Stanley G. Kitchen

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.

Ecological effects of large fires on US landscapes: benefit or catastrophe?

The perception is that todays large fires are an ecological catastrophe because they burn vast areas with high intensities and severities. However, little is known of the ecological impacts of large fires on both historical and contemporary landscapes. The present paper presents a review of the current knowledge of the effects of large fires in the United States by important ecosystems written by regional experts. The ecosystems are (1) ponderosa pine-Douglas-fir, (2) sagebrush-grasslands, (3) pinon-juniper, (4) chaparral, (5) mixed-conifer, and (6) spruce-fir. This review found that large fires were common on most historical western US landscapes and they will continue to be common today with exceptions. Sagebrush ecosystems are currently experiencing larger, more severe, and more frequent large fires compared to historical conditions due to exotic cheatgrass invasions. Historical large fires in south-west ponderosa pine forest created a mixed severity mosaic dominated by non-lethal surface fires while todays large fires are mostly high severity crown fires. While large fires play an important role in landscape ecology for most regions, their importance is much less in the dry pinon-juniper forests and sagebrush-grasslands. Fire management must address the role of large fires in maintaining the health of many US fire-dominated ecosystems.

Differences in native soil ecology associated with invasion of the exotic annual chenopod, Halogeton glomeratus

Various biotic and abiotic components of soil ecology differed significantly across an area where Halogeton glomeratus is invading a native winterfat, [ Krascheninnikovia (= Ceratoides) lanata] community. Nutrient levels were significantly different among the native, ecotone, and exotic-derived soils. NO3, P, K, and Na all increased as the cover of halogeton increased. Only Ca was highest in the winterfat area. A principal components analysis, conducted separately for water-soluble and exchangeable cations, revealed clear separation between halogeton- and winterfat-derived soils. The diversity of soil bacteria was highest in the exotic, intermediate in the ecotone, and lowest in the native community. Although further studies are necessary, our results offer evidence that invasion by halogeton alters soil chemistry and soil ecology, possibly creating conditions that favor halogeton over native plants.

Climate effects on historical fires (1630-1900) in Utah

We inferred climate effects on fire occurrence from 1630 to 1900 for a new set of crossdated fire-scar chronologies from 18 forested sites in Utah and one site in eastern Nevada. Years with regionally synchronous fires (31 years with fire at ≥20% of sites) occurred during drier than average summers and years with no fires at any site (100 years) were wetter than average. Antecedent wet summers were associated with regional-fire years in mixed-conifer and ponderosa pine forest types, possibly by affecting fine fuel amount and continuity. NINO3 (an index of the El Nino–Southern Oscillation, ENSO) was significantly low during regional-fire years (La Ninas) and significantly high during non-fire years (El Ninos). NINO3 also was high during years before regional-fire years. Although regional fire years occurred nearly twice as often as expected when NINO3 and the Pacific Decadal Oscillation were both in their cool (negative) phases, this pattern was not statistically significant. Palmer Drought Severity Index was important for fire occurrence in ponderosa pine and mixed-conifer forests across the study area but ENSO forcing was seen only in south-eastern sites. Results support findings from previous fire and climate studies, including a possible geographic pivot point in Pacific basin teleconnections at ~40°N.

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.

Influence of climate and environment on post-fire recovery of mountain big sagebrush

In arid and semi-arid landscapes around the world, wildfire plays a key role in maintaining species diversity. Dominant plant associations may depend upon particular fire regime characteristics for their persistence. Mountain shrub communities in high-elevation landscapes of the Intermountain West, USA, are strongly influenced by the post-fire recovery dynamics of the obligate-seeding shrub, mountain big sagebrush (Artemisia tridentata Nutt. ssp. vaseyana (Rydb.) Beetle). This species is a short-distance disperser with a short-lived seedbank, leading to highly variable post-fire recovery times (15-100 years). We investigated the relative importance of site productivity and seasonal climate in explaining the variance in recovery time for 36 fires, comprising a fire chrono-sequence (from 1971 to 2007) for the Great BasinandColoradoPlateau.A.t.vaseyanarecoverywaspositivelyrelatedtoprecipitation inthecoolseasonimmediately following fire, likely because deep soil-water recharge that persists throughout the growing season enhances first-year seedling survival. Percentage sand fraction positively correlated with recovery rate yet negatively correlated with live cover in unburnt stands. Our data support the hypothesis that post-fire recovery rate of A. t. vaseyana depends on the climatically controlled ephemerality of the regeneration niche, as is likely true for many arid-land shrub species. Additional keywords: Artemisia tridentata ssp. vaseyana, Colorado Plateau, fire effects, Great Basin, precipitation variability, succession.

Technical Note: A technique for conducting small-plot burn treatments

An experimental design required burn treatments for 10-m2 circular plots. We constructed a fire enclosure for the plots using sheetmetal, electrical conduit, and other commonly available materials. We field tested the enclosure in sagebrush-grass ecosystems in central Nevada and central Utah, and evaluated peak fire temperatures using small metal tags striped with temperature sensitive paint. We obtained average peak surface temperatures of 310, 307, and 381° C in bare ground, under grass, and under shrub microsites, respectively, for the Nevada sites and 253, 299, and 337° C for the same microsites, respectively, in Utah. Subsurface (2-cm depth) temperatures rarely exceeded 79° C, the lowest temperature detectable by our method. The enclosure contained the fire and did not permit escape of any embers or firebrands. The fire enclosure, burn technique and temperature monitoring method used are inexpensive, easily deployed, and desirable for experiments where larger-scale burns are impractical.

Adaptive responses reveal contemporary and future ecotypes in a desert shrub

Interacting threats to ecosystem function, including climate change, wildfire, and invasive species necessitate native plant restoration in desert ecosystems. However, native plant restoration efforts often remain unguided by ecological genetic information. Given that many ecosystems are in flux from climate change, restoration plans need to account for both contemporary and future climates when choosing seed sources. In this study we analyze vegetative responses, including mortality, growth, and carbon isotope ratios in two blackbrush (Coleogyne ramosissima) common gardens that included 26 populations from a range-wide collection. This shrub occupies ecotones between the warm and cold deserts of Mojave and Colorado Plateau ecoregions in western North America. The variation observed in the vegetative responses of blackbrush populations was principally explained by grouping populations by ecoregions and by regression with site-specific climate variables. Aridity weighted by winter minimum temperatures best explained vegetative responses; Colorado Plateau sites were usually colder and drier than Mojave sites. The relationship between climate and vegetative response was mapped within the boundaries of the species-climate space projected for the contemporary climate and for the decade surrounding 2060. The mapped ecological genetic pattern showed that genetic variation could be classified into cool-adapted and warm-adapted ecotypes, with populations often separated by steep dines. These transitions are predicted to occur in both the Mojave Desert and Colorado Plateau ecoregions. While under contemporary conditions the warm-adapted ecotype occupies the majority of climate space, climate projections predict that the cool-adapted ecotype could prevail as the dominant ecotype as the climate space of blackbrush expands into higher elevations and latitudes. This study provides the framework for delineating climate change-responsive seed transfer guidelines, which are needed to inform restoration and management planning. We propose four transfer zones in blackbrush that correspond to areas currently dominated by cool-adapted and warm-adapted ecotypes in each of the two ecoregions.

Multicentury fire and forest histories at 19 sites in Utah and eastern Nevada

Our objective is to provide site-specific fire and forest histories from Utah and eastern Nevada that can be used for land management or additional research. We systematically sampled fire scars and tree-recruitment dates across broad gradients in elevation and forest type at 13 sites in Utah and 1 in eastern Nevada to characterize spatial and temporal variation in historical fire regimes as well as forest structure and composition. We collected similar data non-systematically at five additional sites in Utah. These 19 sites include a broad range of forest types (from pinyon-juniper woodlands to spruce-fir forests) and fire regime types. In this report, we summarize local-scale spatial and temporal variation with site-specific details of historical fire regimes and forests that will be useful for local natural resource and fire management of the individual sites. For each site, we report topography, chronologies of fire and tree recruitment, and properties derived from those chronologies such as time-averaged fire regime parameters (mean fire interval and fire severity) and changes in forest composition and structure that have occurred since the late 1800s.

Learning to Live With Cheatgrass: Giving Up or a Necessary Paradigm Shift?

On the Ground The contemporary flora and fauna of North America represent the survivors of repeated waves of emigration through geologic time mixed with local evolutionary processes. The rate of intercontinental species exchange has increased exponentially during the last 500 years due to intentional and accidental transport by humans. Altered ecosystem composition, structure, and functionality are an inevitable consequence of species migration and naturalization. Highly successful newcomers, such as cheat-grass, should be viewed as permanent additions to North American flora. Researchers, land owners and managers, and policy makers would do well to acknowledge the new realities created by introduced species and focus efforts on 1) limiting new introductions, 2) assessing the variability of impacts across affected ecosystems, and 3) developing reasonable expectations and practices for mitigating effects while preserving core ecosystem functionality.