Climate-Vegetation Atlas of North America

Introduction, Methods, and Sources of Data

Introduction

This atlas presents information on the modern relations between climate and the distributions of 407 plant taxa and biogeographic entities from across North America. Included among these are 115 conifer species, 32 conifer groups (such as the genera and subgenera), 239 hardwood species, and 21 hardwood groups.

The atlas is divided into two volumes. The first volume, U.S. Geological Survey (USGS) Professional Paper 1650-A, contains the Abstract, Introduction (including supporting figures and tables), References Cited, and the atlas pages for conifers. The second volume, USGS Professional Paper 1650-B, contains the Abstract, an abbreviated Introduction (including supporting tables), and the atlas pages for hardwoods. These two volumes are published together and are not available separately. In the first part of each volume, we provide atlas pages of graphical displays of the modern relations between climate and distributions of important trees and shrubs in North America. The graphical displays include (1) maps of geographic distributions of taxa; (2) univariate plots that indicate the presence or absence of the taxon under consideration in relation to single climatic (mean January, July, and annual temperature; mean January, July, and annual precipitation) and bioclimatic (growing degree days, mean temperature of the coldest month, and moisture index) variables; (3) bivariate plots that illustrate the presence and absence of the taxon for various combinations of seasonal temperatures, seasonal precipitation, July temperature (a proxy for growing-season temperature), and annual precipitation; and (4) complex displays that indicate the presence and absence of the taxon in relation to combinations of the three bioclimatic variables.

In the second part of each volume, we provide atlas pages of histograms for each taxon that display the percentage of the total number of grid points for the taxon that occur within a specified range of each climatic or bioclimatic variable. This information permits the user to see where the taxon is most abundant and also to examine the variability of a taxon along specific environmental gradients. In the third part of each volume, the histogram data are presented in tabular form so that users can obtain quantitative information without having to interpret the data from the visual displays.

Methodology and Sources of Data

We used the singular value decomposition least-squares estimation technique (Press and others, 1986) with the modern climatic data to develop regression equations that allowed us to estimate the monthly and annual values for temperature and precipitation at each point on the 25-km grid as functions of location and elevation (figs. 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, and 14) (Lipsitz, 1988; Bartlein and others, 1994). A separate regression equation was used for each target grid point using observations from the vicinity of the target point. Elevation data from the ETOPO5 5-minute topographic grid (Edwards, 1992) were used to provide information for the estimates at each grid point-experimentation revealed that elevation was a reliable predictor of temperature, whereas smoothed elevation worked best for estimation of precipitation. After estimating the monthly temperature and precipitation values, we calculated a series of "bioclimatic" variables that have been demonstrated to more directly control plant distributions than do temperature and precipitation (Prentice and others, 1992; Sykes and others, 1996). The bioclimatic variables employed here are (1) the mean temperature of the coldest month (MTCO, Prentice and others, 1992), (2) growing degree days on a 5°C base (GDD5, Newman, 1980), and (3) a moisture index (MI) that incorporates the full seasonal cycle of precipitation and evapotranspiration (actual evaporation/potential evaporation (AE/PE), based on Thornthwaite and Mather, 1955, 1957; Willmott and others, 1985).

Nearly all of the range maps were obtained from atlases of tree and shrub distributions compiled by Elbert L. Little, Jr. and his associates at the Forest Service of the U.S. Department of Agriculture (Little, 1971, 1976, 1977; Critchfield and Little, 1966). Additional distribution maps were obtained from Bailey (1970), Benson and Darrow (1981) and Yang (1970). The original source for each map is listed in table 1, and table 2 shows the species comprising the groups listed in table 1. For each taxon, we digitized the distribution map and determined at which of the points on the 25-km grid the taxon was present. This information was merged with climate information to create a presence- absence matrix of plant distributions associated with modern climatic (and bioclimatic) data. Graphical displays were then prepared to illustrate the modern geographic distribution of the taxon under consideration and the relations between the plant's distribution and climatic parameters. There have been no mathematical manipulations of the presence-absence data in the following figures and tables. Instead, these figures and tables display the relationships between plant distributions and climate simply by plotting both the presences and absences of the plants in relation to climatic parameters.

Displays of Geographic Distributions and Climatic Relations

Panel "a" on each atlas page (graphical display) illustrates the modern distribution of the taxon on the 25-km grid. Figure 16 illustrates the distributions (taken from the individual atlas pages for these taxa) of four species of pines that inhabit different areas of North America and that presumably have very different climatic tolerances. Each species is portrayed by depicting the grid points where the taxon is present and omitting those where the taxon is absent. In the sample shown here, Pinus edulis inhabits the mesas and mountain slopes of the Colorado Plateau region of the American Southwest; P. engelmannii (Apache pine) lives in subtropical semiarid northwestern Mexico; P. banksiana (jack pine) forms a major constituent of the boreal forest and northern mixed forest of Canada and the Northeastern United States; and P. clausa (sand pine) lives in moist subtropical environments in central Florida.

Panel "b" on each atlas page shows univariate plots of the presence and absence of each taxon relative to single climatic parameters. Each box contains two lines of points: the upper line illustrates climate values at which the taxon is present, whereas the lower line indicates those where it is absent. Together, these two lines represent all grid points from across North America. In all cases, a given plant is absent from many sites where the recording of "presences" indicate that it should be able to survive under the environmental conditions represented by a single variable. This is probably because other climatic and (or) environmental variables, not portrayed in any one univariate view, control the plant's distribution. For example, for the four pine species (fig. 17), the upper three boxes in each panel represent mean annual, January, and July temperature (in °C). The three central boxes represent mean annual, January, and July precipitation (in mm) plotted on a logarithmic scale, which stretches out the distance between points on the lower end of the scale. The lower three boxes represent bioclimatic variables (discussed above) that should be more closely tied to the physiological limits of the plants than are temperature and precipitation (Prentice and others, 1992). The mean temperature of the coldest month (MTCO) represents the coldest conditions experienced by the plant over the course of a year--MTCO is usually, but not always, January in North America. Growing degree days on a 5°C base (GDD5, Newman, 1980) represents the total amount of energy available to the plant through the course of the year (incorporation of information on both the length and intensity of the growing season), and actual evaporation divided by potential evaporation is a moisture index (MI) based on Thornthwaite and Mather (1955, 1957) and implemented by Willmott and others (1985). The latter index incorporates the full seasonal cycle of precipitation and temperature, as well as soil moisture and moisture storage in snowpack. This index is probably a more realistic depiction of the moisture conditions experienced by plants than either seasonal or mean annual precipitation. Regions where moisture is sufficient to maintain evaporation at its potential rate (i.e., humid regions) have moisture-index values close to 1.0, whereas dry regions have moisture-index values somewhat less than 1.0.

The comparison of the four pine species shown in figure 17 illustrates their different adaptations in regard to temperature. Pinus clausa is limited to environments where mean January temperatures are above 10°C and mean July temperatures are in a limited range at the very warm part of the scale. P. engelmannii is also apparently intolerant of freezing temperatures, but it is not as limited by cold as is P. clausa. It also has a larger range of acceptable July temperatures than does P. clausa. P. edulis can survive freezing temperatures but does not survive mean winter temperatures below -10°C. P. banksiana lives in a summer temperature regime that is slightly cooler than that of P. edulis, but it lives under much colder winter climates (well below -10°C). Similar patterns are evident in the comparisons of the MTCO and GDD5 for these taxa.

Panel "c" of figure 15 uses bivariate plots to illustrate potential interactions among climatic variables in controlling plant species' ranges. Here, gray dots represent where the taxon is absent, black dots where it is present. Gray and black together represent the total climate space of North America as depicted for the particular combination of climatic parameters presented. For example, the left-hand box in panel "c" illustrates the presence (or absence) of the species in relation to mean July temperature and mean annual precipitation. For most sites, these two parameters should provide a measure of growing-season temperature versus total moisture through the year. Comparisons among the four pine species in figure 18 shows that, in this "climate space," Pinus clausa inhabits a remarkably small area where mean July temperatures are very warm and mean annual rainfall is very high-these are some of the hottest and wettest environments in North America (it may also be that P. clausa is further limited to sandy substrates, as its name implies).

P. engelmannii, the other subtropical pine in this group of four species, lives under a wider range of summer temperatures and moisture conditions than P. clausa, and it also occupies a much greater geographic range. In contrast, P. banksiana lives in relatively cool summer environments under a wide range of moisture conditions. P. edulis inhabits environments with mean July temperatures between those of P. engelmannii and P. banksiana, but it lives in generally drier regions than either of these two other pines. The central box in panel "c" on figure 15 illustrates the presence (or absence) of the species relative to the seasonal extremes of mean January and July temperature. In the example shown here (fig. 18), P. clausa lives under a narrow band of hot July conditions and a slightly broader range of warm January temperatures. P. engelmannii has a broader range of tolerance for July and January temperatures, although still within the warm end of the spectrum for both months. P. edulis lives in moderately warm July environments with January temperatures generally near freezing, whereas P. banksiana is adapted to moderate July temperatures and very cold January temperatures. The right-hand box in panel "c" (figs. 15 and 18) provides a similar view of species presence (or absence) in relation to mean January and July precipitation. Here, it can be seen that P. engelmannii lives under a strongly summer precipitation dominated regime with little January precipitation; P. clausa receives abundant rainfall in both seasons; P. banksiana lives under a relatively narrow range of July precipitation and a larger range of January precipitation; and P. edulis is adapted to moderate levels of precipitation in both winter and summer.

Panel "d" (fig. 15) incorporates the three bioclimatic variables into a single figure (see Huntley and others, 1995, for a similar presentation of correspondences between climatic parameters and tree distributions in Europe). Each of the four boxes within this panel represents a quartile of the moisture conditions in North America, from the driest quarter of the grid cells on the left to the wettest on the right. Within each box, the presence (black) or absence (gray) of each species is plotted relative to GDD5 and MTCO. In the example shown in figure 19, Pinus clausa lives in a very restricted range in the wettest and warmest environments of North America. In contrast, P. banksiana lives across the spectrum from dry to wet sites but is restricted to locations where MTCO is below freezing and GDD5 is less than approximately 2,500. P. edulis and P. engelmannii both inhabit environments in the lower three quartiles of the moisture index, with P. edulis more common toward the dry end. MTCO and GDD5 differentiate the climates of these two pines, with P. engelmannii restricted to environments with warmer winters and greater total energy inputs (GDD5) over the course of the year.

In terms of precipitation and moisture conditions, the climatic distribution of P. clausa falls at the extreme wet end of the moisture index (>0.9), and the precipitation plots indicate that it receives more moisture in July than in January. The other three pine species survive in a fairly wide range of moisture conditions (as seen in the moisture-index plots), but live in different seasonal precipitation regimes. P. engelmannii lives under moisture conditions ranging from only slightly drier than the moisture-requiring P. clausa to moderately dry environments, with July precipitation dominant over January. P. edulis is the most drought-adapted of the four pines and lives under a nearly even mix of January and July precipitation. P. banksiana lives under moisture conditions that range from moderately dry to very moist, with January and July precipitation nearly equally dominant.

For species with five or fewer occurrences on the North American climate grid (18 conifer species, 9 hardwood species), we identified grid points closest to the digitized polygons that represent the range of each species and used these grid points to approximate climatic parameters. The atlas pages with graphical displays for these species have the text "minimal data-nearest grid points used with environmental parameters" after the species name at the top of the page. These species were treated differently than more abundant species in that (1) large dots were used on the species map to indicate the grid points used in the analysis, (2) "absence data" were not plotted on the displays of distribution against climatic parameters, and (3) the species were not included in the histograms or tables.

Histograms of Relations Between Plant Distributions and Single Climatic Variables

Some taxa (e.g., P. clausa) have very narrow ranges of temperature requirements. Others, such as P. flexilis and P. ponderosa, live under a broader range of temperatures. Some appear to be limited by freezing winter temperatures (look at mean January temperature for P. engelmannii and P. serotina), whereas some require warmer winters (P. clausa). Many taxa have near-Gaussian distributions for temperature response, although some (for example, P. engelmannii) exhibit bimodal responses.

The same format is used in figure 21 to illustrate the species' distributions relative to mean annual, January, and July precipitation (log scale). As with temperature, some species live in narrow, clearly defined ranges (most of the eastern pines), whereas others (especially P. ponderosa) live under a wide range of precipitation regimes. Figure 22 shows the species' distributions in relation to the bioclimatic variables. The patterning for MTCO is very similar to that for mean January temperature. The histograms for GDD5 are similar to those for mean July temperature, but with some differences. P. clausa and P. serotina live under very narrow ranges of July temperatures but have broader ranges relative to GDD5. In regard to the moisture index (actual evaporation divided by potential evaporation), all the pines from eastern North America (except for P. banksiana) are at the wet end of the scale (MI = 0.9 and above). In contrast, western pines live under a variety of moisture conditions but are generally found in much drier habitats than their eastern counterparts.

Tables of Relations of Plant Distributions to Single Climatic Variables

Table 3 illustrates these values for Pinus edulis, and inspection of these values provides a broad characterization of the distribution of this species in relation to climatic parameters. As an example of how to read this information, the first row of data indicates that this pine lives under mean January temperatures that range from -11.4°C (0 percent) to 5.8°C (100 percent), with the median (50 percent) temperature at -2.0°C. Comparisons of the breadth of climatic estimates for the apparent total range (0 to 100 percent) reveal that they are 1.75 to 3.0 times greater than those for the 10- to 90-percent band. This suggests that the total apparent climatic range includes outliers that may represent problems with the original range maps and (or) our estimates of present-day climatic means, and thus the apparent total range is probably overestimating the climatic tolerance of the taxon.

Table 4 provides comparisons of the median (50 percent) values for the distributions of the nine pine species discussed above relative to each of the climatic parameters. The median value represents the midpoint or most likely value for the occurrence of each species and thus provides a good starting point for comparing the climatic tolerances of the taxa. In table 4, the north-to-south gradients of the western pines (the first four species) and of the eastern pines (the last five species) are clearly evident in the temperature, MTCO, and GDD5 data. The precipitation and MI data reveal that the plants live under a variety of moisture regimes and that there are no simple north-to-south gradients in regard to moisture conditions.

Climatic Parameters and Plant Distributions

Acknowledgments

References Cited

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