Forest expansion on serpentine grassland communities: the impact of atmospheric N and land use

Abstract

In the Mid-Atlantic, USA we see a conservation crisis where multifaceted drivers and mesophication are accompanying afforestation on native rare serpentinite grasslands endangering local biodiversity. There are a number of directional natural and anthropogenic drivers, such as succession, acid rain, atmospheric composition, climate change, land use change, and pollutant deposition. The study objective was to explore the timing of expansion of Pinus virginiana and Pinus rigida was influenced by anthropogenic drivers. Comparisons of reconstructed tree establishment was related to climate (growing degree days and mean annual precipitation), pollution deposition (N and S) and site characteristics. Specifically, we hypothesize that traditional serpentine syndrome has been ameliorated by anthropogenic change that act as drivers of loss in species diversity, widespread mesophication, and compositional changes of plant communities.

We established 32 plots (10m x 15m) using stratified random sampling. Cores from 161 trees were sampled and cross-dated and interpreted in conjunction with aerial photographs to derive periods of woody establishment into grasslands. The effects of topographic (slope, inclination, annual direct incident radiation and heatload), soil variables (elemental analysis, pH, soil organic carbon, total nitrogen, depth, texture, δ15N, and δ13C values) and pollutant deposition data (total N and S) on species were examined using ordination and regression. Trends in establishment were compared to historic decadal means of the Palmer Drought Severity Index and mean maximum temperature (Tmax) and minimum temperatures (Tmin). Multiple linear regression was used to establish relationships between potential drivers of woody afforestation.

Diversity (H) is strongly correlated with soil depth, bulk density and negatively related to heavy metal content and heat load. The tree age structure show discontinuous patterns with peaks of regeneration pulses and periods of either low or absent recruitment. The period prior to 1900 is composed of 54% xeric oaks and when all oaks are grouped they comprise 79% of trees from that era, though Q. montana is absent from this group. The period between 1900 and before 1940 records a change in the age-frequency distribution of species with a marked increase in conifers and concomitant decrease of xeric oaks comprising 39% and 19% of all species respectively. Between1950 and 1990 the percent of xeric oaks decreased to 3%, conifers to 13% while Acer spp increased to 19%. The number of shade tolerant species rose from 12.5% in the decades from 1850-1900 to 59% during the interval from 1950-1990. Percentage decadal tree establishment showed strong relationships with climate variables such as PDSI and pollutant deposition. Our preliminary results highlight the importance of anthropogenic driven change affecting species composition in serpentine systems undergoing rapid afforestation.