The Role of Litterfall in Understanding the Ecological Integrity of Endangered Upper Andean Successional Forests

Litterfall is a major pathway of organic matter input to soils and it is critical for nutrient cycling and carbon storage in ecosystems. Although studies have shown that tropical Andean montane forests exhibit an extraordinary biodiversity and are key ecosystems to counteract climate change by serving as global carbon sinks, large areas of the Northern Andes have been transformed for agriculture, and currently secondary forests are common in the landscape. As a consequence, large areas of the Andean region have scarce remnants of old-growth forests. Despite facing significant threats, these areas still lack comprehensive understanding in terms of ecosystem functioning. In this study, we quantified litterfall rate along a successional gradient in tropical upper Andean montane forests in Northern South America. Specifically, our objectives were to: (1) analyze the changes in litterfall rate between successional forests and (2) explore the link between litterfall and belowground attributes. We established 14 permanent plots (20 m × 20 m) encompassing early and late successional forests, near Bogota (Colombia), at altitudes that varied between 2685 to 3140 m. In each plot we randomly placed ten 0.25 m[[sup]]2[[/sup]] litter traps (total n = 140) and collected them every 15–20 days for a year. We measured belowground variables related to plants, soils, and invertebrates in the permanent plots. We found that annual litterfall rate was higher in old-growth (1.92 ± 0.96 g m[[sup]]−2[[/sup]] d[[sup]]−1[[/sup]]) than in secondary forests (1.17 ± 0.49 g m[[sup]]−2[[/sup]] d[[sup]]−1[[/sup]]). In addition, litterfall was positively related to soil organic layer thickness, fine root production, soil respiration, and soil fauna community richness, showing a fundamental association between litterfall and these ecological variables. Our results show that litterfall can give a comprehensive view of the ecological integrity and the recovery of ecosystem function, beyond the traditional focus on changes in species composition with succession. Our findings also highlight that conservation efforts need to include not only old-growth forest remnants, but also secondary forests, to preserve the successional trajectories in these highly transformed upper Andean tropical forests.