The boundary between the western Great Plains and the Rocky Mountains is a prominent biogeographical feature of North America. Over the past century, woodland expansion into former grasslands has been documented in many areas of the west-central Great Plains. Current biogeographical and ecological knowledge indicate that the expansion of woodlands into grasslands can be driven by climate, human land use, and/or species migration, and none of these factors may be entirely independent of the others. This study will disentangle the factors related to this expansion and identify the role of climate. The timing of expansion will be investigated and how the expansion coincides with climate variability over the past three to six centuries across a spatial gradient that extends from western Nebraska to northeastern New Mexico. To do this, areas of woodland expansion with uniform land-use histories across the spatial gradient will be selected for study. Dendroecological techniques will be used to reconstruct the timing of ponderosa pine (Pinus ponderosa) woodland expansion. Then, climate analyses using modern and reconstructed paleoclimatic data will examine variations and trends in climate over annual to centennial time scales. Finally, the study area will be surveyed for woodrat (Neotoma) middens to investigate possible earlier Holocene expansions in the area. The role of climate in 19th and 20th century woodland expansion will be investigated by comparing the timing and location of the expansion to local climate variations and the onset of widespread human land use. The paleoecological and paleoclimatological records will be used to determine whether other expansions have occurred in the past or whether post-1850s vegetation movements are unprecedented in character.
Human impacts on the landscape over the past century can obscure long-term relationships between vegetation and climate. This can lead to the assumption that any recent changes in vegetation dynamics are the result of human activity. Long-term data predating the onset of wide-spread human activity allow us to identify the long-term relationships between climate and woodland expansion and to determine if those relationships have changed over the past century. The results of this project will shed light on the possible role of climate in other cases of woodland expansion, as well as elucidating the potential affects of future climate change on woodland expansion in many regions. An interdisciplinary team will work closely with the U.S. Department of Agriculture and the Shortgrass Steppe Long-Term Ecological Research site to identify long-term management goals and research agendas for woodland areas.
