Does long‐term monitoring of tropical forests using permanent plots provide unbiased results?

AIM: Long‐term ecological research sites have become essential tools in the study of tropical forest dynamics, but the potential impacts of researcher activity on the permanent plots being studied is often ignored. Here we ask whether seemingly benign repeated surveys themselves significantly affect stand dynamics and plant species diversity when implementing long‐term monitoring of permanent plots. LOCATION: Old‐growth semi‐deciduous tropical forest of M'Baïki, Central African Republic. METHODS: We compared demographic parameters of trees (DBH≥9.55 cm) and plant community diversity and composition across three zones of three permanent plots: along trails crossing a plot, in areas adjacent to trails and in the forest interior. We assessed differences in radial growth, mortality, recruitment and species diversity using ANOVA. We examined the dynamics of tree death over time using Cox proportional hazards regression models. We analysed plant species composition using permutational multivariate ANOVA and indicator species analyses. RESULTS: Tree mortality, recruitment and radial growth did not differ among the three zones. Species richness and evenness did not differ among the three zones investigated, but species composition did, with significantly different indicator species between trails and forest interiors. In trails, communities were characterized by light‐demanding small trees and lianas and shade‐tolerant herbs of trampled soils, whilst forest interiors exhibited more shade‐tolerant tree species. CONCLUSIONS: Even in observational studies conducted in the wild, visitors may unintentionally but artificially influence the natural patterns and processes under investigation, an influence whose intensity may depend upon study design, habitat type and natural disturbance regimes. Permanent plots may not be a benign influence on the study system, as is generally assumed, and responses to repeated visitation may place fundamental limits on the questions that can be addressed, especially when species composition is assessed under low levels of natural disturbances or high density of trails.