Adaptation to physiography, geomorphology, topography, slope, and aspect

Physiographic characteristics of a site may play a very important role in positively or negatively modifying regional climatic data in terms of energy flow, temperature, evaporation, rainfall, and soil water budget (through runoff/infiltration and erosion/sedimentation, drainage, the presence or absence of waterlogging, or a satisfactory water table, and so forth). A simple calculation would show, for example, that a northern slope with a 30° dip under 40° latitude N. would have a potential evapotranspiration equal to only one-third of a similar southern slope, and therefore, if rainfall remains similar in both cases, the water budget would be three times greater in the former than in the latter (Le Houu, 1972). The selection of species to be planted is therefore likely to be different in the two cases, if optimum use of the sites is sought.

Physiography may also strongly affect local precipitation if the site is exposed to rain-bearing winds; is in a rain shadow; or is subject to descending, warm, dry winds, similar to the Fof the southern Alps, the samun winds of Iran, the Santa Anas of California, the chinooks of the eastern Rockies, the berg winds of South Africa, the nor'westers of New Zealand, or the zonda of Argentina. Geomorphology and slope may also play an important role in determining rates of erosion or sedimentation and soil depth, thus greatly affecting soil fertility and water budget. Hence, species selection and plant productivity calculations must be modified accordingly (see table 8-1 and figure 8-1).