Have Southern Texas Savannas Been Converted to Woodlands in Recent History?

Abstract

At savanna woodland sites in southern Texas, discrete clusters of woody plants form in herbaceous clearings following the invasion of mesquite (Prosopis glandulosa var. glandulosa), an arborescent legume. The growth rate of these clusters has been shown to vary with precipitation and size. Based on field data and a knowledge of mechanisms of woody-plant successional processes, a simulation model was developed to estimate the rates of growth and development of these woody-plant assemblages on sandy-loam uplands under different precipitation regimes. In the simulation, the establishment of other woody species beneath invading Prosopis occurred within 10-15 yr. As a cluster developed around the Prosopis nucleus, species richness increased rapidly for 35-45 yr and became asymptotic at 10 species per cluster. The estimated age of the oldest Prosopis plant found in clusters was 172-217 yr. However, model-derived size-age relationships predicted that most (90%) clusters and mesquite plants at the site are less than 100 yr old. A lack of field evidence of mortality among large clusters and Prosopis plants suggests that populations are young and expanding geometrically. There was no evidence of density-dependent restrictions on recruitment or expansion. Thus, as new clusters are initiated and existing clusters expand, coalescence to continuous canopy woodlands may eventually occur. Predicted long-term mean radial trunk growth of Prosopis (0.8-1.9 mm/yr) was reasonable in comparison with short-term field measurements on Prosopis in other, more-mesic systems (2-4 mm/yr). Model output was also consistent with historical observations suggesting that the conversion of savannas to woodlands in the Rio Grande Plains has been recent and coincident with both heavy grazing by livestock and seasonal shifts in precipitation that began in the late 1800s. This is in agreement with woody-plant invasions documented in other North American arid and semiarid systems by the direct aging of woody plants.