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The replacement of grasses by shrubs or bare ground (xerification) is a primary form of landscape change in drylands globally with consequences for ecosystem services. The potential for wild herbivores to trigger or reinforce shrubland states may be underappreciated, however, and comparative analyses across herbivore taxa are sparse. We sought to clarify the relative effects of domestic cattle, native rodents, native lagomorphs, and exotic African oryx (Oryx gazella) on a Chihuahuan Desert grassland undergoing shrub encroachment. We then asked whether drought periods, wet season precipitation, or interspecific grass–shrub competition modified herbivore effects to alter plant cover, species diversity, or community composition. We established a long‐term experiment with hierarchical herbivore exclosure treatments and surveyed plant foliar cover over 25 years. Cover of honey mesquite (Prosopis glandulosa) proliferated, responding primarily to climate, and was unaffected by herbivore treatments. Surprisingly, cattle and African oryx exclusion had only marginal effects on perennial grass cover at their current densities. Native lagomorphs interacted with climate to limit perennial grass cover during wet periods. Native rodents strongly decreased plant diversity, decreased evenness, and altered community composition. Overall, we found no evidence of mammalian herbivores facilitating or inhibiting shrub encroachment, but native small mammals interacting with climate drove dynamics of herbaceous plant communities. Ongoing monitoring will determine whether increased perennial grass cover from exclusion of native lagomorphs and rodents slows the transition to a dense shrubland. [ABSTRACT FROM AUTHOR]

Climate change is projected to cause shifts in precipitation regimes globally, leading to intensified periods of precipitation and droughts. Most studies that have explored the influence of changing precipitation regimes on ecosystems have focused on changes in mean annual precipitation, rather than the variance around the mean. Soil fungi are ubiquitous organisms that drive ecosystem processes, but it is unknown how they respond to long‐term increased interannual precipitation variability.Here, we investigated the influence of long‐term increased precipitation variability and host type on soil fungal diversity and community composition in a dryland ecosystem. We collected 300 soil samples from two time points and different host type substrate types at a long‐term precipitation variability experiment at the Jornada Long Term Ecological Research site. Next, we used amplicon sequencing to characterize soil fungal communities.Soil fungal alpha diversity and community composition were strongly affected by host type and sampling year, and increased precipitation variability caused a modest, statistically insignificant, decrease in soil fungal evenness. Furthermore, results from our structural equational model showed that the decrease in grass‐associated soil fungal richness was likely an indirect result of host decline in response to increased precipitation variability.Synthesis. Our work demonstrates effects of increase in interannual precipitation variability on soil fungi, and that plant hosts play a key role in mediating soil fungal responses. [ABSTRACT FROM AUTHOR]

Aims: Grassland-to-shrubland transition is a common form of land degradation in drylands worldwide. It is often attributed to changes in disturbance regimes, particularly overgrazing. A myriad of direct and indirect effects (e.g., accelerated soil erosion) of grazing may favor shrubs over grasses, but their relative importance is unclear. We tested the hypothesis that topsoil "winnowing" by wind erosion would differentially affect grass and shrub seedling establishment to promote shrub recruitment over that of grass. Methods: We monitored germination and seedling growth of contrasting perennial grass (Bouteloua eriopoda, Sporobolus airoides, and Aristida purpurea) and shrub (Prosopis glandulosa, Atriplex canescens, and Larrea tridentata) functional groups on field-collected non-winnowed and winnowed soils under well-watered greenhouse conditions. Results: Non-winnowed soils were finer-textured and had higher nutrient contents than winnowed soils, but based on desorption curves, winnowed soils had more plant-available moisture. Contrary to expectations, seed germination and seedling growth on winnowed and non-winnowed soils were comparable within a given species. The N2-fixing deciduous shrub P. glandulosa was first to emerge and complete germination, and had the greatest biomass accumulation of all species. Conclusions: Germination and early seedling growth of grasses and shrubs on winnowed soils were not adversely nor differentially affected comparing with that observed on non-winnowed soils under well-watered greenhouse conditions. Early germination and rapid growth may give P. glandulosa a competitive advantage over grasses and other shrub species at the establishment stage in grazed grasslands. Field establishment experiments are needed to confirm our findings in these controlled environment trials. [ABSTRACT FROM AUTHOR]

Questions: Reordering of dominant species is an important mechanism of community response to global environmental change. We asked how wildfire (a pulse event) interacts with directional changes in climate (environmental presses) to affect plant community dynamics in a Chihuahuan Desert grassland. Location: Sevilleta National Wildlife Refuge, Socorro County, New Mexico, USA. Methods: Vegetation cover by species was measured twice each year from 1989 to 2019 along two permanently located 400‐m long line intercept transects, one in Chihuahuan Desert grassland, and the second in the ecotone between Chihuahuan Desert and Great Plains grasslands. Trends in community structure were plotted over time, and climate sensitivity functions were used to predict how changes in the Pacific Decadal Oscillation (PDO) affected vegetation dynamics. Results: Community composition was undergoing gradual change in the absence of disturbance in the ecotone and desert grassland. These changes were related to the reordering of abundances between two foundation grasses, Bouteloua eriopoda and B. gracilis, that together account for >80% of above‐ground primary production. However, reordering varied over time in response to wildfire (a pulse) and changes in the PDO (a press). Community dynamics were initially related to the warm and cool phases of the PDO, but in the ecotone these relationships changed following wildfire, which reset the system. Conclusion: Species reordering is an important component of community dynamics in response to ecological presses. However, reordering is a complex, non‐linear process in response to ecological presses that may change over time and interact with pulse disturbances. [ABSTRACT FROM AUTHOR]

In many arid ecosystems, shrub encroachment is coupled to the loss of perennial grasses and associated ecosystem services. Increased native herbivore abundance associated with shrub encroachment can have negative effects on grass restoration. In the Chihuahuan Desert, native herbivore abundance can be two times greater in shrubland states dominated by Prosopis glandulosa (honey mesquite) than in historical Bouteloua eriopoda (black grama) grasslands. We compared the recovery of B. eriopoda patches following disturbance in plots that were exposed to or protected from native herbivores in shrub-dominated, grass-dominated and ecotone (grass and shrub co-dominated) states. We created a disturbance in the center of B. eriopoda grass patches in each treatment quadrat in 2001. The disturbed areas were measured for recruitment and re-establishment of B. eriopoda in 2002, 2005 and 2008. Although mean rodent abundance was generally greater in shrub-dominated states than other states over the study period, reproductive potential of B. eriopoda was similar in shrub and grass dominated states. Additionally, there was no revegetation by B. eriopoda in any state in response to rodent exclusion. Because increased herbivore abundance in shrub-dominated states did not constrain grass revegetation, other factors are likely to be more important constraints on perennial grass recovery in the Chihuahuan Desert. • Small mammal herbivores did not constrain grassland regeneration. • Herbivore exclusion may not be effective in improving restoration of Bouteloua eriopoda grasslands, in some time periods. • Further research into the demography of B. eriopoda ramets and seedlings is needed. [ABSTRACT FROM AUTHOR]

Traditional models of state transition in arid lands emphasize changes in disturbance regimes and abiotic feedbacks that promote the degradation of grassland into shrubland, whereas biotic interactions like competition and facilitation are often overlooked. Here, we conducted an experiment to determine whether shrubs have a positive, neutral, or negative effect on grasses and if these interactions may play a role in grassland–shrubland state transition. Prosopis glandulosa shrub neighbors within 5 m of Bouteloua eriopoda grass patches were left intact (controls) or killed with foliar herbicide, and metrics of grass performance were evaluated over 5 years. We saw no evidence of shrub facilitation of grasses. Instead, grass ANPP responded positively to shrub removal in all years, but more so in years with above-average rainfall. Grass allocation to vegetative reproduction and grass patch size also increased when shrub neighbors were removed. These results demonstrate that biotic interference by shrubs upon grasses reinforce and magnify grazing- and drought-induced abiotic feedbacks during grassland–shrubland transitions. Shrub effects on grass should therefore be considered a key process in desert grassland state transitions. [ABSTRACT FROM AUTHOR]

Global environmental change is altering temperature, precipitation patterns, resource availability, and disturbance regimes. Theory predicts that ecological presses will interact with pulse events to alter ecosystem structure and function. In 2006, we established a long-term, multifactor global change experiment to determine the interactive effects of nighttime warming, increased atmospheric nitrogen (N) deposition, and increased winter precipitation on plant community structure and aboveground net primary production ( ANPP) in a northern Chihuahuan Desert grassland. In 2009, a lightning-caused wildfire burned through the experiment. Here, we report on the interactive effects of these global change drivers on pre- and postfire grassland community structure and ANPP. Our nighttime warming treatment increased winter nighttime air temperatures by an average of 1.1 °C and summer nighttime air temperature by 1.5 °C. Soil N availability was 2.5 times higher in fertilized compared with control plots. Average soil volumetric water content ( VWC) in winter was slightly but significantly higher (13.0% vs. 11.0%) in plots receiving added winter rain relative to controls, and VWC was slightly higher in warmed (14.5%) compared with control (13.5%) plots during the growing season even though surface soil temperatures were significantly higher in warmed plots. Despite these significant treatment effects, ANPP and plant community structure were highly resistant to these global change drivers prior to the fire. Burning reduced the cover of the dominant grasses by more than 75%. Following the fire, forb species richness and biomass increased significantly, particularly in warmed, fertilized plots that received additional winter precipitation. Thus, although unburned grassland showed little initial response to multiple ecological presses, our results demonstrate how a single pulse disturbance can interact with chronic alterations in resource availability to increase ecosystem sensitivity to multiple drivers of global environmental change. [ABSTRACT FROM AUTHOR]

Natural and anthropogenic disturbances are key drivers of vegetation dynamics. The hierarchical-response framework proposes that directional change in communities is driven by chronic resource alterations resulting from global environmental change in the absence of disturbance. Because vegetation is spatially heterogeneous, some local areas within a larger community may be more dynamic than others. Thus, the average rate of change may mask dynamic hotspots and local areas where vegetation remains stable. We used long-term data from two line-intercept transects in undisturbed desert grassland to quantify large-scale community dynamics, small-scale local dynamics, and boundary dynamics of grass patches in the absence of disturbance. We found that directional change in species composition was evident but that the overall rate of change varied spatially. Cover of both dominant grasses, Bouteloua eriopoda and Bouteloua gracilis, increased over the full transects, but most change occurred in localized hotspots. Patch boundaries of the dominant grasses exhibited both stability and local dynamics. Overall, the increasing abundance of B. eriopoda may predispose this grassland to shrub encroachment, whereas locally stable areas may prove resistant to state transition. More generally, global environmental change may be a pervasive driver of vegetation dynamics through localized hotspots of temporal change and spatially varying changes in patch boundaries in the absence of disturbance. [ABSTRACT FROM AUTHOR]

Theoretical models suggest that scale-dependent feedbacks between plant reproductive success and plant patch size govern transitions from highly to sparsely vegetated states in drylands, yet there is scant empirical evidence for these mechanisms. Scale-dependent feedback models suggest that an optimal patch size exists for growth and reproduction of plants and that a threshold patch organization exists below which positive feedbacks between vegetation and resources can break down, leading to critical transitions. We examined the relationship between patch size and plant reproduction using an experiment in a Chihuahuan Desert grassland. We tested the hypothesis that reproductive effort and success of a dominant grass ( Bouteloua eriopoda) would vary predictably with patch size. We found that focal plants in medium-sized patches featured higher rates of grass reproductive success than when plants occupied either large patch interiors or small patches. These patterns support the existence of scale-dependent feedbacks in Chihuahuan Desert grasslands and indicate an optimal patch size for reproductive effort and success in B. eriopoda. We discuss the implications of these results for detecting ecological thresholds in desert grasslands. [ABSTRACT FROM AUTHOR]

For the southwestern United States, climate models project an increase in extreme precipitation events and prolonged dry periods. While most studies emphasize plant functional type response to precipitation variability, it is also important to understand the physiological characteristics of dominant plant species that define plant community composition and, in part, regulate ecosystem response to climate change. We utilized rainout shelters to alter the magnitude and frequency of rainfall and measured the physiological response of the dominant C grasses, Bouteloua eriopoda and Bouteloua gracilis. We hypothesized that: (1) the more drought-adapted B. eriopoda would exhibit faster recovery and higher rates of leaf-level photosynthesis ( A) than B. gracilis, (2) A would be greater under the higher average soil water content in plots receiving 30-mm rainfall events, (3) co-dominance of B. eriopoda and B. gracilis in the ecotone would lead to intra-specific differences from the performance of each species at the site where it was dominant. Throughout the study, soil moisture explained 40-70 % of the variation in A. Consequently, differences in rainfall treatments were not evident from intra-specific physiological function without sufficient divergence in soil moisture. Under low frequency, larger rainfall events B. gracilis exhibited improved water status and longer periods of C gain than B. eriopoda. Results from this study indicate that less frequent and larger rainfall events could provide a competitive advantage to B. gracilis and influence species composition across this arid-semiarid grassland ecotone. [ABSTRACT FROM AUTHOR]

In arid and semi-arid ecosystems, there are legacies of previous-year precipitation on current-year above-ground net primary production. We hypothesized that legacies of past precipitation occur through changes in tiller density, stolon density, tiller growth, axillary bud density and percentage of viable axillary buds. We examined the sensitivity to current- and previous-year precipitation of these grassland structural components in Bouteloua eriopoda, the dominant grass in the northern Chihuahuan Desert., We conducted a rainfall manipulation experiment consisting in −80% reduced precipitation, ambient, +80% increased precipitation treatments that were subjected to one of five precipitation levels in the previous two years (−80% and −50% reduced precipitation, ambient, +50% and +80% increased precipitation). The first two years preconditioned the experimental plots for year three, in which we created wet-to-dry and dry-to-wet transitions. Measurements were taken in year 3., We found that stolon density was the most sensitive to changes in precipitation and that percent-active buds were insensitive., We also found that past precipitation had a significant legacy on grassland structural components regardless of the precipitation received in the current year, and that the legacy occurs mostly through changes in stolon density., Here, we showed that there is a differential sensitivity of structural components to current and past precipitation and supported previous findings that vegetation structure is one of the controls of productivity during precipitation transitions. [ABSTRACT FROM AUTHOR]

Multi-year climatic periods are expected to increase with global change, yet long-term data are often insufficient to document factors leading to ecological responses. We used a suite of long-term datasets (1993-2010) to examine the processes underlying different relationships between aboveground net primary production (ANPP) and precipitation in wet and dry rainfall periods in shrublands and grasslands in the Chihuahuan Desert. We hypothesized that trends in ANPP can be explained by different processes associated with their dominant grasses [ Bouteloua eriopoda (grasslands); Sporobolus flexuosus (shrublands)] and with ecosystem properties that influence soil water dynamics with feedbacks to ANPP. We compared datasets on recruitment and growth for 7 years with no trend in precipitation followed by a 4-year drought and 5 consecutive wet years. We integrated these data in a simulation model to examine the importance of positive feedbacks. In grasslands, ANPP was linearly related to precipitation regardless of rainfall period, primarily as a result of stolon recruitment by B. eriopoda. A lag in responses suggests the importance of legacies associated with stolon density. In shrublands, ANPP was only related to rainfall in the wet period when it increased nonlinearly as the number of wet years increased. Seed availability increased in the first wet year, and seedling establishment occurred 2-4 years later. Increases in biomass, litter and simulated transpiration beginning in the third year corresponded with increases in ANPP. Understanding the processes underlying ecosystem dynamics in multi-year dry or wet periods is expected to improve predictions under directional increases or decreases in rainfall. [ABSTRACT FROM AUTHOR]

Aridland ecosystems are predicted to be responsive to both increases and decreases in precipitation. In addition, chronic droughts may contribute to encroachment of native C shrubs into C-dominated grasslands. We conducted a long-term rainfall manipulation experiment in native grassland, shrubland and the grass-shrub ecotone in the northern Chihuahuan Desert, USA. We evaluated the effects of 5 years of experimental drought and 4 years of water addition on plant community structure and dynamics. We assessed the effects of altered rainfall regimes on the abundance of dominant species as well as on species richness and subdominant grasses, forbs and shrubs. Nonmetric multidimensional scaling and MANOVA were used to quantify changes in species composition in response to chronic addition or reduction of rainfall. We found that drought consistently and strongly decreased cover of Bouteloua eriopoda, the dominant C grass in this system, whereas water addition slightly increased cover, with little variation between years. In contrast, neither chronic drought nor increased rainfall had consistent effects on the cover of Larrea tridentata, the dominant C shrub. Species richness declined in shrub-dominated vegetation in response to drought whereas richness increased or was unaffected by water addition or drought in mixed- and grass-dominated vegetation. Cover of subdominant shrubs, grasses and forbs changed significantly over time, primarily in response to interannual rainfall variability more so than to our experimental rainfall treatments. Nevertheless, drought and water addition shifted the species composition of plant communities in all three vegetation types. Overall, we found that B. eriopoda responded strongly to drought and less so to irrigation, whereas L. tridentata showed limited response to either treatment. The strong decline in grass cover and the resistance of shrub cover to rainfall reduction suggest that chronic drought may be a key factor promoting shrub dominance during encroachment into desert grassland. [ABSTRACT FROM AUTHOR]

Abstract: Conversion of perennial grasslands to shrublands is a desertification process that is important globally. Changes in aboveground ecosystem properties with this conversion have been well-documented, but little is known about how belowground communities are affected, yet these communities may be important drivers of desertification as well as constraints on the reversal of this state change. We examined nematode community structure and feeding as a proxy for soil biotic change across a desertification gradient in southern NM, USA. We had two objectives: (1) to compare nematode trophic structure and species diversity within vegetation states representing different stages of desertification, and (2) to compare nematode community structure between bare and vegetated patches that may be connected via a matrix of endophytic fungi and soil biotic crusts. The gradient included a perennial grassland dominated by Bouteloua eriopoda, the historic dominant in the Chihuahuan Desert, a duneland dominated by the shrub, Prosopis glandulosa, and the ecotone between them. We also sampled a relatively undisturbed, ungrazed B. eriopoda grassland at a nearby site to serve as an end member of our gradient. Nematode communities were sampled using soil cores to depth of 50cm at each location in 2009 and 2010. Results showed that grasslands and mesquite dunelands had different trophic groupings and herbivorous nematode communities with lower species diversity and evenness compared with the ecotone. Nematode trophic structure and herbivore communities were significantly different in all vegetation states with the highest observed diversity in the undisturbed, ungrazed B. eriopoda grassland in 2010. Vegetated and bare ground patches within the two grassland sites had similar herbivore communities, especially species from the family Tylenchinae. However, the mesquite duneland showed the lowest sampled diversity of all sites, but had significantly larger nematode abundances in vegetated dunes than interdune areas that are void of vegetation and soil biotic crusts where bacteriovores dominated. Decreased nematode trophic structure and species diversity in the Jornada black grama grassland samples compared with the undisturbed grassland illustrate the effect of desertification on the soil biotic community. Our results show that nematodes can be used to identify changes in belowground community structure based on trophic interactions. Large-scale disturbances like desertification can have consequences on the diversity and soil biotic functioning at finer spatial scales. [Copyright &y& Elsevier]

Abstract: Global environmental change is altering temperature, precipitation patterns, and resource availability in aridland ecosystems. In 2006, we established a multifactor global change experiment to determine the interactive effects of nighttime warming, increased atmospheric N deposition, and more frequent occurrence of El Niño years on plant community dynamics in a northern Chihuahuan Desert grassland. Here we only report the results of warming and N addition from the first monsoon growing season prior to the imposition of the precipitation treatments. Our passive nighttime warming treatment increased daily minimum temperatures by 1.4–3.0°C. Fertilization increased NO3 as measured with Root Simulator Probes, by 28% and NO3 in warmed plots was 12% higher than in control plots. After one monsoon season we found that warming significantly increased total plant cover. Responses among dominant species varied, however. Warming significantly increased cover of Bouteloua eriopoda (P <0.05) and tended to increase cover of the shrub Gutierrezia sarothrae (P =0.064), whereas N addition significantly increased the cover of Bouteloua gracilis (P <0.05). Our results demonstrate that rapid soil and plant community responses can occur in response to nighttime warming and N fertilization during one monsoon season in this semi-arid desert grassland. The sensitivity of this desert system to key drivers of global environmental change suggests that further increases in temperature and atmospheric deposition will likely alter community structure and ecosystem functioning in this and other aridland ecosystems. [Copyright &y& Elsevier]

Abstract: We used a daily time step, multi-layer simulation model of soil water dynamics to integrate effects of soils, vegetation, and climate on the recruitment of Bouteloua eriopoda (black grama), the historically dominant grass in the Chihuahuan Desert. We simulated landscapes at the Jornada ARS-LTER site with heterogeneous soil properties to compare: (1) a grass-dominated landscape in 1858 with the current shrub-dominated landscape (i.e., a change in vegetation structure), and (2) the current shrub-dominated landscape with future landscapes over a range of climate scenarios associated with the North American monsoon (i.e., a change in climate). A historic shift from high productivity grasslands to low productivity shrublands decreased simulated recruitment for most of the site; the amount of reduction depended on location-specific soil properties and changes in production. In some cases, soil properties interacted with vegetation structure: soils high in clay content maintained high recruitment even with a decrease in production. Wetter summers would increase recruitment in all vegetation types. Drier summers below 25% of current rainfall would decrease recruitment to negligible values (<0.03) throughout the landscape. We used our results to identify the conditions where recruitment of B. eriopoda is possible with and without significant modifications to soil and vegetation. [Copyright &y& Elsevier]

Abstract: A prescribed burn resulted in significant decreases in canopy cover of the grasses: Bouteloua eriopoda, Sporobolus flexuosus, and Aristida purpurea. One year post-burn, basal cover of B. eriopoda remained significantly lower in burned patches than in unburned areas but there were no differences in basal cover of the other perennial grasses. Only one species of the 14 summer annual species occurred in both burned and unburned plots. There were six species of spring annuals in burned patches but no spring annuals in the unburned grassland ten months post-burn. Fire killed 100% of the snakeweed shrubs (Gutierrezia sarothrae), 77% of the Ephedra torreyana shrubs, and 36% of the Yucca elata. All mesquite shrubs that were top-killed by fire, resprouted one month post-burn. Fire had no effect on abundance and species richness of rodents. There were fewer wolf spider, Geolycosa spp. burrows in burned areas than in unburned grassland. The area and volume of soil in termite galleries and sheeting were significantly larger in the unburned grassland than in the burned areas. [Copyright &y& Elsevier]

Abstract: To better understand the effects of fire in the Chihuahuan desert, gas exchange characteristics of two dominant grass species, Bouteloua eriopoda and Aristida purpurea, and soil nitrogen availability were studied in response to prescribed fire at the Jornada Experimental Range in southern New Mexico. Burned and unburned plant individuals were measured before and after fire. Rates of net photosynthesis and stomatal conductance were highest in burned individuals, with those of A. purpurea exceeding B. eriopoda. Soil nitrogen supply rates increased compared to unburned controls. Similar to other grasslands where fire is common, physiological characteristics of vegetation responded positively. These adaptations indicate that fire may be beneficial in the preservation and restoration of native grasses. [Copyright &y& Elsevier]

Abstract: The literature on aboveground litter decomposition in arid environments, which is dominated by short-term studies (one- to three-year duration), shows no consistent relationship between mass loss rates and factors controlling decomposition in mesic environments. For ten years, we repeated a decomposition study of five species in four arid to semi-arid biomes in central New Mexico. Annual precipitation during the study ranged from 60% to 150% of the long-term mean. Mass remaining after one year of decomposition varied widely between species: Bouteloua eriopoda (47–85%), Bouteloua gracilis (31–87%), Juniperus monosperma (62–85%), Larrea tridentata (14–67%), and Oryzopsis hymenoides (22–88%). No consistent results were found relating percent mass change during several intra-annual intervals with initial litter C:N or current, cumulative or antecedent precipitation for any species at any site. The relative rates of decomposition were L. tridentata⪢O. hymenoides>B. eriopoda=J. monosperma>B. gracilis. Except for percent lignin, litter quality measures were not good predictors of relative mass loss rates among species. Although a few significant relationships between decomposition and precipitation and litter quality were found, our study results were overwhelmingly unsupportive of these factors having major impacts on litter decomposition processes in central New Mexico. [Copyright &y& Elsevier]

Aboveground net primary production (ANPP) dynamics are a key element in the understanding of ecosystem processes. For semiarid environments, the pulse-reserve framework links ANPP to variable and unpredictable precipitation events contingent on surficial hydrology, soil moisture dynamics, biodiversity structure, trophic dynamics, and landscape context. Consequently, ANPP may be decoupled periodically from processes such as decomposition and may be subjected to complex feedbacks and thresholds at broader scales. As currently formulated, the pulse-reserve framework may not encompass the breadth of ANPP response to seasonal patterns of precipitation and heat inputs. Accordingly, we examined a 6-year (1999–2004), seasonal record of ANPP with respect to precipitation, soil moisture dynamics, and functional groups in a black grama ( Bouteloua eriopoda) grassland and a creosotebush ( Larrea tridentata) shrubland in the northern Chihuahuan Desert. Annual ANPP was similar in the grassland (51.1 g/m2) and shrubland (59.2 g/m2) and positively correlated with annual precipitation. ANPP differed among communities with respect to life forms and functional groups and responses to abiotic drivers. In keeping with the pulse-reserve model, ANPP in black grama grassland was dominated by warm-season C4 grasses and subshrubs that responded to large, transient summer storms and associated soil moisture in the upper 30 cm. In contrast, ANPP in creosotebush shrubland occasionally responded to summer moisture, but the predominant pattern was slower, non-pulsed growth of cool-season C3 shrubs during spring, in response to winter soil moisture accumulation and the breaking of cold dormancy. Overall, production in this Chihuahuan Desert ecosystem reflected a mix of warm-temperate arid land pulse dynamics during the summer monsoon and non-pulsed dynamics in spring driven by winter soil moisture accumulation similar to that of cool-temperate regions. [ABSTRACT FROM AUTHOR]

Question: Does shrub invasion at ecotones indirectly limit grass establishment by increasing mammalian seedling herbivory? Location: Chihuahuan Desert, New Mexico, USA. Methods: We tested the hypothesis that herbivore-related mortality of seedlings of the dominant perennial grass Bouteloua eriopoda would be highest in shrub-dominated portions of grassland-shrubland ecotones. We tested the hypothesis in two Chihuahuan Desert sites featuring similar shrub encroachment patterns but different shrub species, grass cover, and different abundances of small mammals. Within each site we transplanted B. eriopoda seedlings to grass-dominated, middle, and shrubdominated positions of replicate ecotones during the time of year (mid-summer) when they would naturally appear and monitored seedling fates. We estimated population size/activity of putative small mammal herbivores. Results: Seedlings were killed by mammals in greater numbers in shrubland than in grassland or middle ecotone positions at the site with large herbivore numbers. At the site with low herbivore numbers, most seedlings were killed in middle ecotone positions. The abundance patterns of herbivores did not parallel patterns of seedling herbivory across the ecotones or between sites. Conclusions: Seedling herbivory is an important process and is related to vegetation composition, but the mechanisms underlying the relationship are not clear. We speculate that variation in small mammal foraging behavior may contribute to seedling herbivory patterns. Restoration strategies in the Chihuahuan Desert need to account for the abundance and/or behavior of native herbivores. [ABSTRACT FROM AUTHOR]

Relationships involving fire and perennial grasses are controversial in Chihuahuan Desert grasslands of southern New Mexico, USA. Research suggests that fire delays the resprouting of perennial grasses well after two growing seasons. However, such results are confounded by livestock grazing, soil erosion, and drought. Additionally, post-fire grass responses may depend on initial clone size. We evaluated the effects of fire, grazing, and clone size on Bouteloua eriopoda (black grama) in southern New Mexico grasslands. Four 2-ha plots were established in each of four sites. Fire and grazing were applied or not applied in 1999 such that four treatment combinations were assigned randomly to plots within each site. Within each plot, small (0–10 cm2 basal area), medium (10–30 cm2), and large ( > 30 cm2) clones were initially mapped in five 0.91-m2 quadrats where grass attributes and litter cover were evaluated before and at the end of two growing seasons following fire. Maximum fire temperature was also measured. At a population level, canopy and litter cover were each approximately 50% less in burned than unburned areas. However, compared to initial levels, canopy height had increased by 10% at the end of the study, regardless of fire. At a clonal level, basal cover reductions were attributed mostly to large clones that survived fire. Smaller clone densities had decreased by as much as 19% in burned compared to unburned areas, and fire reduced the basal cover of medium clones. Basal and canopy cover, recruitment, and clone basal area decreased with increased fire temperatures. Almost all responses were independent of grazing, and interactive effects of grazing and fire were not detected. Fire did not kill all perennial grass clones, regardless of size. However, rapid responses were likely influenced by above-average precipitation after fire. Future studies in desert grasslands should examine how perennial grass dynamics are affected by fire, precipitation patterns, and interactions with grazing. [ABSTRACT FROM AUTHOR]

Abstract: The roles of microbes in shaping plant communities have historically been underestimated. Recent improvements in our abilities to detect, identify, and monitor microbial inhabitants of plant tissues are increasing our appreciation of the complex microbial dynamics in arid and semi-arid ecosystems. Microbial endophytes can modify plants at genetic, physiologic, and ecologic levels. These modifications induce profound changes in how plants respond to their environments, with potential consequences in terms of spatial variation in vegetation dynamics. Microscale examination of Bouteloua eriopoda (black grama) and Atriplex canescens (fourwing saltbush) collected in the northern Chihuahuan Desert revealed diverse fungal communities associated with individual plants at the cellular and subcellular levels. To explore thresholds of plant fitness defined by microbial communities, endophytes from B. eriopoda were transferred to Sporobolus cryptandrus (sand dropseed), and endophytes from S. cryptandrus and A. canescens were transferred to B. eriopoda. Dramatic, whole-plant differences in morphology and biomass between treated and untreated plants were observed. Treated plants were generally larger, with greater reproductive potential than untreated controls. We hypothesize that these transformed plants will continue to out-perform their native counterparts, propagating changes from the plant–microbe interface to ecologically significant scales. [Copyright &y& Elsevier]

Abstract: We conducted experiments to examine microsite and rodent influence on mesquite (Prosopis glandulosa) germination and survival in Chihuahuan Desert black-grama (Bouteloua eriopoda) grassland. Simulated caches with mesquite seeds were placed in undisturbed grassland, burned grassland patches with reduced grass cover, and on the periphery of kangaroo rat mounds. Rodent access to cached seeds was controlled by covering half of the caches with mesh domes. Kangaroo rats destroyed dung pats containing scarified mesquite seeds and excavated all seeds in simulated caches during a year in which no grasses or forbs set seed. Germination was the highest in unburned grassland, but over-winter survival was only observed across experiments in burned grassland microsites. A large proportion of seed caches were excavated (1.1–5.5% of the rodent-excluded caches and between 15.6% and 21.1% of the control caches). There were seasonal differences in numbers of emergent seedlings, with a peak in September 2002 following late summer rains. There were no significant differences in numbers of germinants in burned and unburned grassland microsites, but significantly fewer mesquite seeds germinated on banner-tail kangaroo rat mounds. Caches excavated by rodents yielded 46.6% of germinants. This study suggests that scarification of mesquite seeds is not necessary for germination and heteromyid rodents may be responsible for increased mesquite density in grasslands with some mature mesquite plants. The data also suggest that competition between mesquite seedlings and grasses in black-grama grassland is weak because of the physical characteristics of this system. [Copyright &y& Elsevier]

We measured CO2 and evapotranspiration (ET) fluxes above a Chihuahuan desert grassland from 1996 through 2001. Averaged across six years, this ecosystem was a source (positive flux) of CO2 in every month. Over that period, sustained periods of carbon uptake (negative flux) were rare. Averaged across all months, we estimated that CO2 was being released into the atmosphere at a rate of +1.5g CO2m-2d-1 (+0.40g Cm-2d-1) or +533g CO2m-2yr-1 (+145.3g Cm-2yr-1). Average ET was 299mmyr-1 with a monthly range of 0.15mmd-1 in December to 2.15mmd-1 in August. [Copyright &y& Elsevier]

Question: In the same landscape context - at a desert grassland- shrubland transition zone, how does subdominant plant abundance vary in microsites around dominant grasses and shrubs? Location: Sevilleta LTER, New Mexico, USA (34°21' N; 106°53' W; 1650 m a.s.l.). Methods: We compared the distribution of subdominant plants in canopy, canopy edge and interspace microsites around individual shrubs (Larrea tridentata) and grasses (Bouteloua eriopoda) at a transition zone that has been encroached by shrubs within the past 50 - 100 a. Plots of variable size according to microsite type and dominant plant size were sampled. Results: Subdominant abundance was higher in microsites around L. tridentata shrubs than in microsites around B. eriopoda. Furthermore, differences in species abundance and composition were higher among microsites around grasses than among microsites around shrubs. The distribution of subdominants was mostly explained by their phenological characteristics, which indicates the importance of temporal variation in resources to their persistence. Conclusions: This study of coexistence patterns around dominants revealed ecological contrasts between two dominant life forms, but other factors (such as disturbances) have to be taken into consideration to evaluate landscape-scale diversity. [ABSTRACT FROM AUTHOR]

To understand the coupled water and energy cycles in semiarid environments, we measured temporal fluctuations of evapotranspiration (ET) and identified key sources of the observed variability. Flux measurements are made using the Bowen ratio method, accompanied by measurements of soil moisture and radiation. We present data from semiarid grassland and shrubland sites, situated within 2 km of each other in New Mexico. The study includes three summer monsoon seasons. Midday available energy ( Q a) is higher at the grassland than at the shrubland by 20% or 70 W m−2 because of differences in net radiation (Rn) and soil heat flux (G). At both sites, midday evaporative fraction and daily ET are strongly correlated with surface soil moisture (0-5cm) but poorly correlated with water content at greater depths or averaged throughout the entire root zone. The sensitivity of EF to 0-5cm is 30% lower at the grassland site. The differences in Q a and EF cancel, yielding similar time series of ET at the two sites. Decreases in 0-5cm, ET, and EF following rainfall events are rapid: exponential time constants are less than 3 days. With the exception of the largest storms, infiltration following rainfall events only wets the top 10 cm of soil. Therefore the surface soil layer is the primary reservoir for water storage and source for ET during the monsoon season, suggesting that direct evaporation is a large component of ET. Given these results, predicting ET based on root zone-averaged soil moisture is inappropriate in the semiarid environments studied here. [ABSTRACT FROM AUTHOR]

We used the patch arid land simulator (PALS-FT) – a simple, mechanistic ecosystem model – to explore long-term variation in evapotranspiration (ET) as a function of variability in rainfall and plant functional type (FT) at a warm desert site in southern New Mexico. PALS-FT predicts soil evaporation and plant transpiration of a canopy composed of five principal plant FTs: annuals, perennial forbs, C4 grasses, sub-shrubs, and evergreen shrubs. For each FT, the fractional contribution to transpiration depends upon phenological activity and cover as well as daily leaf stomatal conductance, which is a function of plant water potential, calculated from root-weighted soil water potential in six soil layers. Simulations of water loss from two plant community types (grass- vs. shrub-dominated) were carried out for the Jornada Basin, New Mexico, using 100 years of daily precipitation data (1891–1990). In order to emphasize variability associated with rainfall and fundamental differences in FT composition between communities, the seasonal patterns cover of perennials were held constant from year to year. Because the relative amount of year to year cover of winter and summer annual species is highly variable in this ecosystem, we examined their influence on model predictions of ET by allowing their cover to be variable, fixed, or absent. Over the entire 100-yr period, total annual ET is highly correlated with total annual rainfall in both community types, although T and E alone are less strongly correlated with rainfall, and variation in transpiration is nearly 3 times greater than evaporation and 2 times greater than variation in rainfall (CV of rainfall = 35%). Water use shows a relatively high similarity between the grass- and shrub-dominated communities, with a 100-yr average T/ET of 34% for both communities. However, based on a year-by-year comparison between communities, T/ET was significantly greater in the grass-dominated community, reflecting the fact that over the long term more than half of the rain occurs in the summer and is used slightly more efficiently (T¿E) by the C4-grass community than the shrub community, although we found some rainfall patterns that resulted in much greater T/ET in the shrub community in a given year. Percent of water lost as transpiration (T/ET) suggests that while there is a general trend toward increased T/ET with rainfall in both community types, T/ET is extremely variable over the 100-yr simulation, especially for normal and below normal amounts of rainfall (T/ET values range from 1 to 58% for the grass-dominated site and 6 to 60% for the shrub-dominated site). These predictions suggest that because of the relatively shallow distribution of soil water, there is little opportunity for vertical partitioning of the soil water resource by differential rooting depths of the plant FTs, in contrast to the two-layer hypothesis of Walter (1971). However, functional types may avoid competition by keying on particular `windows' of moisture availability via differences in phenologies. We found very little differences in average, long-term model predictions of T, E, and ET when annual plant cover was variable, fixed, or absent. The results of our simulations help reconcile some of the disparate conclusions drawn from experimental studies about the relative contribution of transpiration vs. evaporation to total evapotranspiration, primarily by revealing the great year-to-year variability that is possible. [ABSTRACT FROM AUTHOR]

Vegetation throughout the southwestern United States has changed from perennial grassland to woody shrubland over the past century. Previous studies on the development of ‘islands of fertility’ focused primarily on only the most limiting, plant-essential element, soil nitrogen (N). The research presented here addressed the question of whether other plant-essential elements, namely phosphorus (P) and potassium (K), showed similar concentration gradients under the desert shrub Larrea tridentata, creosotebush. It also examined whether the spatial distribution of N, P, and K differed from that of essential, but non-limiting nutrients, namely calcium (Ca), magnesium (Mg), and sulfur (S), and non-essential elements, namely sodium (Na), chloride (Cl), and fluoride (F). Within adjacent grassland and shrubland plots, surface soils were collected under and between vegetation and analyzed for a suite of soil nutrients. Soil nutrient distribution followed a uniform pattern that mirrored the spatial homogeneity of bunchgrasses in the grassland, but followed a patchy distribution that mirrored the spatial heterogeneity of individual shrubs in the shrubland. The main differences were that in the grassland, all elements were uniformly distributed, but in the shrubland the plant-essential elements, nitrogen, phosphorus, and potassium, were concentrated under the shrub canopy, and the non-limiting and non-essential elements were either concentrated in the intershrub spaces or were equally concentrated under shrubs and in the interspaces. Our results show how vegetation shifts from grassland to shrubland contribute to long-term, widespread change in the structure and function of desert ecosystems. [ABSTRACT FROM AUTHOR]

Temporal fluctuations in plant species coexistence are key to understanding ecosystem state transitions and long‐term maintenance of species diversity. Although plant microbiomes can alter plant competition in short‐term experiments, their relevance to natural temporal patterns in plant communities is unresolved. In a semiarid grassland, the frequency and magnitude of change in plant species composition through time varied from relatively static to highly dynamic among patches across the landscape. We field tested whether these alternative successional trajectories correlated with alternative plant–soil interactions. In temporally stable patches, we found negative plant–soil feedbacks, where plants grew worse with conspecific than heterospecific soil biota—a mechanism that maintains stability in mathematical models. In contrast, feedbacks in temporally dynamic patches were neutral to positive. Importantly, the magnitude of feedbacks depended on plant frequency, enabling plant species to increase in cover when rare, which theory predicts will promote long‐term, stable coexistence. Although our study does not determine the direction of causality, our results reveal a novel link between plant—microbe interactions and temporal stability of plant species coexistence and help to explain 20+ yr of plant abundance dynamics at the patch‐to‐landscape scales. [ABSTRACT FROM AUTHOR]