Regional impacts of agricultural land use history on forest vegetation and soils: Comparing primary and post-agricultural forests in Northern New Jersey

Abstract

We studied the effects of agricultural land use history on present-day forest conditions at large spatial and temporal scales. We used historic forest maps from the late 1800s to document the spatial extent of primary and post-agricultural forest cover in the 8080 km2 area of northern New Jersey outside the coastal plain, measuring the structure and composition of their vegetation and soils in 72 paired plots, and comparing the responses to agricultural land use history across regional and landscape-level environmental gradients. We estimated a 25% net increase in forest cover from the late 1800’s to 2015, with a 26% decline in primary forests during the same time. As little as 5–14% of the land area of some ecoregions are occupied by primary forests today. Post-agricultural forests comprise >46% of total forest cover and up to 72–77% in some ecoregions. Forest vegetation and soils differed significantly in the two forest types, with 8–11x greater invasive shrub and liana cover in post-ag forests, 2–5x more invasive species than native species, and reduced tree regeneration in seedling and sapling size classes (37–63%). Post-ag forest soils had a higher pH and associated base mineral concentrations (Ca, Mg), lower nutrients (NH4, %N), electrical conductivity and organic content (%C, % organic matter), and changes to soil texture (lower % sand, higher % silt and clay). Variation in these changes across large spatial and temporal scales showed few to no relationships with bedrock/surface geology, glacial history, soils, topography, forest age class or integrity metrics, indicating the robustness of these changes across a wide range of environmental contexts and conditions. Comparisons to past primary forest conditions in the same region (1948–1973) showed even greater deficits in tree regeneration and increases in invasive abundance in post-ag forests than present-day comparisons suggested. The results indicate the significance of agricultural history as a major driver of long-term and large-scale forest change. Greater protection of primary forests is needed along with prioritizing the respective needs of these different forest land use history types for stewardship and restoration in the future.