220.127.116.11. Midlatitudes of Western North America
The predominance of conifers in western North American forests arose in the middle and late Tertiary period as a result of long-term global cooling, tecton-ism, and changes in atmospheric composition and weathering rates (Barron, 1985; Raymo et al., 1988; Ruddiman and Kutzbach, 1989, 1991). In western North America, summer-dry conditions were intensified with regional tectonism and rain shadow develop ment in the late Tertiary period (Barnosky, 1987). Precipitation gradients today are latitudinal, longitudinal (reflecting north-south trending mountain ranges), and altitudinal. Winter precipitation from storms that develop in the vicinity of the Aleutian Low is greatest in the Coast Range, Cascade Range, and Sierra Nevada, and levels decrease eastward and southward. The East Pacific subtropical high-pressure system determines the intensity of summer drought along a latitudinal gradient that also shifts seasonally.
Rain forest extends from northern California to British Columbia and as far north as Kodiak Island in Alaska (Alaback and Pojar, 1997). At low latitudes, warm temperate rain forest occurs in a region of mild, wet winters and cool, dry summers. From southern Oregon to central Vancouver Island, the climate is cool to mild, and snow is uncommon at low elevations and abundant at high elevations. Most rainfall occurs in fall and winter, and less than 10% falls in summer. The seasonal rain forest of this region supports Pseudot-suga, Tsuga heterophylla, Abies grandis, and Thuja plica-ta, and deciduous broad-leaved species are confined to riparian settings and areas of disturbance (Alaback and Pojar, 1997). The driest areas of rain forest, including southeast Vancouver Island, the San Juan Islands, southern Puget Lowland, and Willamette Valley, support oak (Quercus garryana) and isolated grassland. At high elevations in the south, coastal ranges support Tsuga mertensiana, Chamaecyparis nootkatensis, and Abies spp. The perhumid temperate forest from central Vancouver Island to southeastern Alaska is a diverse landscape of wetlands, forest, and subalpine meadows as a result of wet, cool conditions throughout the year. Picea sitchensis, T. heterophylla, A. amabilis, Thuja plicata, and C. nootkatensis are the dominant trees, with a rich understory of ferns and ericaceous taxa. At high elevations in the north, Tsuga heterophyl-la and other cold-intolerant species are replaced by T. mertensiana and C. nootkatensis, and finally by tundra and heath. Fires are frequent in the southern summer-dry ecosystems, but are rare in the more humid zones in the north.
East of the Cascade Range and Coast Range, forests contain more xerophytic elements as a result of greater continentality and summer drought. The interior Coast Range of British Columbia is covered by forests of Pi-nus ponderosa, P. contorta, Pseudotsuga, A. grandis, and Larix occidentalis. Picea engelmannii, Pinus albicaulis, and A. lasiocarpa are present at high elevations. The northern Rocky Mountains support forests of Pinus ponderosa, P. contorta, and P. flexilis at low and middle elevations and forests with P. albicaulis, A. lasiocarpa, and Picea engelmannii at high elevations. Where maritime air penetrates inland, coastal species extend eastward to the Continental Divide in northwestern Montana and southwestern British Columbia and occur together with interior, xerophytic species. In the Sierra Nevada of California, Pinus contorta, P. jeffreyi, Pseudotsuga, A. magnifica, Pinus longaeva, and P. flexilis are the dominant conifers. The lowlands support grassland and oak savanna in mesic settings, and farther east sagebrush steppe is dominated by Artemisia tridentata, other composites, and chenopods. Alpine tundra is present on high peaks throughout the West.
South America's temperate forests are dominated by evergreen, small-leaved hardwoods, while conifers play only a minor role (Kalin Arroyo et al., 1993). Deciduous forests, dominated by different species of Nothofa-gus, are restricted to high altitudes and high latitudes. This southern temperate flora has evolved from Gondwanan stock, which developed when Southern Hemisphere land areas were connected (Markgraf et al., 1995, 1996). During the middle Tertiary period, neotropical elements were added to the flora, and Andean tectonism in the late Tertiary period resulted in migration of new taxa from the Northern Hemisphere primarily to alpine and subalpine elevations (Moore, 1983; Simpson, 1983). While North American herbaceous taxa migrated to the southernmost tip of South America, North American tree taxa, such as Alnus and Juglans, reached only latitude 27°S. Thus, the southern temperate arboreal flora never received an influx of the northern temperate flora. The high species diversity and endemism that characterize the South American forests reflect their long history of isolation and the relatively small changes in climate during glacial-interglacial oscillations in mountainous regions (Markgraf et al., 1995; Villagrán, 1988; Villagrán and Armesto, 1993; Kalin Arroyo et al., 1993).
Present-day vegetation is distributed along two precipitation gradients associated with latitude and orography (Lawford, 1996). The latitudinal gradient is related to the seasonal shift of the southern westerly storm tracks. Precipitation from latitude 30° to 40°S falls primarily in winter and ranges from 350 mm/ year in Santiago to 2300 mm/year in Puerto Montt. South of latitude 40°S, precipitation reaches a maximum of over 5000 mm/year between latitudes 45° and 50°S, where the southern westerlies dominate year-round. The second, west-east precipitation gradient across the Andes is extremely steep, from 2000 mm/year on the Andean divide in, e.g., Futaleufu, to 800 mm/ year in Bariloche— a distance of less than 50 km.
Vegetation types west of the Andes from north to south are (Donoso 1993, 1996): (1) sclerophyllous forests and woodlands between latitudes 25° and 30°S with Lithraea caustica, Quillaja saponaria, Cryptocarpa alba, and Peumus boldus; (2) semideciduous forests between latitude 30° and 40°S with Nothofagus obliqua, Aextoxicon punctatum, Laurelia spp., and Persea lingue; (3) species-rich, warm-temperate Valdivian rain forest between latitudes 40° and 45°S with N. dombeyi, L. sempervirens, L. philippiana, Eucryphia cordifolia, Caldcluvia paniculata, Lomatia ferruginea, and Weinmannia tricho-sperma; and (4) cool-temperate Patagonian and Magellanic rain forests south of latitude 45°S with N. betu-loides, several genera and species of Myrtaceae, Fitzroya cupressoides, Podocarpus nubigena, and Drimys winteri. Nonforested vegetation types include the Magellanic moorland in regions of poor drainage, high precipitation, and high winds along the southwest coast of Chile, with Astelia pumila, Donatia fascicularis, and other bog taxa restricted to the Southern Hemisphere; Andean high-elevation, cushion-forming vegetation with Bolax gummifera, Abrotanella emarginata, Drapetes musco-sus, and Azorella spp.; and steppe and steppe-scrub east of the Andes, with bunchgrasses and many primarily shrubby species of Asteraceae.
The temperate coastal rain forests bordering the Pacific Ocean in both North and South America produce deep litter and stable habitats rich in microorganisms and invertebrates. Hatch (1971), in the series The Beetles of the Pacific Northwest, laid the foundation for faunistic studies in the coastal regions of western North America. Parsons et al. (1991) and Kavanaugh (1992) are among the recent faunistic studies that provide detailed ecological information from specific geographic regions in Oregon and British Columbia. The insect fauna of the coastal forests is diverse and rich in endemic taxa at the subspecies, species, generic, and even tribal and subfamilial levels. Kavanaugh (1988) proposed that species survived the last glaciation in a series of disjunct refugia, the most significant of which lay along the coast of southern Washington, Oregon, and northern California. Separate refugia also may have existed in the Aleutian Islands, Kodiak Island, coastal southeastern Alaska, and the Queen Charlotte Islands, as well as a much larger Beringian refugium in interior Alaska, Yukon Territory, and northern British Columbia.
The taxonomy and ecology of the southern South America beetle fauna are less well known. There are monographic treatments for some taxa (e.g., Jeannel 1962 for the trechine carabids) and biogeographic analyses (e.g., Kuschel 1969 for the Coleoptera of southern South America), but few faunistic and ecological studies are available. Ashworth and Hoganson (1987) and Ashworth et al. (1991) provided analyses of the beetle faunas along montane transects in the Chilean Lake and Chilean Channels, and Niemela (1990) reported on the distribution of Carabidae along a vege-tational transect in Tierra del Fuego.
The southern South American fauna is especially distinctive because of its high degree of endemism. Darlington (1965), in his classic the Biogeography of the Southern End of the World, showed that ground beetles displayed endemism to the level of tribe. For example, species in the tribe Migadopini do not occur in the Northern Hemisphere and have a typical Gondwanan distribution in southern South America, including the Falkland Islands, Tasmania and southeastern Australia, and New Zealand and the Auckland Islands. A similar distribution is observed in the most abundant of southern South American weevils (subfamilies Cylindrorhininae and Rhytirhininae [Kuschel, 1969] and the families Protocujidae, Chalcodryidae, Nemo-nychidae, and Belidae). These groups are either mono-typic or are represented by a small number of genera that are inhabitants of Nothofagus forest. High en-demism within the Southern Hemisphere beetles is attributed to their greater isolation in a smaller, more fragmented land area.
Although the southern and northern faunas are distinctive, there are a number of similarities between them. In both faunas, species richness declines polewards, and the faunas west and east of the mountains are very different. The dominant scavengers of the western rain forest are carabids, but in the eastern xeric forests and steppes, they are replaced by tenebrionids. The Patagonian tenebrionids are a particularly distinctive group of beetles with high endemism, especially in the tribes Nycteliini, Scotobiini, and Praocini. The Carabidae serve to illustrate some faunal similarities between the wet forests of the different hemispheres. In both forests, the dominant predators are relatively large, flightless ground beetles of the supertribe Cara-bitae; they include the brightly colored Ceroglossini in Chile and the snail-eating Cychrini in the Pacific Northwest. Carabids in the tribes Trechini and Broscini are also well represented in these forests compared with other environments. In the tribe Broscini, the ground beetle Creobius eydouxi Guerin of Chile is superficially similar to Zacotus matthewsi LeConte of the Pacific Northwest; both are moderately sized flightless beetles that have a metallic luster and fused elytra.
Most of the morphologic similarities are at a superficial level and can perhaps be best explained by evolutionary convergence. However, there are also probably ancient evolutionary links between some of the taxa. Erwin (1985) discussed the biogeographic rela tionships within the Trachypachidae, a beetle family that is considered ancestral to the Carabidae because it shares characteristics with both the Hydradephaga and the Geadephaga. In forests from British Columbia to California, the family is represented by the genus Tra-chypachus, and in the forests of southern Chile, it is represented by its sister genus Systolosoma. Erwin considered these genera to be ancient relicts that have been disjunct in their existing geographic ranges since the early Tertiary period.
Thus, despite the long distances separating the regions and the differences in the particular controls of regional climate, the western midlatitudes in both hemispheres are remarkably similar in terms of their seasonal variations of climate, the structure of the vegetation, and their general biogeographic characteristics. As we discuss in Section 21.3, this interhemispher-ic similarity is also a feature of the late Quaternary climatic histories of the two regions.
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