Introduction

The modern relations between the presence or abundance of plant taxa and climatic parameters can be used to estimate past climatic conditions and to model potential changes in plant distributions due to climate change. To facilitate these reconstructions and modeling exercises, we constructed a 25-km equal-area gridded dataset of plant distributions and climatic parameters for North America north of lat 25°N. (Thompson and others, 1999a, 1999b, 2000, 2006, 2007). Here we provide the datasets and methodologies that will enable researchers to obtain climatic estimates from a list of plant taxa present in a fossil assemblage by the Mutual Climatic Range (MCR) technique (Iversen, 1944; Grichuk, 1969).

MCR is a relatively simple approach that involves comparing modern climatic tolerances of taxa present in a fossil assemblage to determine the range of a climatic parameter where all of the constituent taxa could coexist. The method is notable in that it does not have many of the requirements of other approaches to paleoenvironmental reconstruction. Specifically, it does not require that (1) taxa present in the fossil assemblage live together today, (2) data are proportional or quantified in any manner other than a list of taxa that are present, or (3) the fossil list include all taxa present at the site when the assemblage was formed.

The MCR approach has been used to estimate late Neogene and Quaternary paleoclimatic conditions from across the globe based on insect (Elias, 1997; Elias and Matthews, 2002), mollusk (Moine and others, 2002), and ostracode (Horne, 2007) assemblages. Similarly, since the middle of the 20th century, the technique has been widely applied in the reconstruction of past climatic parameters from paleobotanical data (for example, Grichuk, 1969, 1984; Iversen, 1944; Mosbrugger and Utescher, 1997; Pross and others, 2000; Sinka and Atkinson, 1999). In North America, paleoclimatic estimates have been obtained by applying the MCR technique to assemblages of plant remains from packrat (Neotoma spp.) middens in the arid Western United States (Sharpe, 2002; Thompson and others, 1999c, 2008).

In many cases, paleoclimatic reconstructions based on the application of the MCR technique to paleobotanical data result in the delimitation of a broad range of values for a given climatic parameter where the taxa in a given fossil assemblage could possibly coexist. To reduce the breadth of this estimated range, we devised a modified version of the MCR technique that weights the information contributed by each taxon based on the modern population distribution of this taxon relative to the climatic parameter under consideration. We refer to the traditional MCR method as the unweighted approach (MCR_un) and to our new variant as the weighted approach (MCR_wt).

In the MCR_wt approach, weights are applied to data from each taxon to decrease the effects of extreme values in the modern calibration dataset on the climatic estimate. For a given increment in the climatic parameter under consideration, the weights are summed across the taxa present, and the range of the maximum sum of weights identifies where there is the greatest overlap of the 10th to 90th percentiles of the constituent taxa in an assemblage. The midpoint of the range of the maximum sum of weights is then taken as the estimate of the given climatic parameter. The matrices included in this report provide paleoclimatic investigators with the data required to obtain their own climatic estimates from North American plant assemblages, by both the MCR_un and MCR_wt approaches. These data can also be used in the simulation of changes in the distributions of plant species related to potential future changes in climate (for example, Shafer and others, 2001).