The effect of human impact and environmental conditions on Polylepis forest and on the world’s highest mistletoe infestation

Forests dominated by the genus Polylepis (Rosaceae) are highly threatened due to anthropogenic impacts and climate change. Some species such as Polylepis flavipila are also affected by hemiparasitic plants such as Tristerix chodatianus (Loranthaceae) that induce progressive damage to the host canopy, altering the structure and functioning of the forest. In this paper, we focused on evaluating the parasitism pattern of T. chodatianus on Polylepis populations, as well as how human impact and environmental factors affect the Polylepis flavipila population structure. We carried this out along an altitudinal gradient in Nor Yauyos Cochas Landscape Reserve, Laraos, Lima-Peru. We laid out three transects in each forest (Shutco and Chaqsii-Chaqsii) from the bottom of their current distribution to the tree line, totaling a sampled area of 3.44 ha. Phenotypic data were taken for all individuals (parasitized and non-parasitized) with a diameter at ground height ≥ 5 cm. Environmental factors (bare cover, slope, rock cover, grass cover, and elevation) and human impact (trails, overgrazing, and felling) variables were also collected. Our results showed that the higher the elevation is, the shorter are the trees and with the increase of the slope, the diameter decreases and the frequency of trees increases. Moreover, human impact (trails and overgrazing) affected negatively the dasometric characteristics of P. flavipila. Regarding the patterns of parasitism, we observed that the larger trees (in diameter and height) had a greater presence of T. chodatianus and, as the elevation increased, the proportion of parasitized trees decreased. In conclusion, our results showed that P. flavipila forest is threatened by several factors, including human impact and T. chodatianus infestation. Future evaluations on forest dynamics would clarify the observed pattern.