An important message concerning changes in abiotic factors with elevation is the realization that decreasing ambient pressure is the only physical property unaffected by microclimatic effects. All others (e.g., temperature, sunlight, wind, long-wave radiation, water and nutrient relations) can be strongly influenced by topography, microsite, and plant form at any altitude. Natural variability in these factors can substantially lower, or raise, the effective altitude of a microsite at any given altitude. In the Northern Hemisphere, south-facing, wind-sheltered microsites can effectively match conditions at an altitude thousands of meters lower, while similar north-facing microsites, sheltered from sun but not the cold night sky, could generate increases in effective altitude. Even smaller microsites around a fallen tree stem or an exposed boulder can result in effectively different altitudes based on differences in sunlight exposure and temperatures. Additionally, changes in leaf orientation of the plant can create different levels of sky and wind exposure, two primary factors influencing microclimate at any altitude. Leaf and plant aggregation (close spacing) and height patterns can also influence microclimate due to the potentially strong boundary layer effects on temperature and ambient gas concentrations. Thus, microclimate effects can significantly impact fundamental gas exchange processes at any altitude, with the exception of ambient pressure effects on molecular diffusion. In contrast to the potential effects of microsite and plant form on effective altitude, individual plants cannot escape the ambient pressure of their respective altitudes (only negligible changes in ambient pressure due to weather fronts). Thus, lower ambient pressure and more rapid molecular diffusion are the only immutable abiotic factors associated with increasing altitude, one that is not dependent on microsite/microclimate effects.
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