculture.
Water is key to human society, food security, agriculture, and natural ecosystems. Land use and changes in climate are altering the quantity and quality of available water. Our long-term monitoring of watersheds at Tanguro offers a unique opportunity to understand how these dynamics are reshaping freshwater ecosystems. In an agricultural landscape, watersheds are often heavily modified and impacted.
Our research studies how deforestation, healthy riparian forest buffers, dams, and reservoirs affect river characteristics.
Intensifying farming in tropical Brazil can increase food production and consequently spare standing forest from being cleared. Increasing crop yields, or growing multiple crops in one year, may require higher use of fertilizers. However, higher fertilizer use will degrade the regional and global environment if it increases leaching of nitrogen and phosphorus to streams and rivers, or if it increases the emissions of nitrous oxide (a powerful greenhouse gas) from farm soils, which occurs in many regions of intensive cropping in temperate regions.
At Tanguro, we examine indicators of nitrogen loss to determine the magnitude and sustainability of current nitrogen losses and how they vary in predictable ways across soils and farming practices. This information can be used to develop recommendations for fertilizer practices that minimize nitrogen losses, reduce environmental impacts, and increase the sustainability of continuous intensive cropping.
Regarding phosphorus, oxisol (highly weathered) soils with high phosphorus absorption capacity are widespread in Brazil. Brazilian soybean producers commonly fertilize with approximately twice as much phosphorus as is harvested in soybean to counter low phosphorus availability within highly weathered soils. This has led to the accumulation of phosphorus in the soil, especially during the 2000s and 2010s, but the degree to which producers can capitalize on this residual soil phosphorus stock to offset fertilizer inputs remains unclear. Using field trials, we are testing the effect of residual soil phosphorus in plots under different management systems.
Amazon forests are facing a number of threats simultaneously, including agricultural expansion, fires, logging, climate change, and weak environmental policy enforcement. These disturbances have transformed many tropical forests into fragmented and degraded environments and reduced their natural resilience to disturbances. Degraded forests are prone to climate-induced forest dieback, especially along forest edges that are adjacent to hot, dry agricultural fields. The more extensive the edges, the more prone a forest patch is to degradation and the more carbon is released to the atmosphere.
Fire is not a natural feature in the Amazon landscape. It is almost always a result of human activities related to deforestation or managing pasture lands. Fire is also one of the primary causes of forest degradation in the Amazon. Since 2004, we have conducted a unique experiment on 150 hectares of forest, burning the forests at intervals for 8 years and monitoring it ever since. This is the largest fire experiment in the tropics and its results continue to yield new insights into forests’ resilience and susceptibility to fire, how that interacts with a changing climate, and the pathways of recovery that may occur.
Our studies also extend to the pantropics and the effects of compound disturbances—when more than one degradation driver operates alongside others—on forest carbon cycling. Recent droughts have temporarily changed tropical forests from a net carbon sink to a net carbon source on timescales varying from at least a few months to a few years. It is still unclear how long drought legacies can persist, and we need a better understanding of the recovery time of drought-disturbed forests. Drought-fire impacts are significantly greater along forest edges, and trees become fuel during droughts, increasing forest flammability.
Structural and functional changes in tropical forests can lead to declines in carbon stocks and sequestration rates, with major implications for the global climate system. Therefore, quantifying and attributing changes in ecosystem characteristics to different drivers are key to understanding the feedbacks between tropical forests and climate. Improving our understanding of tropical forest responses to future perturbations related to changing climate, drought, and fire frequency and intensity is key. The information can help inform effective mitigation and adaptation strategies that take into account the complex interactions between tropical forests, climate change, droughts, wildfires, and carbon cycling.
Biodiversity supports ecological resilience. At Tanguro, we study how species diversity informs us of and contributes to ecosystem equilibrium. Our research has shown us that in these disturbed landscapes, plant and animal species diversity may not decrease, but instead shift to novel species and assemblages that are more resilient to repeated disturbance.
We also see that large mammals like tapirs (Tapirus terrestris), which prefer a mosaic landscape, play a key role in forest regeneration, dispersing three times more seeds in degraded forests than in undisturbed forests. Importantly, their large size allows them to disperse large seeds, which grow into large trees that have a higher potential for carbon accumulation. The dung in tapir latrines attracts secondary dispersers like dung beetles and ants, which help spread the seeds to places more suitable for plant recruitment and survivorship. Tapirs also have been shown to change soil nitrification rates via their excreta and by trampling the forest floor, which promotes soil turning and can contribute to forest regeneration by increasing soil nutrient availability.
Brazil is the largest soybean producer in the world, yet sustaining agricultural production depends on a healthy environment and stable climate. Regional and global climate changes are jeopardizing this huge agricultural system. For the last 10 years, our team of scientists has been studying the vulnerabilities and strengths of the agricultural system in the Amazon Cerrado region, given a rapidly changing climate and shifting market and policy contexts.
Our main findings raise a potentially grave concern for the near future. We show that the area with suitable climate for crop production in Brazil is shrinking and will continue to do so at an accelerating rate. Irrigation, cultivar development, and forest conservation may be the only near-term options for avoiding catastrophic crop and financial losses, given that greenhouse gas emissions will likely continue increasing as a result of inadequate climate policy. We are working together with farmers, stakeholders, and scientists from different backgrounds to evaluate and develop adaptation and mitigation strategies.