Landscape dynamics and ecological change in the Southern Appalachians
COWEETA LTER, historical land use, land-use change, Southern Appalachians, invasive plants, forest herbs, bird community, ecosystem services
Our studies as part of the Coweeta LTER site have focused on how historic and contemporary patterns of land use affect flora, fauna and ecosystem processes in Southern Appalachian forests. Our early work documented differential responses among native herbs to historical land use and contemporary landscape patterns (e.g., Pearson et al. 1998). We extended that work to consider effects of land-use change on forest communities, to explore mechanisms that might explain the distributions of native forest herbs, and to relate distributions of non-native invasive plant species to land-use patterns. A key conclusion of our research to date is that historical land use has had a long-lasting legacy on the vegetation and soils of the Southern Appalachian forests. Using broad-scale analyses of the region, we intersected changes in housing density with the spatial pattern of forest communities, as dictated by the steep environmental gradients, to see whether there was differential risk. Cove hardwood forest communities were found to be particularly vulnerable to increasing development, as these communities are naturally fragmented and located in sheltered slopes at middle elevations (Turner et al. 2003b). We also delved more deeply into the potential mechanisms that might explain the herb spatial distributions. Many species that were absent or rare in locations that were used historically for agriculture (e.g., bloodroot, bellwort, Trillium) were long-lived perennials dispersed by ants over short distances (< 2 m). We asked whether a lack of dispersers might explain the distribution of these herbs, but results showed that the species ants required for dispersal were present across the study sites (Mitchell et al. 2002). Thus, lack of dispersers could not explain herb distributions - although dispersal rates would be slow, especially if populations had been locally extirpated and were distant from seed sources.
We focused next on whether soils and growing conditions might be affected by land-use history. Although there were some differences in mean size of nutrient pools with land-use history, there were striking differences in the variance of soil nutrients (Fraterrigo et al. 2005). In previously farmed sites, there was less total variance, and most variance was among plots rather than within plots. In reference sites, total variance was greater, and most variance was within plots (thus at finer scales) not among plots. Thus, it was the spatial structure of soil nutrients that revealed a key land-use legacy. Spatial variation in plant densities were largely driven by soil patterns because detrending the patterns with soil nutrients removed the effects of different land-use histories on the spatial patterns of the herbaceous species (Fraterrigo et al. 2006c). Soil microbial communities were also distinct among land-use histories. Previously farmed sites have less fungi, more Gm- bacteria, whereas reference sites have more fungi (Fraterrigo et al. 2006a). These differences were present even though total microbial biomass and litterfall did not vary. Finally, we found that herbaceous plants exhibit different patterns of growth and biomass allocation at sites with different land use histories. In references sites where total herb cover is higher, plants allocate more biomass to stems rather than leaves compared to sites previously used for agriculture or logging (Fraterrigo et al. 2006b). Conversely, there is more allocation to leaves, and less to stems in previously farmed areas (where total herb cover is lower and soil phosphorus is higher). In sum, these studies clearly demonstrated long-term effects of historical land use manifest in soils and forest herbs.
Additionally, support from the LTER program is acknowledged in several general articles that are not tied directly to our empirical studies. These include a contribution to the special feature about LTER in BioScience (Turner et al. 2003a), an invited annual review article on landscape ecology (Turner 2005a) and a contribution to a special feature in Ecology on landscape ecology in North America (Turner 2005b).
We next extended our vegetation studies to non-native invasive plants that are shade tolerant and hence have the potential to impact this densely forested region. Our field data revealed that Celastrus orbiculatus (oriental bittersweet, a woody vine native to east Asia) and Microstegium vimineum (Japanese stiltgrass, a shade-adapted C-4 grass) were the invasives that were encountered most freqeuntly. As part of his doctoral dissertation, Tom Albright found that oriental bittersweet was more likely to be found in locations closer to its introduction locus (near Asheville, NC), at lower elevations, with greater forest edge habitat and near roads (Albright et al. in press). Furthermore, oriental bittersweet appears to be expanding its distribution in the Southern Appalachians. Analyses of the regional distribution of Microstegium, using data from the Carolina Vegetation Survey, revealed that that, at broad scales, areas with high human activity and low forest cover were at highest risk of M. vimineum invasion (Anderson et al., in review). At fine spatial scales, the probability of M. vimineum presence increased with increasing native species richness and followed a soil fertility gradient. Spatial parameters suggested that areas within a 3-km distance of established locations, including high fertile sites with well-established native plant communities, were at an elevated risk of invasion.
Doctoral student Tim Kuhman focused on how land-use history influences the presence and abundance of non-native invasive plants. This work included a field study of the effects of land-use history on invasive species presence and abundance at the Bent Creek Experimental Forest, for which land-use history is well known and mapped. These data revealed a very strong positive association between land-use history and invasive species occurrence, along with intriguing differences in both overstory composition and the evergreen understory (which is drastically reduced in areas that were farmed historically) (Kuhman et al. 2011). Experimental germination studies were conducted with Celastrus seeds to identify the factors that could lead to increased success of invasives in sites that were previously cultivated (Kuhman et al. in press). To determine whether recent exurban development is facilitating the spread of non-native plants, Kuhman also sampled the presence and abundance of non-native invasive plants within 25 watersheds varying in housing development patterns (Kuhman et al. 2010). Collectively, our studies demonstrate strong influences of both past and present land use on invasion of intact forest communities by shade-tolerant, non-native plants.
Albright, T. P., D. P. Anderson, N. S. Keuler, S. M. Pearson and M. G. Turner. 2009. The spatial legacy of introduction: Celastrus orbiculatus in the southern Appalachians, USA. Journal of Applied Ecology 46:1229-1236.
Anderson, D. P., M. G. Turner, S. M. Pearson, T. P. Albright, R. K. Peet and A. Wieben. Predicting Microstegium vimineum invasion in intact forests of the southern Blue Ridge Mountains, U.S.A. (In review).
Fraterrigo, J. M., M. G. Turner, S. M. Pearson, and P. Dixon. 2005. Effects of past land use on spatial heterogeneity of soil nutrients in Southern Appalachian forests. Ecological Monographs 75:215-230.
Fraterrigo, J. M., T. C. Balser and M. G. Turner. 2006a. Microbial community variation and its relationship with nitrogen mineralization in historically altered forests. Ecology 87:570-579.
Fraterrigo, J. M., M. G. Turner and S. M. Pearson. 2006b. Plant allocation and growth in the herb layer of historically altered forests. Journal of Ecology 94:548-557.
Fraterrigo, J. M., M. G. Turner and S. M. Pearson. 2006c. Interactions between past land use, life-history traits and understory spatial heterogeneity. Landscape Ecology 21:777-790.
Fraterrigo, J. M., S. M. Pearson and M. G. Turner. 2009. Land-use history and the response of understory herbaceous plants to nitrogen fertilization. Forest Ecology and Management 257:2182-2188.
Fraterrigo, J. M., S. M. Pearson and M. G. Turner. 2009. Joint effects of habitat configuration and temporal stochasticity on population dynamics. Landscape Ecology 24:863-877.
Hicks, N. G. and S. M. Pearson. 2003. Salamander diversity and abundance in forests with alternative land-use histories in the southern Blue Ridge Mountains. Forest Ecology and Management 177:117-130.
Kuhman, T. R., S. M. Pearson and M. G. Turner. 2010. Effects of land-use history and the contemporary landscape on non-native plant invasion at local and regional scales in the forest-dominated southern Appalachians. Landscape Ecology 25:1433-1445.
Kuhman T. R., S. M. Pearson and M. G. Turner. 2011. Agricultural land-use history increases non-native plant invasion in a Southern Appalachian forest a century after abandonment. Canadian Journal of Forest Research 41:920-929.
Kuhman, T. R., S. M. Pearson and M. G. Turner. Disentangling factors related to land-use history that facilitate non-native plant invasion: a field experiment with Celastrus orbiculatus. Biological Invasions (In press).
Mitchell, C. E., M. G. Turner and S. M. Pearson. 2002. Effects of historical land use and forest patch size on myrmecochores and ant communities. Ecological Applications 12:1364-1377.
Pearson, S. M., A. B. Smith, and M. G. Turner. 1998. Forest fragmentation, land use, and cove-forest herbs in the French Broad River Basin. Castanea 63:382-395.
Turner, M. G. 2005a. Landscape ecology: what is the state of the science? Annual Review of Ecology, Evolution and Systematics 36:319-344.
Turner, M. G. 2005b. Landscape ecology in North America: past, present and future. Ecology 86:1967-1974.
Turner, M. G., S. Collins, A. Lugo, J. Magnuson, S. Rupp and F. Swanson. 2003a. Long-term ecological research on disturbance and ecological response. BioScience 53:46-56.
Turner, M. G., S. M. Pearson, P. Bolstad and D. N. Wear. 2003b. Effects of land-cover change on spatial pattern of forest communities in the southern Appalachian Mountains (USA). Landscape Ecology 18:449-464.
This research is funded by the LTER program of the National Science Foundation.