Nutrient enrichment – an emerging threat to tropical forests

Tropical forests, often referred to as the "lungs of the Earth", are essential for sustaining life on our planet. They provide clean air, water and unparalleled biodiversity. While deforestation due to slash-and-burn agriculture, mining and logging remains the most recognised threat, less visible but equally dangerous forces are at work. A new study reveals that nutrient enrichment – driven by human activities such as agriculture and fossil fuel combustion – poses a significant risk to the delicate dynamics of tropical forests.

The research, conducted by an international team of scientists from the University of Kaiserlautern-Landau (RPTU), the HAWK University of Applied Sciences and Arts Hildesheim/Holzminden/Goettingen, both in Germany, and the HUN-REN Centre for Ecological Research in Hungary, focuses on how nutrient deposition affects tropical tree seedlings' growth and biomass accumulation. Their findings, published in Current Forestry Reports last December, show that this phenomenon can potentially disrupt forest composition and resilience, particularly in the face of global climate change.

By synthesising data from 59 studies conducted across tropical regions world-wide, the researchers employed meta-analysis to uncover broad patterns of nutrient effects. Their analysis revealed that nutrient addition significantly boosted tree seedling growth, with shoot biomass increasing by an average of 26 per cent and growth rates by 14 per cent. Notably, the combination of nitrogen (N), phosphorus (P), and potassium (K) produced the most pronounced effects, driving growth rate increases of up to 27 per cent. These impacts were particularly evident in seasonally dry sites, where growth rates surged by 38 per cent and shoot biomass by an impressive 70 per cent. Lead author Dr Daisy Cárate Tandalla explains: “NPK are fundamental nutrients for plant growth. However, many tropical soils are nutrient-limited. Adding these nutrients disproportionately benefits fast-growing, competitive species, potentially shifting forest composition.”

Human activities dramatically altering natural nutrient cycles 

While volcanic activity and wildfires have historically contributed to nutrient deposition, agriculture and fossil fuel burning have intensified and expanded this process to even the most remote tropical regions. These nutrient inputs can give a competitive edge to certain tree species, leading to homogenised forests with fewer species – a trend that threatens biodiversity and ecosystem stability. “These changes may reduce species diversity across entire food chains and weaken forest resilience in the face of climate change,” warns senior author Dr Péter Batáry. “The loss of diversity also diminishes the forests' ability to adapt to environmental stressors.”

Attention to nutrient management urgently needed

The study also highlights the complexity of tropical forest research. “The studies we reviewed used a mix of methods – greenhouse pot experiments, transplantation trials and in-situ fertiliser applications,” notes co-author Dr Jürgen Homeier from the HAWK. “Identifying seedlings to the species level remains a significant challenge due to the extraordinary diversity and similarity of young tropical trees.”

The findings underscore the need for urgent attention to nutrient management in tropical regions. While nutrient deposition may seem like a localised issue, its impacts ripple through global ecosystems, affecting biodiversity, carbon storage and the planet's overall health. Tropical forests are a cornerstone of life on Earth, and preserving their complexity and resilience is crucial. This study is a timely reminder that even remote human activities can have far-reaching consequences for the natural world.

(RPTU)/wi)

Reference:
Cárate Tandalla, D., Homeier, J. & Batáry, P.:  Responses of tropical tree seedlings to nutrient addition: a meta-analysis to understand future changes in tropical forest dynamics. Current Forestry Reports; December 2024