Autogenic succession in a subtropical savanna: conversion of grassland to thorn woodland

Dense thorn woodlands occupy what are thought to have been grasslands and savannas prior to settlement of the Rio Grande Plains of Texas. To assess the physiognomic stability of the two-phase landscapes, cluster size, density and cover were quantified for 1941, 1960, and 1983 from aerial photographs. Results indicate: 1) mesquite Prosopis glandulosa invaded grasslands and served as the nucleus of cluster organization on upland sites; 2) woody plant community development has been highly punctuated by variations in precipitation; 3) clusters >5 m² in area are persistent features of the landscape; and 4) the present two-phase pattern is moving toward a monophasic woodland as new clusters are initiated and existing clusters expand and coalesce. As a result, 5) shrub clusters on uplands represent an intermediate stage in the conversion of grassland to woodland, and 6) closed-canopy woodlands on more mesic sites appear to represent portions of the landscape where this has already occurred. -from Authors