Biogeographical and Historical Contexts

The La Copita Research Area (27° 40'N; 98° 12'W; elevation = 75-90 m ASL) is situated in the northeastern portion of the North American Tamaulipan Biotic Province (Blair, 1950) in the

Rio Grande Plains of southern Texas. The potential natural vegetation of this region has been classified as Prosopis- Acacia -Andropogon-Setaria savanna (Kiichler, 1964). However, the contemporary vegetation is subtropical thorn woodland (McLendon, 1991) and occupies about 12 million ha in Texas alone (Jones, 1975). The shrubs and small trees at the study site are characteristic of dry tropical and subtropical zones in Mexico, Central America, South America (Chaco, Caatinga, Caldenal), Africa, Australia, India, and southeast Asia. In many instances, it is believed that these vegetation types have replaced grasslands over large areas since the 1800s (Table 1). Current vegetation at the La Copita site, which has been grazed by domestic livestock since the late 1800s, consists of savanna parklands in sandy loam uplands that grade into closed-canopy woodlands in clay loam lowland drainages. All wooded landscape elements (upland shrub clusters and groves; lowland playa and drainage woodlands) are typically dominated by the leguminous tree Prosopis glandulosa in the overstory, with an understory mixture of evergreen, winter-deciduous, and summer-deciduous shrubs. Climate of the region is subtropical (mean annual temperature 22.4 °C) with warm, moist winters and hot, dry summers. Mean annual rainfall is 720 mm and highly variable (CV = 35%).

Reports from settlers indicate that much of southern Texas was grassland or open savanna in the mid-1800s (Inglis, 1964). Historical aerial photography demonstrates that woody plant cover on La Copita increased from 10% in 1941 to 40% in 1983 (Archer et al, 1988). S13C and radiocarbon analyses of soil organic carbon have confirmed that C, trees and shrubs have displaced C4 grasses in upland and lowland portions of the landscape in the past 100 years (Boutton et al, 1998). Plant growth (Archer, 1989) and transition probability models (Scanlan and Archer, 1991), substantiated by tree ring analysis (Boutton et al, 1998), indicate that most trees on the site have established over the past 100 years. The suc-cessional processes involved in woody plant community development and topoedaphic controls over spatial patterns of tree /shrub expansion have been elucidated (Archer, 1995b). Armed with information from these prior studies, we are now poised to ascertain the biogeochemical consequences of succession from grassland to woodland.

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