Legacies of the agricultural past in the forested present: an assessment of historical land-use effects on rich mesic forests

Aim and location The research investigated the long-term effects of human disturbance, namely nineteenth century agricultural land-use, on the modern species composition, structure and distribution of Rich Mesic Forests (RMF) in western Massachusetts, USA. RMF are a species-rich north-eastern variant of the Mixed Mesophytic Forest Type of eastern North America. Methods Land-use history patterns were reconstructed for two towns (c. 16,000 ha) from the onset of widespread European settlement and agricultural land-use in the late eighteenth century until present. Vegetation and a range of environmental variables were sampled in sixty-one 10 x 10 m plots in thirty-four forest stands with varying histories of human disturbance. Vegetation data were ordinated (DCA) to identify patterns of variation and related environmental and historical factors. The distribution patterns of individual taxa in relation to historical land-use and environmental factors were analysed using G-tests of independence and logistic regression. Associations between species secondary forest colonization ability and life history characteristics (e.g. diaspore dispersal mode, degree of vegetative spread) were assessed. Results Persistent compositional differences were documented between the vegetation of primary forests and post-agricultural, secondary forests indicating that distribution patterns for many plant species still reflect the open, agricultural environment of the nineteenth century, despite the current predominance of forest cover in the study area. A major factor driving modern vegetation patterns in RMF is the ability and rate of colonization by forest herbs. In particular, species with seeds lacking morphological adaptations for dispersal (barochores) and those which produce seeds with elaiosomes to encourage ant dispersal (myrmecochores) are less frequent in secondary forests. Environmental differences between primary and secondary forests, although present, appear to be less important in influencing species distribution patterns. Main conclusions Widespread agricultural land-use represents a novel disturbance in the naturally forested ecosystems of eastern North America with long-term impacts on plant community composition and structure. Many secondary forest sites that are environmentally suitable for RMF vegetation do not support the suite of plant species typical of this community type, apparently because of the dispersal limitations of certain forest herbs. These poorly dispersed herb taxa are well adapted for growth in stable forest ecosystems characterized by local, small-scale disturbance (e.g. gap-phase dynamics), yet are maladapted for rapid population recovery and recolonization following severe disturbance (e. g. agricultural land-use).