Mitchel P. McClaran

Ecosystem resilience despite large-scale altered hydroclimatic conditions

Climate change is predicted to increase both drought frequency and duration, and when coupled with substantial warming, will establish a new hydroclimatological model for many regions. Large-scale, warm droughts have recently occurred in North America, Africa, Europe, Amazonia and Australia, resulting in major effects on terrestrial ecosystems, carbon balance and food security. Here we compare the functional response of above-ground net primary production to contrasting hydroclimatic periods in the late twentieth century (1975–1998), and drier, warmer conditions in the early twenty-first century (2000–2009) in the Northern and Southern Hemispheres. We find a common ecosystem water-use efficiency (WUEe: above-ground net primary production/evapotranspiration) across biomes ranging from grassland to forest that indicates an intrinsic system sensitivity to water availability across rainfall regimes, regardless of hydroclimatic conditions. We found higher WUEe in drier years that increased significantly with drought to a maximum WUEe across all biomes; and a minimum native state in wetter years that was common across hydroclimatic periods. This indicates biome-scale resilience to the interannual variability associated with the early twenty-first century drought—that is, the capacity to tolerate low, annual precipitation and to respond to subsequent periods of favourable water balance. These findings provide a conceptual model of ecosystem properties at the decadal scale applicable to the widespread altered hydroclimatic conditions that are predicted for later this century. Understanding the hydroclimatic threshold that will break down ecosystem resilience and alter maximum WUEe may allow us to predict land-surface consequences as large regions become more arid, starting with water-limited, low-productivity grasslands.

Spread of introduced Lehmann lovegrass Eragrostis lehmanniana Nees. in Southern Arizona, USA

Abstract Through seeding and subsequent spread, Lehmann lovegrass Eragrostis lehmanniana Nees. has transformed the structure and function of at least 145 000 ha of semi-desert grassland in southern Arizona since its introduction in 1932. Our study of its spread on the Santa Rita Experimental Range since 1954 showed that by 1989 it was present on >85% of 75 relatively widely dispersed non-seeded permanent plots and accounted for >40% of all perennial grass plants on these plots. In addition, it represented >50% of the perennial grass plants on sites 90% of the grass biomass and produce 2–4 times more biomass annually than native grass vegetation. In the future, strong consideration of these plant composition changes, and the likely decrease in wildlife diversity and increase in fire frequencies accompanying Lehmann lovegrass dominance, should be made before seeding the species.

Woody plants in grasslands: post-encroachment stand dynamics.

Woody plant abundance is widely recognized to have increased in savannas and grasslands worldwide. The lack of information on the rates, dynamics, and extent of increases in shrub abundance is a major source of uncertainty in assessing how this vegetation change has influenced biogeochemical cycles. Projecting future consequences of woody cover change on ecosystem function will require knowledge of where shrub cover in present-day stands lies relative to the realizable maximum for a given soil type within a bioclimatic region. We used time-series aerial photography (1936, 1966, and 1996) and field studies to quantify cover and biomass of velvet mesquite (Prosopis velutina Woot.) following its proliferation in a semidesert grassland of Arizona. Mapping of individual shrubs indicated an encroachment phase characterized by high rates of bare patch colonization. Upon entering a stabilization phase, shrub cover increases associated with recruitment and canopy expansion were largely offset by contractions in canopy area of other shrub patches. Instances of shrub disappearance coincided with a period of below-average rainfall (1936-1966). Overall, shrub cover (mean +/- SE) on sandy uplands with few and widely scattered shrubs in 1902 was dynamically stable over the 1936-1996 period averaging approximately 35% +/- 5%. Shrub cover on clayey uplands in 1936 was 17% +/- 2% but subsequently increased twofold to levels comparable to those on sandy uplands by 1966 (36% +/- 7%). Cover on both soils then decreased slightly between 1966 and 1996 to 28% +/- 3%. Thus, soil properties influenced the rate at which landscapes reached a dynamic equilibrium, but not the apparent endpoint. Although sandy and clayey landscapes appear to have stabilized at comparable levels of cover, shrub biomass was 1.4 times greater on clayey soils. Declines in shrub cover between 1966 and 1996 were accompanied by a shift to smaller patch sizes on both sandy and clayey landscapes. Dynamics observed during the stabilization phase suggest that density-dependent regulation may be in play. If woody cover has transitioned from directional increases to a dynamic equilibrium, biomass projections will require monitoring and modeling patch dynamics and stand structure rather than simply changes in total cover.

Isotopic analysis of tooth enamel carbonate from modern North American feral horses: implications for paleoenvironmental reconstructions

The accuracy of paleoenvironmental reconstructions based on isotope analyses of equid teeth is currently uncertain because the exact relationship between the isotope composition of modern feral equids and their environment has not been thoroughly studied. We analyzed the carbon and oxygen isotope values (δ13C and δ18O) of tooth enamel carbonate and the δ13C values of fecal samples from modern feral horses. We compared those values with the δ13C values of local vegetation and the δ18O values of local waters. Herds were studied in two contrasting localities: eastern Oregon, where grasslands consisted of 100% C3 species, and New Mexico, where >95% of the grasses were C4 species. Carbon isotope analyses of fecal material and tooth enamel suggest that horses consumed primarily grass, but some New Mexico horses also consumed significant amounts of shrubs and/or forbs. Microhistological analyses of fecal samples show that Oregon horses consumed 95% grass, and Oregon enamel δ13C values are consistent with a diet containing 100% C3 plants. Microhistological analyses of fecal samples from New Mexico indicate a diet averaging 75% grass, while enamel δ13C values suggest that diets averaged 85% C4 plants (range=72–97%). Thus, reconstructions of the C3/C4 ratio of grasses in ancient grasslands that are based on the δ13C values of fossil equid teeth may underestimate the abundance of C4 grasses. The mean δ18O values of tooth enamel paralleled the trends observed in the mean δ18O values of precipitation. However, the mean δ18O values of enamel carbonate from Oregon and New Mexico differed by only 3.3‰, which is less than the difference in the mean δ18O values of precipitation (6.5‰). In addition, the range of δ18O values within New Mexico enamel samples (6.5‰) was greater than the difference between mean enamel δ18O values at each site. Calculated values for the δ18O of water ingested by horses are 2–3‰ more positive than mean δ18O values for corresponding precipitation, suggesting that horses consumed waters that were enriched in 18O due to evaporation. While our results confirm that local climatic and hydrological conditions can influence the δ18O values of equid enamel, they also show that the δ18O values of equid teeth are not always a direct proxy for the isotope ratios of precipitation.

SPREAD OF INTRODUCED LEHMANN LOVEGRASS ALONG A GRAZING INTENSITY GRADIENT

1. Changes in density of Lehmann lovegrass and native grasses, and the proportion of lovegrass present along a livestock grazing intensity gradient were measured on six occasions in permanent plots during 1972-90, on the Santa Rita Experimental Range, Arizona, USA. The gradient included grazing exclosures and plots radiating away from a cattle watering point. Lehmann lovegrass spread from sowings done during 1945-59 4km from the water point and an average 1 8 km from the exclosures. 2. Lovegrass density increased with time but was not affected by different grazing intensities. Native grass density decreased, and lovegrass relative abundance increased with time and as grazing intensity increased. Lovegrass density and relative abundance did not differ between adjacent ungrazed and grazed areas. 3. Livestock grazing was not necessary for Lehmann lovegrass to spread, but the lovegrass relative abundance was greater at higher grazing intensities.

Landscape patterns of vegetation change indicated by soil carbon isotope composition

Vegetation change, particularly from the grass to shrub life form, is a critical issue on the world’s semiarid rangelands. Stable carbon isotope (y 13 C) values and associated radiocarbon ages from soil organic matter (SOM) were used to evaluate vegetation change across five landscape positions at a small enclosed basin in southeastern Arizona. Light and dense SOM fractions were separated to distinguish recent vegetation changes. The direction and timing of vegetation change differed with landscape position along a gentle elevation gradient from the basin outlet to a nearby volcanic ridge top. C4 perennial grasses have dominated the basin outlet, center, and toe slope landscape positions since at least 5000–6000 years BP, except for the dominance of C3 plants at the bottom of the outlet excavation at 5000 years BP. This isotopic change is associated with rounded cobbles that may have been a stream channel, suggesting the presence of C3 herbaceous or woody riparian vegetation. On midslope and ridge top landscape positions, where semidesert shrubs now dominate, the proportion of plants with C4 metabolism calculated from mass balance mixing formulas decreased from approximately 60% as recently as 400 years BP to only 1.5% observed today. The light SOM fraction from mid-slope and ridge top surface soil horizons was approximately 30% C4 and had a post-bomb date, suggesting that the conversion from grass to shrub occurred over the last several decades. D 2003 Elsevier B.V. All rights reserved.

Desert grassland dynamics estimated from carbon isotopes in grass phytoliths and soil organic matter

. We document the potential for using carbon isotopes in both soil organic matter (SOM) and grass phytoliths in soil to increase the temporal and taxonomic resolutions of long term vegetation dynamics. Carbon isotope values from both SOM and phytoliths are expected to describe both the age of material through 14C dating, and the photosynthetic pathway of the source plant material through ratios of 12C/13C. Taxonomic resolution is increased because the phytoliths examined are specific to grasses, whereas the SOM reflects the contribution of all the vegetation. Temporal resolution is increased because phytoliths are less mobile in the soil profile than SOM, and can therefore provide older dates from the same soil depth. Our results, from a desert grassland site in southwestern North America, largely confirm these expectations, and show that C4 species have dominated the grass composition for the last 8000 yr, C3 non-grass vegetation increased about 100–350 yrBP, and no significant C3 grass or non-grass vegetation existed between 350–2000 yr BP.

Viewpoint: implications of participatory democracy for public land planning.

Non-traditional, collaborative public park approaches such as coordinated resource management have been proposed to improve the public participation process used in public land planning on rangelands. Either implicitly or explicitly, most advocates of such non-traditional approaches to public participation seem to embrace a participatory democracy model of governance. Whether or not this model for decision-making can practicably be implemented, given our current institution and leaal frameworks for public lands management, has not been closely examined. Criticisms of the traditional public participation process are catagorize into 5 main issues: efficacy; representation and access; information exchange and learning; continuity of participation; and decision-making authority. We use these categories to evaluate the feasibility of implementing participatory democracy-based decision-making in public lands planning. Although there is some statutory and regulatory authority for participatory democracy in public land planning, there are a number of logistical, legal, and even philosophical challenges to its application that warrant further consideration.

Functional response of U.S. grasslands to the early 21st-century drought

Grasslands across the United States play a key role in regional livelihood and national food security. Yet, it is still unclear how this important resource will respond to the prolonged warm droughts and more intense rainfall events predicted with climate change. The early 21st-century drought in the southwestern United States resulted in hydroclimatic conditions that are similar to those expected with future climate change. We investigated the impact of the early 21st-century drought on aboveground net primary production (ANPP) of six desert and plains grasslands dominated by C4 (warm season) grasses in terms of significant deviations between observed and expected ANPP. In desert grasslands, drought-induced grass mortality led to shifts in the functional response to annual total precipitation (P(T)), and in some cases, new species assemblages occurred that included invasive species. In contrast, the ANPP in plains grasslands exhibited a strong linear function of the current-year P(T) and the previous-year ANPP, despite prolonged warm drought. We used these results to disentangle the impacts of interannual total precipitation, intra-annual precipitation patterns, and grassland abundance on ANPP, and thus generalize the functional response of C4 grasslands to predicted climate change. This will allow managers to plan for predictable shifts in resources associated with climate change related to fire risk, loss of forage, and ecosystem services.

Empirical assessment of state‐and‐transition models with a long‐term vegetation record from the Sonoran Desert

Resilience-based frameworks, including state-and-transition models (STM), are being increasingly called upon to inform policy and guide ecosystem management, particularly in rangelands. Yet, multiple challenges impede their effective implementation: (1) paucity of empirical tests of resilience concepts, such as alternative states and thresholds, and (2) heavy reliance on expert models, which are seldom tested against empirical data. We developed an analytical protocol to identify unique plant communities and their transitions, and applied it to a long-term vegetation record from the Sonoran Desert (1953-2009). We assessed whether empirical trends were consistent with resilience concepts, and evaluated how they may inform the construction and interpretation of expert STMs. Seven statistically distinct plant communities were identified based on the cover of 22 plant species in 68 permanent transects. We recorded 253 instances of community transitions, associated with changes in species composition between successive samplings. Expectedly, transitions were more frequent among proximate communities with similar species pools than among distant communities. But unexpectedly, communities and transitions were not strongly constrained by soil type and topography. Only 18 transitions featured disproportionately large compositional turnover (species dissimilarity ranged between 0.54 and 0.68), and these were closely associated with communities that were dominated by the common shrub (burroweed, Haplopappus tenuisecta); indicating that only some, and not all, communities may be prone to large compositional change. Temporal dynamics in individual transects illustrated four general trajectories: stability, nondirectional drift, reversibility, and directional shifts that were not reversed even after 2-3 decades. The frequency of transitions and the accompanying species dissimilarity were both positively correlated with fluctuation in precipitation, indicating that climatic drivers require more attention in STMs. Many features of the expert models, including the number of communities and participant species, were consistent with empirical trends, but expert models underrepresented recent increases in cacti while overemphasizing the introduced Lehmanns lovegrass (Eragrostis lehmanniana). Quantification of communities and transitions within long-term vegetation records presents several quantitative metrics such as transition frequency, magnitude of accompanying compositional change, presence of unidirectional trajectories, and lack of reversibility within various timescales, which can clarify resilience concepts and inform the construction and interpretation of STMs.