Constraints for the restoration of tropical alpine vegetation on degraded slopes of the Venezuelan Andes

Background: In the Venezuelan high Andes, wheat cultivation on steep slopes since colonial times has caused the degradation of extensive areas. After several decades of abandonment, natural succession has not allowed the spontaneous recovery of the original alpine vegetation. Aims: Our aim was to identify a degradation gradient, based on vegetation structure, and relate this gradient to different environmental factors (grazing intensity, physical soil degradation, distance to seed sources and loss of soil fertility) that could be preventing spontaneous succession. Methods: Vegetation structure and 22 environmental variables were sampled on 92 slopes with different degrees of degradation. Multivariate statistics were used to relate the state of the vegetation to environmental factors. Results: Six types of vegetation were identified and ranked along a degradation gradient. Several soil properties, for example C, N, clay content and cation-exchange capacity, decreased by as much as 50% along this gradient, while other variables, such as current grazing intensity were only slightly correlated to the degradation gradient. Conclusions: Vegetation and soil properties were clearly related to a strong degradation gradient, caused by former wheat cultivation and overgrazing, with symptoms of desertification. Restoration practices have to be directed towards re-establishing soil organic matter and its functional role in regulating water balance and nutrient cycling.