Landscape-scale vegetation dynamics inferred from spatial patterns of soil δ13C in a subtropical savanna parkland

Grasslands and savannas around the world have experienced woody plant encroachment during the past 100 years, but we know little regarding the manner in which woody plants spread across the landscape. We used soil delta C-13, aerial photography, and geostatistics to quantify patterns of woody encroachment in a 160 x 100 m georeferenced grid subdivided into 10 x 10 m cells in a savanna parkland landscape in southern Texas. delta C-13 contour maps revealed that centers of closed contours coincided with centers of woody patches, and that larger woody patches developed from smaller woody plant clusters that spread laterally and coalesced. Areas where woody patches were expanding into grassland were characterized by low densities of soil delta C-13 contour lines, and indicated the direction and extent of woody encroachment. Conversely, areas with high contour densities represented grassland-woodland boundaries that were temporally stable. Indeed, aerial photos from 1930, 1941, 1982, and 2003 confirmed that woody patches with low spatial variability in delta C-13 corresponded to areas where woody plants had encroached during the past 30-75 years. While aerial photos can only record vegetation cover at the photo acquisition time, kriged maps of soil delta C-13 allowed us to accurately reconstruct long-term temporal dynamics of woody plant encroachment into grassland. This approach can reliably reconstruct landscape-scale vegetation changes in areas where historical aerial photography or satellite imagery are unavailable and provides a strong spatial context for studies aimed at understanding the functional consequences of vegetation change.