Old-Growth Definition for Southwestern Subtropical Upland Forests

摘要

Mainly evergreen, broad-leaved forests in the Southwestern United States are211u001erestricted to the Lower Rio Grande Valley of Texas. The soils and long growing 211u001eseason make this region valuable cropland, and thus almost all of the area once 211u001eoccupied by this forest type has been converted to row crops. Remaining old-211u001egrowth forests are usually dominated by some combination of the broad-leaved 211u001eevergreen Texas ebony and a host of other species. Few quantitative studies have 211u001edescribed the composition of this forest type, and, likewise, little is known of 211u001ethe dynamics. Droughts, flooding regime, and fire were large-scale disturbance 211u001efactors. Now, the Rio Grande is used extensively for irrigation, and flooding is 211u001econtrolled. Therefore, the presettlement water regime has been greatly altered, 211u001eand vegetation of the remaining forest fragments is also adjusting to the new 211u001emoisture regime. Some areas are wetter and some drier than in historical times. 211u001eMany of the fragments that remain have already been incorporated into public 211u001eownership by the U.S. Fish and Wildlife Service or the Texas Parks and Wildlife 211u001eDepartment