Document: FLI-3-59-3

Ecosystem-level impacts of mesquite (Prosopis glandulosa) encroachment on C and N pools of herbaceous vegetation and soils in a temperate savanna.

HUGHES, R.F.* ¹, C.R.MCMURTRY ², G.P.ASNER ¹, S.R.ARCHER ² and C.A.WESSMAN ¹

University of Colorado, Boulder, Colorado 80309 ¹
Texas A & M University, College Station, Texas 77843 ²

Abstract:
Mesquite, a nitrogen-fixing tree species, has expanded across substantial portions of North Texas rangelands during the past 100 years. This long-term expansion is representative of woody encroachment occurring across arid and semiarid systems worldwide, and is a phenomenon which has the capacity to subsantially alter C and N storage. Our research quantified the dynamics of herbaceous and soil C and N pools in response to varying levels of mesquite biomass in North Texas (i.e., among a series of study sites where aboveground mesquite biomass ranged from 3,000 kg/ha to 42,000 kg/ha). While aboveground C and N pools in mesquite biomass increased by 14 times along this gradient, C and N pools in aboveground herbaceous biomass (i.e., standing live, standing dead, and litter) doubled along the gradient (from 1560 to 3358 kg C/ha and from 49 to 101 kg N/ha), and C and N pools in surface soils (0-10 cm depth) increased by only 33% and 45%, respectively. Annual accumulation rates of C and N in herbaceous biomass (i.e., standing live and standing dead at peak biomass) were 6 times greater than accumulations in mesquite biomass at the low end of the range in mesquite biomass, but were only 9 and 12% of accumulations in mesquite at the high end of the range. Combined pools of C and N (i.e., aboveground mesquite, aboveground herbaceous, and surface soils) ranged from 1618 to 2496 kg N/ha and from 18926 to 42254 kg C/ha across the series of study sites. Surface soils dominated N pools across the sites, accounting for between 77 and 94% of the combined N pools. In contrast, soils accounted for between 48 and 83% of combined C pools. Mesquite accounted for 8 to 47% of combined C pools, and herbaceous biomass accounted for between 4 and 15% of those pools. At the individual tree scale, mesquite influenced C and N pools in both surface soils and herbaceous biomass. Surface soil C and N pools were 14 and 12% greater, respectively, at points under mesquite canopies compared to those away from mesquite canopies. Similarly, C and N concentrations were significantly higher in herbaceous tissue sampled under versus away from mesquite canopies, and C and N mass in herbaceous biomass was a much as 18% higher under versus away from mesquite canopies.

Keywords: Texas, Mesquite, woody encroachment, carbon, nitrogen, soils, herbaceous biomass

This abstract is being presented at: 8:45 AM in session:
Oral Session #25: Organic Matter Dynamics in Ecosystems.