H. Wayne Polley

Nonlinear grassland responses to past and future atmospheric CO2

Carbon sequestration in soil organic matter may moderate increases in atmospheric CO2 concentrations (Ca) as Ca increases to more than 500 µmol mol-1 this century from interglacial levels of less than 200 µmol mol-1 (refs 1–6). However, such carbon storage depends on feedbacks between plant responses to Ca and nutrient availability. Here we present evidence that soil carbon storage and nitrogen cycling in a grassland ecosystem are much more responsive to increases in past Ca than to those forecast for the coming century. Along a continuous gradient of 200 to 550 µmol mol-1 (refs 9, 10), increased Ca promoted higher photosynthetic rates and altered plant tissue chemistry. Soil carbon was lost at subambient Ca, but was unchanged at elevated Ca where losses of old soil carbon offset increases in new carbon. Along the experimental gradient in Ca there was a nonlinear, threefold decrease in nitrogen availability. The differences in sensitivity of carbon storage to historical and future Ca and increased nutrient limitation suggest that the passive sequestration of carbon in soils may have been important historically, but the ability of soils to continue as sinks is limited.

Biodiversity Increases the Resistance of Ecosystem Productivity to Climate Extremes

It remains unclear whether biodiversity buffers ecosystems against climate extremes, which are becoming increasingly frequent worldwide. Early results suggested that the ecosystem productivity of diverse grassland plant communities was more resistant, changing less during drought, and more resilient, recovering more quickly after drought, than that of depauperate communities. However, subsequent experimental tests produced mixed results. Here we use data from 46 experiments that manipulated grassland plant diversity to test whether biodiversity provides resistance during and resilience after climate events. We show that biodiversity increased ecosystem resistance for a broad range of climate events, including wet or dry, moderate or extreme, and brief or prolonged events. Across all studies and climate events, the productivity of low-diversity communities with one or two species changed by approximately 50% during climate events, whereas that of high-diversity communities with 16–32 species was more resistant, changing by only approximately 25%. By a year after each climate event, ecosystem productivity had often fully recovered, or overshot, normal levels of productivity in both high- and low-diversity communities, leading to no detectable dependence of ecosystem resilience on biodiversity. Our results suggest that biodiversity mainly stabilizes ecosystem productivity, and productivity-dependent ecosystem services, by increasing resistance to climate events. Anthropogenic environmental changes that drive biodiversity loss thus seem likely to decrease ecosystem stability, and restoration of biodiversity to increase it, mainly by changing the resistance of ecosystem productivity to climate events.

Biodiversity, productivity and the temporal stability of productivity: patterns and processes

Theory predicts that the temporal stability of productivity, measured as the ratio of the mean to the standard deviation of community biomass, increases with species richness and evenness. We used experimental species mixtures of grassland plants to test this hypothesis and identified the mechanisms involved. Additionally, we tested whether biodiversity, productivity and temporal stability were similarly influenced by particular types of species interactions. We found that productivity was less variable among years in plots planted with more species. Temporal stability did not depend on whether the species were planted equally abundant (high evenness) or not (realistically low evenness). Greater richness increased temporal stability by increasing overyielding, asynchrony of species fluctuations and statistical averaging. Species interactions that favoured unproductive species increased both biodiversity and temporal stability. Species interactions that resulted in niche partitioning or facilitation increased both productivity and temporal stability. Thus, species interactions can promote biodiversity and ecosystem services.

Increasing CO2 and plant-plant interactions: effects on natural vegetation

Plant species and functional groups of species show marked differences in photosynthesis and growth in relation to rising atmospheric CO2 concentrations through the range of the 30 % increase of there cent past and the 100 % increase since the last glaciation. A large shift was found in the compositional mix of 26 species of C3’s and 17 species of C4’s grown from a native soil seed bank in a competitive mode along a CO2 gradient that approximated the CO2 increase of the past 150 years and before. The biomass of C3’s increased from near zero to 50 % of the total while that of the C4’s was reduced 25 % as CO2 levels approached current ambient. The proposition that acclimation to rising CO2 will largely negate the fertilization effect of higher CO2 levels on C3’s is not supported. No signs of photo synthetic acclimation were evident for Avena sativa, Prosopis glandulosa, and Schizachyrium scoparium plants grown in subambient CO2. The effects of changing CO2 levels on vegetation since the last glaciation are thought to have been at least as great, if not greater, than those which should be expected for a doubling of current CO2 levels. Atmospheric CO2 concentrations below 200 ppm are thought to have been instrumental in the rise of the C4 grasslands of North America and other extensive C4 grasslands and savannas of the world. Dramatic invasion of these areas by woody C3 species are accompanying the historical increase in atmospheric CO2 concentration now in progress.

Species Richness and the Temporal Stability of Biomass Production: A New Analysis of Recent Biodiversity Experiments

The relationship between biological diversity and ecological stability has fascinated ecologists for decades. Determining the generality of this relationship, and discovering the mechanisms that underlie it, are vitally important for ecosystem management. Here, we investigate how species richness affects the temporal stability of biomass production by reanalyzing 27 recent biodiversity experiments conducted with primary producers. We find that, in grasslands, increasing species richness stabilizes whole-community biomass but destabilizes the dynamics of constituent populations. Community biomass is stabilized because species richness impacts mean biomass more strongly than its variance. In algal communities, species richness has a minimal effect on community stability because richness affects the mean and variance of biomass nearly equally. Using a new measure of synchrony among species, we find that for both grasslands and algae, temporal correlations in species biomass are lower when species are grown together in polyculture than when grown alone in monoculture. These results suggest that interspecific interactions tend to stabilize community biomass in diverse communities. Contrary to prevailing theory, we found no evidence that species’ responses to environmental variation in monoculture predicted the strength of diversity’s stabilizing effect. Together, these results deepen our understanding of when and why increasing species richness stabilizes community biomass.

Increasing CO2 : comparative responses of the C4 grass Schizachyrium and grassland invader Prosopis

The woody C3 Prosopis glandulosa (honey mesquite) and C4 perennial grass Schizachyrium scoparium (little bluestem) were grown along a gradient of daytime carbon dioxide concentrations from near 340 to 200 Mmol/mol air in a 38 m long controlled environment chamber. We sought to determine effects of historical and prehistorical in- creases in atmospheric CO2 concentration on growth, resource use, and competitive in- teractions of a species representative ofC4-dominated grasslands in the southwestern United States and the invasive legume P. glandulosa. Increasing CO2 concentration stimulated N2 fixation by individually grown P. glandulosa and elicited in C3 seedlings a similar relative increase in leaf intercellular CO2 concentration, net assimilation rate, and intrinsic water use efficiency (leaf net assimilation rate/stomatal conductance). Aboveground biomass of P. glandulosa was not altered by CO2 concentration, but belowground biomass and whole- plant water and nitrogen use efficiencies increased linearly with CO2 concentration in seedlings that were grown alone. Biomass produced by P. glandulosa that was grown with S. scoparium was not affected by CO2 concentration. Stomatal conductance declined and leaf assimilation rates of S. scoparium at near maximum incident light increased at higher CO2 concentration, but there was no effect of CO2 concentration on biomass production or whole-plant water use efficiency of the C4 grass. Rising CO2 concentration, especially the 27% increase since the beginning of the 19th century, may have contributed to more abundant P. glandulosa on C4 grasslands by stimulating the shrubs growth or reducing the amount of resources that the C3 required. Much of the potential response of P. glandulosa to CO2 concentration, however, appears to be contingent on the shrubs escaping compe- tition with neighboring grasses.

Aboveground productivity and root-shoot allocation differ between native and introduced grass species

Plant species in grasslands are often separated into groups (C4 and C3 grasses, and forbs) with presumed links to ecosystem functioning. Each of these in turn can be separated into native and introduced (i.e., exotic) species. Although numerous studies have compared plant traits between the traditional groups of grasses and forbs, fewer have compared native versus introduced species. Introduced grass species, which were often introduced to prevent erosion or to improve grazing opportunities, have become common or even dominant species in grasslands. By virtue of their abundances, introduced species may alter ecosystems if they differ from natives in growth and allocation patterns. Introduced grasses were probably selected nonrandomly from the source population for forage (aboveground) productivity. Based on this expectation, aboveground production is predicted to be greater and root mass fraction to be smaller in introduced than native species. We compared root and shoot distribution and tissue quality between introduced and native C4 grass species in the Blackland Prairie region of Central Texas, USA, and then compared differences to the more well-studied divergence between C4 grasses and forbs. Comparisons were made in experimental monocultures planted with equal-sized transplants on a common soil type and at the same density. Aboveground productivity and C:N ratios were higher, on average, in native grasses than in native forbs, as expected. Native and introduced grasses had comparable amounts of shallow root biomass and tissue C:N ratios. However, aboveground productivity and total N were lower and deep root biomass and root mass fraction were greater in native than introduced grasses. These differences in average biomass distribution and N could be important to ecosystems in cases where native and introduced grasses have been exchanged. Our results indicate that native–introduced status may be important when interpreting species effects on grassland processes like productivity and plant N accumulation.

Climate Change and North American Rangelands: Trends, Projections, and Implications

Abstract The amplified “greenhouse effect” associated with increasing concentrations of greenhouse gases has increased atmospheric temperature by 1°C since industrialization (around 1750), and it is anticipated to cause an additional 2°C increase by mid-century. Increased biospheric warming is also projected to modify the amount and distribution of annual precipitation and increase the occurrence of both drought and heat waves. The ecological consequences of climate change will vary substantially among ecoregions because of regional differences in antecedent environmental conditions; the rate and magnitude of change in the primary climate change drivers, including elevated carbon dioxide (CO2), warming and precipitation modification; and nonadditive effects among climate drivers. Elevated atmospheric CO2 will directly stimulate plant growth and reduce negative effects of drying in a warmer climate by increasing plant water use efficiency; however, the CO2 effect is mediated by environmental conditions, especially soil water availability. Warming and drying are anticipated to reduce soil water availability, net primary productivity, and other ecosystem processes in the southern Great Plains, the Southwest, and northern Mexico, but warmer and generally wetter conditions will likely enhance these processes in the northern Plains and southern Canada. The Northwest will warm considerably, but annual precipitation is projected to change little despite a large decrease in summer precipitation. Reduced winter snowpack and earlier snowmelt will affect hydrology and riparian systems in the Northwest. Specific consequences of climate change will be numerous and varied and include modifications to forage quantity and quality and livestock production systems, soil C content, fire regimes, livestock metabolism, and plant community composition and species distributions, including range contraction and expansion of invasive species. Recent trends and model projections indicate continued directional change and increasing variability in climate that will substantially affect the provision of ecosystem services on North American rangelands.

POTENTIAL NITROGEN CONSTRAINTS ON SOIL CARBON SEQUESTRATION UNDER LOW AND ELEVATED ATMOSPHERIC CO2

The interaction between nitrogen cycling and carbon sequestration is critical in predicting the consequences of anthropogenic increases in atmospheric CO2 (hereafter, Ca). The progressive N limitation (PNL) theory predicts that carbon sequestration in plants and soils with rising Ca may be constrained by the availability of nitrogen in many ecosystems. Here we report on the interaction between C and N dynamics during a four-year field experiment in which an intact C3/C4 grassland was exposed to a gradient in Ca from 200 to 560 micromol/mol. There were strong species effects on decomposition dynamics, with C loss positively correlated and N mineralization negatively correlated with Ca for litter of the C3 forb Solanum dimidiatum, whereas decomposition of litter from the C4 grass Bothriochloa ischaemum was unresponsive to Ca. Both soil microbial biomass and soil respiration rates exhibited a nonlinear response to Ca, reaching a maximum at approximately 440 micromol/mol Ca. We found a general movement of N out of soil organic matter and into aboveground plant biomass with increased Ca. Within soils we found evidence of C loss from recalcitrant soil C fractions with narrow C:N ratios to more labile soil fractions with broader C:N ratios, potentially due to decreases in N availability. The observed reallocation of N from soil to plants over the last three years of the experiment supports the PNL theory that reductions in N availability with rising Ca could initially be overcome by a transfer of N from low C:N ratio fractions to those with higher C:N ratios. Although the transfer of N allowed plant production to increase with increasing Ca, there was no net soil C sequestration at elevated Ca, presumably because relatively stable C is being decomposed to meet microbial and plant N requirements. Ultimately, if the C gained by increased plant production is rapidly lost through decomposition, the shift in N from older soil organic matter to rapidly decomposing plant tissue may limit net C sequestration with increased plant production.

Realistically Low Species Evenness Does Not Alter Grassland Species-Richness-Productivity Relationships

Biodiversity is declining worldwide from reductions in both species richness and evenness. Field experiments have shown that primary productivity is often reduced when richness of plant species is lowered. However, experiments testing richness effects have used evenness levels that are much higher than normally encountered in plant com- munities and have been based on the assumption that species extinctions are random. We experimentally varied, for the first time, both species richness (1-8 perennial species/m 2 ) and species evenness (near maximal vs. realistically low) in grassland plots. Net primary productivity (NPP) and ecosystem CO2 uptake declined when richness was reduced, and reductions were similar between evenness treatments. Richness effects were associated more with a selection effect than with complementarity (found only with high evenness). Im- portantly, extinctions in plots during the second year were not random, but were greater at low than at high evenness (i.e., with increased rarity) and in species with low aboveground growth rates. Thus, species evenness can affect grassland ecosystem processes indirectly by affecting species richness, and it will be imperative to understand how nonrandom extinctions affect NPP in future studies. Our results indicate that richness studies may not be biased by using mixtures with artificially high evenness levels, but the results also demonstrate that results from these studies are directly applicable only to communities in which plant extinctions are random.