Trade-offs at applying tree nucleation to restore degraded high Andean forests in Colombia

Tree islands, the more recognizable type of applied nucleation, and passive restoration have been proven as cost-effective restoration strategies. Comparison between them is a priority, particularly through paired experiments in ecosystems wherein restoration research is scarce, such as high forest in the northern Andes (high Andean forests [HAF]). This ecosystem is important given its environmental benefits but also because vegetation development is more limited by abiotic conditions, unlike tropical moist forests. To address these gaps, we set up an experiment in a logged pine plantation comparing the effect of nuclei and passive restoration on vegetation structure and diversity as well as natural regeneration. In this case, we divided emergent shoots into three functional groups related to restoration goals: invasive herbs, páramo species, and woody recruits. After 3 years, nuclei had higher coverage, height, and diversity, but this was exclusively due to planted trees, as nuclei did not affect the abundance of invasive herbs and páramo species. More importantly, the abundance and richness of woody recruits were kept low and constant under nuclei, likely as a by-product of plantation density. Conversely, it gradually increased at passive restoration plots. Since nuclei contribute to structure and diversity and passive restoration contributes to woody recruits, we propose a combination of both to restore HAF. Possible changes in the direction of the interaction between planted trees and recruits confirm the need for long-term monitoring to accurately determine the potential of tree nuclei in HAF.