Herbaceous Production and Soil Nitrogen after Mesquite Mortality in Southern Great Plains (US) Grassland

Abstract

In the mixed C3/C4 grassland of the southern Great Plains, United States, the invasive woody legume, honey mesquite (Prosopis glandulosa), affects grass production and composition differently beneath the canopy (subcanopy) than in spaces between trees (intercanopy) due in part to the dominant presence of C3 Texas wintergrass (Nassella leucotricha) beneath the mesquite canopy and soil enrichment from N-fixation by mesquite. This arrangement, unlike most Prosopis systems worldwide that have C4 grass or C3 subshrub understories, uniquely affects grass production spatially and seasonally during mesquite expansion and possibly after anthropogenic removal of mesquite. We compared herbaceous and soil N responses in subcanopy and intercanopy microsites during the first 2 yr following a root-killing herbicide mesquite treatment. Perennial grass (PGR) and total herbaceous (THB) production were greater in treated than untreated intercanopy and subcanopy microsites at 1-yr post treatment, with Texas wintergrass comprising the largest portion of PGR. In yr 2, PGR production declined in both treated microsites with no differences between treatments. However, THB production remained greater in treated than untreated microsites due mainly to increased annual forb production that supplanted PGR production from yr 1. Increased annual forb production in treated microsites in yr 2 was likely due to high rainfall in the fall of yr 1 that stimulated forb seed germination, increased light from the loss of shading by mesquite, and soil inorganic N that increased from yr 1 to yr 2. Pretreatment spatial heterogeneity of herbaceous composition and soil N, caused by mesquite, affected post-treatment patterns of herbaceous production. The unexpected replacement of PGR by annual forbs in yr 2 revealed that grass forage production following brush control can deviate markedly from predicted models under certain conditions.