Vegetation dynamics during last 35,000 years at a cold desert locale: preferential loss of forbs with increased aridity

Paleoecological records are an important source of data to better understand ecological responses to climate. To help understand vegetation-climate relationships in the Great Basin cold desert of North America, we analyzed 154 plant taxa from 52 fossil woodrat middens that spanned both an 800-m elevation gradient and the last 35,000 yr (35 ka) within a single study area. Vegetation was analyzed by community assemblage (CA), by plant functional type, and by individual species. Concordant with a predominant trend of increased aridity since glacial maximum, CA change was largely unidirectional despite centennial-scale cycles of climate variation. Richness of forb and other herbaceous plant functional types peaked from 15 to 26 ka during glacial maximum and then gradually decreased from similar to 15 ka to present. Analysis of individual taxa indicates that once a herbaceous species was lost from the study area, that species did not re-occur. In contrast, woody (shrubs and trees) species richness peaked from 8 to 15 ka during and following the Bolling-Allerod rapid warming. However, most tree taxa that established during this period of warming were subsequently lost as climate became more arid after the beginning of Mid-Holocene Temperature Maximum warming similar to 8 ka. These shifts in plant functional types decreased relative richness of forbs compared to shrubs (i.e., decreased forb/shrub ratio), and these shifts continue to the present despite intervening cycles of climate cooling and warming. We conclude that the relative importance of herbaceous species in current CAs of the Great Basin cold desert has been decreasing for the last 15 ka. We also speculate that decreased richness of herbaceous species has, in part, provided opportunities for exotic species to establish and proliferate in the Great Basin during the last 100 yr. Thus, observations of past vegetation change suggest that increased aridity with future climate warming will continue to favor woody vegetation over herbaceous species at our cold desert locale, at least until invasive species displace native shrub species.