PALEOBOTANICAL EVIDENCE FOR THE COMPLEXITY OF HOLOCENE CLIMATIC VARIABILITY WITH ELEVATION IN THE INTERIOR WESTERN UNITED STATES

In the Interior western United States, the landscape is characterized by high physiographic relief, pronounced regional and local rain shadows, and gradients in temperature and precipitation across space and elevation. There is a strong gradient in the seasonality of precipitation across the region, with cool season moisture from the North Pacific dominant to the northwest, and warm season moisture from southerly sources associated with the North America monsoon (NAM) to the southeast. In addition, the seasonality of precipitation can vary considerably with elevation across short distances (for example, lower elevation sites may have a higher proportion of summer rainfall than nearby higher elevation sites).

Fossil pollen and plant macrofossils from lake and wetlands, combined with macrofossils from packrat middens and remnant wood from above modern upper tree line, provide evidence that complex changes in the distributions of plant species and in the composition of plant communities occurred throughout the Holocene in this region. Rapid warming at the beginning of the Holocene led to the development of desert plant communities and to the northward and upslope migration of many woodland and forest species. Warmer-than-historic climatic conditions occurred across the region during parts of the early (EH, ~11.7 to 7.5 cal ka) and middle Holocene (MH, ~7.5 to 4.0 cal ka), but the timing of maximum warmth varied across space and elevation. In addition, in some settings warmer conditions during the EH or MH were substantially drier than present, whereas at some sites to the south and east it appears that moisture levels were higher than at present (perhaps due to an enhanced NAM)

Based on downslope movements of woodland taxa and changes in the composition of forest communities, cooler and moister conditions in many parts of the region occurred during the late Holocene (~4.0 cal ka to present), with the coolest conditions during the neoglacial (NG, ~4 to 2 cal ka) and the Little Ice Age (~ 0.55 to 0.25 cal ka). However, upper tree lines were higher than present through the NG at some sites, providing seemingly contradictory evidence of warmer-than-historic summer temperatures, and emphasizing the importance of understanding differential responses to climate change across the elevation gradient in this region.