The Hawaiian Islands, despite their relatively small land mass, support a great diverstiy of natural communities. More than 175 different natural communities have been recognized by scientists during recent attempts to classify Hawaiian ecosystems (The Nature Conservancy of Hawai`i, 1987). The large number of distinct native communities, almost all of them unique to the Hawaiian Islands, have developed because of the great variation in elevation, rainfall, substrate, topography, and exposure that occurs even over short distances in Hawai`i. For the purposes of generalization, plant communities may be grouped together into several major ecological zones, delineated by elevation and moisture. Five of these zones are presented in "Under the Hawaiian Sky:" alpine, subalpine, montane mesic, rainforest, and dryland forest. Other types of zones include bogs, coastal, wetlands, and grasslands. The location of ecological zones is controlled in part by the Hawaiian climate. A very important element in the climate here is the prevalence of the northeasterly trade winds, which bring moisture-laden air to the windward slopes of the Islands. With increasing elevation, air temperature decreases. Because cold air is less capable of holding moisture, water precipitates out as rain on the northeast-facing slope of the Islands. The wettest regions are the windward summits and upper slopes of Kaua`i, O`ahu, and Moloka`i, and a band between 2,000-5,000 feet on Maui and Hawai`i. Maui and Hawai`i have a temperature inversion layer between 5,000-7,000 feet, where a mass of air slightly warmer than that below, prevents lower air masses from moving higher. Because more rain falls below this layer, the upper elevations (alpine and subalpine) of Maui and Hawai`i are very dry, often receiving less than 20 inches of precipitation annually. Leeward mountain slopes and low areas to the southwest of tall mountains are dry because they are in "rain shadows," where the windward slopes intercept the most air and precipitation before it reaches the leeward side. This effect also explains why the low islands of Ni`ihau and Kaho`olawe are dry; these islands are in the rain shadows formed by Kaua`i and East Maui.
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