The composition and structure of modern forests have been influenced by land use practices such as agriculture and the introduction of non-native species. Successional forest stands are often characterized by a large percentage of non-native species that colonize early after abandonment. The goal of this project was to describe the abundance and population dynamics of exotic species as legacy effects of agriculture and determine how they change through time as forests develop. We compared trajectories of abundance between stands that were formerly cultivated and pastured, two common land uses that differ in the extent of soil disturbance and the presence structural characteristics that influence recolonization. We tested the following hypotheses: 1) Abundance of non-native species, both individually and as a group, will decline through time as the forest environment becomes less suitable for them; 2) Abundance and spatial distributions of non-native species will differ between formerly cultivated and pastured sites due to contrasts between the two practices that may influence establishment and spread of species after abandonment.
Results/Conclusions
We described species abundance through time with a paired chronosequence consisting of forest stands growing on abandoned pastures and cultivated fields. Stands were grouped into four age classes, and a group of minimally-disturbed mature forest patches were included as a control. Stand age and land-use history were determined using aerial photographs and site characteristics. Species composition, soil characteristics, and environmental variables were recorded in 2-meter plots within sites.
Abundance of non-native species declined significantly through the chronosquence and was generally significantly higher in formerly cultivated than pastured sites. The non-native proportion of the total community was significantly higher in cultivated sites 61-80 years after canopy closure than in the control group, which indicates a persistent legacy of cultivation. The dynamics of individual species varied according to life history traits and habitat preferences. Species such as Celastrus orbiculatus and Ligustrum vulgare largely declined after 40 years, but species adapted to forest conditions such as Duchesnea indica are capable of surviving and spreading for decades after canopy closure. Abundance of individual species within sites was linked to environmental variables such as canopy openness and soil moisture. No change in spatial patterning of non-native species was apparent.