Climate is fundamentally important to the distribution of plants and defining the suitability of plant species to specific areas. This section provides information on climate classification systems and spatial analysis and modeling and how these relate to species suitability and selection.

Classification Systems

Many attempts have been made to classify the climates on Earth into a comprehensive and comprehensible system. One of the earliest began with Aristotle and his discussion of Temperate, Torrid, and Frigid Zones. The system that is in almost universal use now is the Köppen system, developed in 1928 by German climatologist and amateur botanist Wladimir Köppen.

The Köppen System

The modified Köppen system uses letters to denote the six major climate regions and their 24 sub-classifications. These regions are based on average monthly temperature and precipitation values. While it does not take full account of factors such as cloudiness, solar radiation, wind, or even extremes in temperature, it still remains a useful system. Although the individual zones are shown (Figure 1) with clearly delineated boundaries, it is important to note that the system is just a guide to average climate trends and that the areas between zones represent a gradual transition between climates.

Köppen spent time updating and modifying his system of climate classification right up until his until his death at 94 in 1940. Since then it has been modified by a number of geographers, most notably the late Glen Trewartha of University of Wisconsin, whose version is probably in the widest use today.

The modified Köppen classification uses six letters to divide the world into six major climate regions, based on average annual precipitation, average monthly precipitation, and average monthly temperature.

  • A for tropical humid
  • B for dry
  • C for mild mid-latitude
  • D for severe mid-latitude
  • E for polar
  • H for highland (this classification was added after Köppen created his system)

Each category is further divided into sub-categories based on temperature and precipitation (Table 1).

For example, the U.S. states located along the Gulf of Mexico are designated as "Cfa." The "C" represents the "mild mid-latitude" category, the second letter "f" stands for the German word feucht or "moist," and the third letter "a" indicates that the average temperature of the warmest month is above 72°F (22°C). Thus, "Cfa" gives us a good indication of the climate of this region, a mild mid-latitude climate with no dry season and a hot summer.

Köppen also used vegetation to aid in climate classification, including tropical rainforest, tropical wet and dry season vegetation, low-latitude steppe, low-latitude desert, Sclerophyll forest, mid-latitude deciduous forest, Boreal forest, and tundra vegetation.

Spatial Modeling of Climate

More recently, climate station data has been used to create spatial models of climate elements including precipitation, minimum and maximum temperature, relative humidity, and solar radiation. These models have typically used distance weighting approaches that work well for flat conditions. Mountainous areas with detailed topography and coastal regions, however, have been poorly modeled by these purely statistical approaches. PRISM was developed to address these issues and allow for detailed, accurate modeling of real-world conditions. The climate spatial layers used in this project were developed with PRISM.

New Approach to Species Suitability Mapping

This project uses a new approach to species suitability mapping. Instead of using climate classification systems that combine various climate factors into zones and then listing species that occur in those zones, we are modeling and mapping the spatial distribution of each climate factor and defining quantitative plant tolerances for each factor. We believe that this approach allows for more explicit definitions of the spatial variability and causal factors related to plant suitability.