Uncertainties Associated with Climatic Estimates from Plant Macrofossil Assemblages in the Southwestern United States: Evaluations Based on Comparisons of Modern Observed and Estimated Values for Climatic Variables, and Application to Late Quaternary Assemblages

Abstract:

Plant macrofossils from packrat (Neotoma spp.) middens provide evidence of vegetation changes that have been used to infer aspects of differences between modern and past climates in the southwestern United States (the "Southwest"). This is a dry region of complex physiography with sharp elevational changes resulting in diverse microclimates over short distances.

Although it is a relatively straightforward process to obtain quantitative estimates from plant macrofossil assemblages, it can be difficult to assess associated uncertainties. For the Southwest we evaluated aspects of potential uncertainties by comparing observed climates with estimates based on modern midden assemblages, and on virtual assemblages from gridded modern plant distribution data. We compared anomalies (estimated minus observed values) produced by four quantitative estimation approaches for winter temperature, summer temperature, and annual precipitation; presenting these results as correlations, average absolute anomalies, and the degrees of bias. In addition, we assessed the influences on anomalies of: 1) the number of taxa in an assemblage, and 2) the geographic, elevational, and vegetational context of the assemblages. 

Our findings indicate that much of uncertainty in climatic estimates is related to how modern climate is depicted and how it is then estimated at individual locations. Important issues here include: what time span in the historic record is selected to represent interannual and interdecadal variability, and how is climate interpolated to a specific location. Other results include: 1) modern assemblages with relatively few plant taxa provide estimates with larger anomalies and more bias; and, 2) none of the approaches can accurately estimate true extreme values in the climate data. Overall, as a very rough guide, the range of uncertainty for climate estimates from modern plant assemblages is ± 2°C for temperature, and ± 50mm for annual precipitation. Climatic estimates from Holocene middens largely fall within these bands of uncertainly, whereas those from the Last Glacial Maximum exceed these bands and are colder and generally wetter than today.