Have southern Texas savannas been converted to woodlands in recent history?

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 varies with precipitation and size. A simulation model was developed in which 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. Estimated age of the oldest Prosopis plant found in clusters was 172-217 yr, but model-derived size-age relationships predicted that 90% of clusters and mesquite plants at the site are <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 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. -from Author