Secondary production of the central rangeland region of the United States.

Rangelands are the dominant land use across a broad swath of central North America where they span a wide gradient, from <350 to >900 mm, in mean annual precipitation. Substantial efforts have examined temporal and spatial variation in aboveground net primary production (ANPP) to precipitation (PPT) across this gradient. In contrast, net secondary productivity (NSP, e.g., primary consumer production) has not been evaluated analogously. However, livestock production, which is a form of NSP or primary consumer production supported by primary production, is the dominant non-cultivated land use and an integral economic driver in these regions. Here, we used long-term (mean length = 19 years) ANPP and NSP data from six research sites across the Central Great Plains with a history of a conservative stocking to determine resource (i.e., PPT)-productivity relationships, NSP sensitivities to dry-year precipitation, and regional trophic efficiencies (e.g., NSP:ANPP ratio). PPT-ANPP relationships were linear for both temporal (site-based) and spatial (among site) gradients. The spatial PPT-NSP model revealed that PPT mediated a saturating relationship for NSP as sites became more mesic, a finding that contrasts with many plant-based PPT-ANPP relationships. A saturating response to high growing-season precipitation suggests biogeochemical rather than vegetation growth constraints may govern NSP (i.e., large herbivore production). Differential sensitivity in NSP to dry years demonstrated that the primary consumer production response heightened as sites became more xeric. Although sensitivity generally decreased with increasing precipitation as predicted from known PPT-ANPP relationships, evidence suggests that the dominant species' identity and traits influenced secondary production efficiency. Non-native northern mixed-grass prairie was outperformed by native Central Great Plains rangeland in sensitivity to dry years and efficiency in converting ANPP to NSP. A more comprehensive understanding of the mechanisms leading to differences in producer and consumer responses will require multisite experiments to assess biotic and abiotic determinants of multi-trophic level efficiency and sensitivity.

Contrasting responses of woody and grassland ecosystems to increased CO2 as water supply varies.

Experiments show that elevated atmospheric CO2 (eCO2) often enhances plant photosynthesis and productivity, yet this effect varies substantially and may be climate sensitive. Understanding if, where and how water supply regulates CO2 enhancement is critical for projecting terrestrial responses to increasing atmospheric CO2 and climate change. Here, using data from 14 long-term ecosystem-scale CO2 experiments, we show that the eCO2 enhancement of annual aboveground net primary productivity is sensitive to annual precipitation and that this sensitivity differs between woody and grassland ecosystems. During wetter years, CO2 enhancement increases in woody ecosystems but declines in grass-dominated systems. Consistent with this difference, woody ecosystems can increase leaf area index in wetter years more effectively under eCO2 than can grassland ecosystems. Overall, and across different precipitation regimes, woody systems had markedly stronger CO2 enhancement (24%) than grasslands (13%). We developed an empirical relationship to quantify aboveground net primary productivity enhancement on the basis of changes in leaf area index, providing a new approach for evaluating eCO2 impacts on the productivity of terrestrial ecosystems.

Elevated atmospheric carbon dioxide increases soil carbon.

The general lack of significant changes in mineral soil C stocks during CO2 -enrichment experiments has cast doubt on predictions that increased soil C can partially offset rising atmospheric CO2 concentrations. Here, we show, through meta-analysis techniques, that these experiments collectively exhibited a 5.6% increase in soil C over 2-9 years, at a median rate of 19 g C m-2 yr-1 . We also measured C accrual in deciduous forest and grassland soils, at rates exceeding 40 g C m-2 yr-1 for 5-8 years, because both systems responded to CO2 enrichment with large increases in root production. Even though native C stocks were relatively large, over half of the accrued C at both sites was incorporated into microaggregates, which protect C and increase its longevity. Our data, in combination with the meta-analysis, demonstrate the potential for mineral soils in diverse temperate ecosystems to store additional C in response to CO2 enrichment.

Biomass Production in a Tallgrass Prairie Ecosystem Exposed to Ambient and Elevated CO"2.

Responses to elevated CO"2 have not been measured for natural grassland ecosystems. Global carbon budgets will likely be affected by changes in biomass production and allocation in the major terrestrial ecosystems. Whether ecosystems sequester or release excess carbon to the atmosphere will partly determine the extent and rate that atmospheric CO"2 concentration rises. Elevated CO"2 also may change plant community species composition and water status. We determined above- and belowground biomass production, plant community species composition, and measured and modeled water status of a tallgrass prairie ecosystem in Kansas exposed to ambient and twice-ambient CO"2 concentrations in open-top chambers during the entire growing season from 1989 through 1991. Dominant species were Andropogon gerardii, A. scoparius, and Sorghastrum nutans (C"4 metabolism) and Poa pratensis (C"3). Aboveground biomass and leaf area were estimated by periodic sampling throughout the growing season in 1989 and 1990. In 1991, peak biomass and leaf area were estimated by an early August harvest. Relative root production among treatments was estimated using root ingrowth bags which remained in place throughout the growing season. Latent heat flux was simulated with and without water stress. Botanical composition was estimated annually. Compared to ambient CO"2 levels, elevated CO"2 increased production of C"4 grass species, but not of C"3 grass species. composition of C"4 grasses did not change, but Poa pratensis (C"3) declined, and C"3 forbs increased in the stand with elevated CO"2 compared to ambient. Open-top chambers appeared to reduce latent heat flux and increase water-use efficiency similar to the elevated CO"2 treatment when water stress was not severe, but under severe water stress, the chamber effect on water-use efficiency was limited. In natural ecosystems with periodic moisture stress, increased water-use efficiency under elevated CO"2 apparently would have a greater impact on productivity irrespective of photosynthetic pathway.