Ponderosa pine (Pinus ponderosa) has been a small component of the central Great Plains ecosystem for centuries; however at some point it expanded beyond protected scarps, ravines, and outcroppings and developed into woodlands that occupy large areas of the landscape. We undertook a study of the timing of expansion and infilling of ponderosa woodland in the central Great Plains to document when and how the woodlands moved into the grasslands and how this movement was influenced by climate. We hypothesized that moisture availability would be positively correlated with tree recruitment and that pine regeneration would be more robust during wet periods and in areas with higher moisture availability. Our study area encompassed a 300-400 km wide band in the central Great Plains extending from northern New Mexico and Oklahoma, through Colorado and Kansas, to central Wyoming and Nebraska. We collect cores and cross sections from all trees, seedlings, and saplings in fixed-area sample plots to reconstruct trends in woodland initiation and tree recruitment in 11 sites. Reconstructed decadal recruitment patterns since the late 1700s were compared with reconstructed and instrumental Palmer Drought Severity Index (PDSI). Reconstructed annual pine recruitment since 1985 was compared with monthly, seasonal, and annual trends in temperature, precipitation, aridity index and PDSI.
Results/Conclusions Woodlands in the western portion of the central Great Plains expanded into the region over 200 years ago and expansion moved eastward through the 19th century. Modern woodland basal area and density cannot be explained by site-specific characteristics such as latitude, longitude, elevation, average precipitation, or age of woodlands. However, throughout the region the woodland population is stable and self-sustaining. Woodland dynamics were most synchronous in the mid-20th century when recruitment was low and the area was experiencing an extended drought. Over 40% of reconstructed recruitment occurred in the most recent two decades. During this time pine establishment was positively and significantly correlated with annual moisture availability, indicating that climate plays a strong role in recent woodland demography. Woodlands are a stable component of the plains ecosystem and their presence in the region should be incorporated into human perception of the landscape. Additionally, climate has influenced past and present dynamics of woodlands and should be recognize when considering the future of the trees within the plains.
