Woodland expansion is a highly debated topic documented on numerous continents threatening grassland and savanna ecosystems. The Great Plains, USA contains several land cover types such as grasslands, croplands, deciduous forests, shrublands and is expanding in energy development while the remaining grasslands within the region are climatically suitable for woodland expansion. Evidence exists showing how fire can reduce woody cover and restrict woodland expansion. The effect of fire on plant biomass however may be reduced in fragmented landscapes. Our main goal for this study was to better understand woody plant dynamics within the fragmented grasslands of the Great Plains, USA. This information would improve the global understanding of woodland expansion. We therefore calculated woody plant cover and height potential (defined as the 95th percentile) with respect to fire and mean annual precipitation (MAP) within a piece-wise regression framework. Thereafter, we calculated fragmentation indices (# patches, proportion of the landscape, largest patch index, edge density, effective mesh size, patch cohesion index) in areas classified as grasslands within three sampling areas (10km2, 360km2 and 3600km2) in >2000 sampling sites to identify the degree to which fragmentation could influence woody plant dynamics in encroached grasslands.
Results/Conclusions
We found that woody cover and plant height potential was generally restricted in semi-arid areas (<800mm MAP) while woody cover could range between 0-100% in mesic areas (>800mm MAP) even though the effect of fire was prominent in mesic areas. The fragmentation indices grassland edge density and proportion were significant drivers of woody plant cover potential. That is, areas with higher number of edges and lower grassland proportion tended to contain higher than average % woody cover. High edge density in a landscape is indicative of fragmentation, and confirms that most grassland areas are fragmented in the Great Plains, USA. Our study highlights that areas characterized with high number of edges, low proportion of grasslands and receiving relatively high MAP are susceptible to further woodland encroachment. This is a particular problem in areas that do not experience high/frequent fire activity. This talk synthesizes the culmination of several studies highlighting the interaction between % woody cover potential (woody plant biomass) that is driven by largely climate but can be restricted by fire activity, which in turn is affected by landscape fragmentation.