Although local increases in woody plant cover have been documented in arid and semiarid ecosystems worldwide, there have been few long‐term, large‐scale analyses of changes in woody plant cover and aboveground carbon (C) stocks. We used historical aerial photography, contemporary Landsat satellite data, field observations, and image analysis techniques to assess spatially specific changes in woody vegetation cover and aboveground C stocks between 1937 and 1999 in a 400‐km2 region of northern Texas, USA. Changes in land cover were then related to topo‐edaphic setting and historical land‐use practices. Mechanical or chemical brush management occurred over much of the region in the 1940–1950s. Rangelands not targeted for brush management experienced woody cover increases of up to 500% in 63 years. Areas managed with herbicides, mechanical treatments or fire exhibited a wide range of woody cover changes relative to 1937 (−75% to + 280%), depending on soil type and time since last management action. At the integrated regional scale, there was a net 30% increase in woody plant cover over the 63‐year period. Regional increases were greatest in riparian corridors (33%) and shallow clay uplands (26%) and least on upland clay loams (15%). Allometric relationships between canopy cover and aboveground biomass were used to estimate net aboveground C storage changes in upland (nonriparian) portions of regional landscapes. Carbon stocks increased from 380 g C m−2 in 1937 to 500 g C m−2 in 1999, a 32% net increase across the 400 km2 region over the 63‐year period. These plant C storage change estimates are highly conservative in that they did not include the substantial increases in woody plant cover observed within riparian landscape elements. Results are discussed in terms of implications for ‘carbon accounting’ and the global C cycle.

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 1 herbaceous) of sites with the highest Prosopis basal area were 1.2 Â and 4.0 Â 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

No abstract

Woody plant encroachment has degraded grassland and savanna ecosystems worldwide by decreasing herbaceous production and diversity, and altering these physiognomies toward woodlands. This study evaluated the long-term efficacy of fire and herbicide restoration strategies used in the southern Great Plains to reduce Honey mesquite (Prosopis glandulosa) dominance, restore a grassland/savanna physiognomy, and increase herbaceous production and diversity. Three treatments were evaluated: high-intensity winter fire, aerial spray of clopyralid 1 triclopyr (C 1 T), and aerial spray of clopyralid and were compared to untreated mesquite woodland (control). Post-treatment mesquite stand physiognomy was different between fire (low mortality, high basal sprouting), C 1 T (high mortality, high basal sprouting of surviving plants), and clopyralid (moderate mortality, low basal sprouting of surviving plants) treatments. From 6 to 8 years post-treatment, herbaceous production was increased in C 1 T and clopyralid treatments but not in the fire treatment. Mesquite regrowth in the fire treatment exerted a competitive influence that limited herbaceous production. Herbaceous functional group diversity was increased in fire and C 1 T treatments due to a decrease in C 3 perennial grass dominance and an increase in C 4 perennial grasses and/or C 3 forbs. Treatments that maintained mesquite overstory (control and clopyralid) had lower herbaceous diversity due to C 3 perennial grass dominance and lower C 4 perennial grass cover. The clopyralid treatment demonstrated greatest potential for long-term restoration of southern Great Plains savanna by reducing mesquite canopy cover to historic levels, limiting mesquite basal regrowth and increasing grass production.

Woody vegetation has encroached into areas once dominated by herbaceous land cover in arid and semiarid regions of the southwestern United States and around the world, resulting in documented changes to the biophysical and biogeochemical structure of these ecosystems during the past century. In North Texas rangelands, encroaching mesquite (Prosopis glandulosa var. glandulosa), a known nitrogen (N)-fixing species, has caused changes in aboveground biomass, which, in turn, have influenced carbon (C) and N storage in surface soils. However, the effect on N oxide (nitric-NO and nitrous-N 2 O oxide) emissions from the soils was unknown. We examined biotic (vegetation type and soil organic and inorganic N dynamics) and abiotic (soil moisture, temperature, and soil texture) controls over soil NO and N 2 O emissions across a gradient of aboveground Prosopis biomass growing on two soil types. Soil N oxide fluxes were dominated by NO emissions produced during nitrification. Aboveground biomass was the best spatial predictor of NO emissions, with values increasing 20-fold (0.04-0.78 mg NO-N·m Ϫ2 ·d Ϫ1 ) across a 70-fold biomass gradient (5-350 g/m 2 ). Emissions also covaried with soil pH and clay content. Microsite position, under or between the mesquite canopies, did not influence NO emission rates. NO fluxes were four times higher from clay loam than from shallow clay soils; however, soil N properties (total organic N and extractable inorganic N) and cycling rates (mineralization and nitrification) did not differ significantly across the sites. Temporally, NO emissions and nitrification potential were positively correlated with temperature, with precipitation events elevating NO emissions fourfold over a 24-h period and producing small amounts of N 2 O. We conclude that mesquite encroachment in these grasslands increases NO emissions in a spatially explicit manner influenced by the aboveground biomass and soil type, which is then temporally mediated primarily by temperature and secondarily by precipitation.