Introduction

The term "ecoregion" is used to denote a mappable generalized complex of ecosystem and associated abiotic factors (Loveland and Merchant, 2004). In North America, a variety of ecoregion classification systems are being used by land-management agencies and conservation groups as the organizational frameworks for planning and resource-management programs. Climate is a major control on the distributions of plant species that are, in large part, the basis for defining ecoregions and mapping their boundaries. Three of the preceding volumes of this atlas (Thompson and others, 1999a, 1999b, 2000) explored the continental-scale relations between climate and the geographic ranges of woody plant species in North America. The fourth volume (Thompson and others, 2006) examined the relations between climate and plant species' ranges in Alaska, along with an exploration of the climatic characteristics of ecoregions within this State.

This volume examines the relations between climate and the distributions of (1) Küchler's (1985) "potential natural vegetation" categories for the 48 contiguous States of the United States of America, (2) Bailey's (1997, 1998) ecoregions of North America, and (3) World Wildlife Fund's (Ricketts and others, 1999) ecoregions of North America. The Küchler "potential natural vegetation" categories were defined prior to the development of the concept of ecoregions; for the sake of simplicity, we refer to those categories as ecoregions in this volume. For these analyses, we constructed a 25-km equal-area grid of modern climatic and bioclimatic parameters for North America from instrumental weather records. We generated a digital representation of the geographic distribution of each ecoregion from published sources, and the presence or absence of each ecoregion was then determined for each point on the 25-km grid, thus providing a basis for comparison of the climatic data with the geographic distribution of each ecoregion. The climatic (mean January, July, and annual temperature and precipitation) and bioclimatic data used here are the same as those used in the previous volumes of this atlas (see Thompson and others, 1999a for methodology). The bioclimatic parameters are mean temperature of the coldest month (MTCO), growing degree days on a 5°C base (GDD5), and a moisture index that equals actual evaporation divided by potential evaporation (AE/PE). The relations between climate and these distributions are presented in graphical and tabular form. As described in Thompson and others (1999a), the tables provide the range, quartile, and other data for each ecoregion in regard to each climate parameter. For ecoregions with five or fewer grid points, only the minimum and maximum values are given for each parameter (and asterisks are shown for the other values).

This volume has five sections: (1) this Introduction, (2) index maps of the gridded distributions of the ecoregions under each of the three classification systems, (3) graphical displays for each ecoregion that include univariate, bivariate, and trivariate plots of the presence or absence of the entity in relation to climatic and bioclimatic variables, (4) histograms that display the percentage of the total number of grid points for the ecoregion that occur within a specified range of each climatic or bioclimatic variable, and (5) tables that permit users to obtain quantitative information on the relations between the distribution of each ecoregion and climatic or bioclimatic parameters (these tables are in Microsoft Excel format and are designed to be opened independently, instead of viewed in a web browser).