Arboreal Bioregions

The arboreal biome includes the areas supporting forests, as opposed to only individual trees. Patches of meadows, rocky outcrops, or swamps may occur because of local ecological conditions, and although they are mostly treeless, they still form part of the arboreal biome. Temperature and humidity mainly determine the type of forest occuring in an area. Only the large zonal types are briefly characterized; most are more or less disjunct today.

hylaea A name orginally proposed for the Amazonian rain forest, Hylaea is now widely used to designate all tropical evergreen rain forests—dense, multi-storied forests, with little light reaching the forest floor. Animals and plants are adapted to favorable conditions such as temperatures, precipitation, and air humidity that are continuously high. Biodiversity is very high, probably partly because of the presumedly continuous existence of tropical rain forests over exceptionally long periods of time. Processing of shed plant material is fast, and little detritus accumulates on the forest floor. Recent studies using fumigation techniques have shown that most insects inhabit the tree crowns. From an amazingly large number of undescribed species discovered by this method, the total number of existing insect species would be 35 million; more broadly based estimates range from 10 to 30 million species of insects.

The climate supporting the Hylaea is basically nonseasonal. However, seasonal snowmelt in the Andean headwaters of the Amazon results in a seasonal discharge regime that leads to months-long seasonal flooding of vast rain forest areas and drastic seasonal changes of conditions for all life. Similar situations may occur elsewhere. Evergreen tropical rain forests exist in parts of South America, in Central America, in a discontinuous belt across equatorial Africa, and in parts of Southeast Asia, from whence they extend into tropical northeastern Australia, where only small remnants remain.

FIGURE 2 Main biomes. [Modified from a map of vegetation zones in Cox, B. C., and Moore, P. D. (1985). "Einführung in die Biogeographie." UTB 1408. Gustav Fischer Verlag © Spektrum Akademischer Verlag, Heidelberg.]

silvaea The term "Silvaea" refers to summer green deciduous broad-leafed forests, which, like the Hylaea, were much more widespread in the Tertiary than they are today. They occur in oceanic-to-suboceanic subtropical-to-temperate areas, mainly in eastern North America, in central Europe and the northern portion of southern Europe and Asia Minor, and in eastern China, Korea, and Japan. Adequate humidity is permanently available, and the species-rich vegetation offers protection against wind and radiation. During the vegetation period, the biota experiences favorable temperatures. In autumn, insects withdraw and most are inactive during winter, which may be frosty, although the soil does not freeze to a great depth. Strictly seasonal leaf shedding provides enormous amounts of dead plant material. Because low temperatures reduce production less than decomposition, much detritus accumulates, which provides habitat and food for many specialists among the diverse arthropod and insect fauna.

The evergreen temperate rain forests are located in restricted areas of the Southern Hemisphere, especially in Patagonia, southeastern Australia (including Tasmania), and New Zealand. They are in many ways similar to the two previously mentioned types but geographically disjunct, except along the Australian east coast where tropical and temperate rain forests meet and intergrade. Large tropical and subtropical areas with monsoon climate support forests that are only seasonally green (Fig. 2).

scleraea Hard-leafed trees and shrubs dominate in the Scleraea in subtropical areas with rainy, mild winters and hot, dry summers. This evergreen forest type occurs not only along the western borders of the large landmasses, mainly in California and the European Mediterranean region, but also in middle Chile, the southwestern Cape of Africa, and southwestern Australia. Thick bark, as well as hard, reflectant, and often wax-covered leaf surfaces, or feltlike, often rolled-in leaf undersides and other modifications protect the deeply rooted plants against summer heat and dryness; winters tend to be wet and cool, but frost is rare. Life cycles are fitted to this pronouncedly seasonal regime, and some specialized insects also move to protected underground habitats where there are minute, blind, soil-inhabiting ground beetles and rove beetles (Carabidae and Staphylinidae). Plant cover is diverse and sufficiently dense to provide food and protection so that overall conditions for insect life are good; insect diversity is therefore very high.

taiga A small number of conifer species form a belt of northern evergreen forests known as taiga. The taiga ranges from North America through northern Asia to Scandinavia. Summer temperatures during the short vegetation period may be high, but the duration and severity of winters, and also the relative monotony of vegetation, limit the number of insect taxa. During the Pleistocene, the taiga was displaced southward; the more southern montane conifer forests in the

Northern Hemisphere date back to this period. Biodiversity in the taiga is generally low; a few species dominate and may inhabit vast areas.

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