Cattle grazing exclusion and precipitation are key factors associated with early-stage tree carbon productivity in reforested Andean montane forests

There are increasing initiatives in tropical regions to mitigate global warming and conserve species through reforestation with native tree species. However, there are knowledge gaps regarding how widespread factors affect tree carbon productivity of reforested sites in tropical Andean forests. This study investigated potential factors affecting tree carbon productivity in young reforested sites (5 – 10 years old) in the Andes of Ecuador. We established a network of observational forest plots distributed from 343 to 3247 m above sea level, across 22 reforested sites to investigate how abiotic (e.g., climate, soil, topography), biotic (i.e., planted species richness), and land use management conditions (including historical land-use and post-transplantation cattle grazing) affected tree carbon productivity. In each plot, we estimated stem biomass, and characterised soil properties and grazing pressure. We used plot coordinates to characterize climate, elevation and topography using information from global and local databases. Using linear mixed models, we found high variation in tree carbon productivity (0.03–9.23 Mg ha⁻¹ year⁻¹). This variability was significantly linked to cattle grazing and mean annual precipitation. Sites with higher mean annual precipitation had higher productivity than drier sites and those without cattle grazing exhibited greater tree carbon productivity compared to grazed sites (a mean of 2.21 vs 0.48 Mg ha⁻¹ year⁻¹, respectively). Sites without cattle grazing encompassed the full range of observed variation, and there was some limited evidence that in ungrazed circumstances increased planted species richness was associated with greater productivity at this early stage of woodland development. Primarily, our findings suggest that excluding cattle grazing and selecting sites for reforestation based on their mean annual precipitation can significantly enhance early-year tree carbon productivity in reforestation efforts in the tropical Andes. The extent to which these results hold in later years will require continued monitoring and/or experimental testing, but they do provide straightforward management guidelines that can be implemented during the early years of reforestation in the Andes to enhance carbon sequestration.