Managing the realized niche through ground disturbance improve regeneration of early-seral species in high-graded old-growth forests
High grading, by removing mostly trees with superior growth and timber quality characteristics, is an important source of productivity and biodiversity loss in many forests in developing countries. After high-grading, little attention is paid to tree regeneration and understory vegetation, which often are dominated for long time periods by grasses, herbs, or shrubs species. In such settings, managing understory vegetation through ground disturbance has been proposed as a restoration tool to encourage tree regeneration. In this context, we investigated the effectiveness of ground disturbance in high-graded Nothofagus old-growth forests in the Chilean Andes. In these forests, the understory is dominated by dense tickets of native bamboo (i.e., Chusquea culeou E. Desv.), which prevents the regeneration of early- and late seral tree species. We tested whether ground disturbance can be applied to expand the realized niche for tree species regeneration and at the same time constrain it for bamboo. Second, we investigated direction and magnitude of niche shifts, and what ecological and environmental factors are responsible for these shifts, using multivariate ordination (NMS) and habitat modeling (NPMR) to understand the niche spaces for different seral functional groups (i.e., short-life and long-life early- and late-seral groups, and bamboo cover, separately).
We concluded that the niche concept can provide useful insights in understanding the role of ground disturbance as a restoration tool in high-graded forests. The niche concept allowed us to understand what factors most influence in determining the cover of undesirable bamboo species and limit the regeneration of desirable early- and late- seral trees species, and how these factors are modified after ground disturbance. Specifically, the results show that ground disturbance expands the realized niche for early-seral species, and constrained it for bamboo and late-seral species. These patterns are supported by the impact of ground disturbance on litter cover (-) and soil water content (+), which affect the bamboo cover (-). On the other hand, presence of early-seral species is associated with high soil water content (+) and exposed mineral soil (+). Surprisingly, light availability had less influence on tree regeneration, even on early-seral species. In summary, this operational scale management experiment provided an opportunity to link ecological concepts to restoration practices and gained insights into practical solutions for restoring high-graded forests.