Herbaceous diversity and composition in mixed Juniperus Pinchotii-Prosopis Glandulosa communities in Southern mixed-grass prairie

Abstract

ABSTRACTHerbaceous biomass and composition were measured across a continuum of increasing cover of mixed Prosopis glandulosa and Juniperus pinchotii woody plant from 1996 to 2003. The study site in northwest Texas had no history of woody plants treatment and comprised trees up to 6 m tall with cover averaging 65%. A strong, negative, linear relationship between woody plant cover and herbaceous biomass was measured (p = 0.0001). Precipitation differences resulted in 2.7 fold variation in herbaceous biomass between years (p = 0.003) and affected the slope of the decline of herbaceous biomass associated with increasing woody plant cover (p = 0.034). Within any year the slope of decline of herb biomass was parallel for Prosopis and Juniperus cover (p = 0.843). Relative to zero woody cover, 100% Prosopis cover reduced herbaceous biomass by 56% (p = 0.0001) while 100% Juniperus cover reduced herbaceous biomass 73% (p = 0.0001). At 100% cover Prosopis had twice the herbaceous biomass compared to Juniperus (p = 0.0021) but both species reduced herbaceous biomass by ± 1300 kg ha−1. Warm season herb biomass decreased with increasing Prosopis or Juniperus cover while cool season herbaceous biomass was minimally influenced by increases of either species. Multivariate analysis indicated vegetation associations differed between tree species. Vegetation differed (p = 0.001) between low (<25%) and high (>75%) Prosopis cover (dissimilarity R = 0.98), between low and high Juniperus cover (R = 0.81). Ten herbaceous species consistently decreased and 2 consistently increased with increasing woody plant cover.