Changes in aboveground primary production and carbon and nitrogen pools accompanying woody plant encroachment in a temperate savanna

When woody plant abundance increases in grasslands and savannas, a phenomenon widely observed worldwide, there is considerable uncertainty as to whether aboveground net primary productivity (ANPP) and ecosystem carbon (C) and nitrogen (N) pools increase, decrease, or remain the same. We estimated ANPP and C and N pools in aboveground vegetation and surface soils on shallow clay and clay loam soils undergoing encroachment by Prosopis glandulosa in the Southern Great Plains of the United States. Aboveground Prosopis C and N mass increased linearly, and ANPP increased logarithmically, with stand age on clay loam soils; on shallow clays, Prosopis C and N mass and ANPP all increased linearly with stand age. We found no evidence of an asymptote in trajectories of C and N accumulation or ANPP on either soil type even following 68 years of stand development. Production and accumulation rates were lower on shallow clay sites relative to clay loam sites, suggesting strong edaphic control of C and N accumulation associated with woody plant encroachment. Response of herbaceous C mass to Prosopis stand development also differed between soil types. Herbaceous C declined with increasing aboveground Prosopis C on clay loams, but increased with increasing Prosopis C on shallow clays. Total ANPP (Prosopis+herbaceous) of sites with the highest Prosopis basal area were 1.2x and 4.0x greater than those with the lowest Prosopis basal area on clay loam and shallow clay soils, respectively. Prosopis ANPP more than offset declines in herbaceous ANPP on clay loams and added to increased herbaceous ANPP on shallow clays. Although aboveground C and N pools increased substantially with Prosopis stand development, we found no corresponding change in surface soil C and N pools (0-10 cm). Overall, our findings indicate that Prosopis stand development significantly increases ecosystem C and N storage/cycling, and the magnitude of these impacts varied with stand age, soil type and functional plant traits.