Search for: All records

Abstract Climate change drivers elicit ecosystem responses that vary through time. We propose that the mismatch between equilibrium and transient ecosystem response defines ecological acclimation. As precipitation is expected to shift under climate change, we asked how both increases and decreases in water availability elicit ecological acclimation of the N cycle and how long does this acclimation take. Using foliar δ¹⁵N as a proxy for N cycling processes, we found that the slope of foliar δ¹⁵N decreases with mean annual precipitation across continents. However, within a desert grassland, slopes of interannual foliar and soil δ¹⁵N increased with precipitation amount. Using precipitation manipulation field experiments, we then assessed trends in foliar and soil δ¹⁵N as duration of the precipitation manipulation increased, from 5 to 14 years. When parsed temporally, the δ¹⁵N-precipitation slopes showed initially increasing trends that decreased after 14 years of precipitation manipulation. The difference in directionality of spatial δ¹⁵N-precipitation slope compared to within-site, experimental δ¹⁵N-precipitation slopes revealed potential acclimation of the N cycle. Furthermore, we estimated rates of ecological acclimation—defined as convergence time for within-site δ¹⁵N-precipitation slopes to match the global model—to range from 11 to 18 years. We conclude N cycling is changing with precipitation amount and duration of the altered precipitation regime. We hypothesize that fast and slow ecological mechanisms—such as microbial processes and shifts in plant species dominance, respectively—explain ecological acclimation of N cycling responses to climate change. As a result, spatial models must be interpreted with caution when forecasting future responses to climate change. 

Abstract Microbes play critical roles in dryland ecosystems, driving nutrient cycling, soil stability, and plant interactions. Despite their ecological importance, few studies have examined how microbial communities respond to vegetation changes in arid landscapes. In the northern extent of the Chihuahuan Desert, the encroachment of woody shrubs into grasslands has been occurring since the 1800s, largely driven by extensive livestock grazing and increased drought levels. In this study, we investigated how microbial communities respond to both biotic (i.e., vegetation) and abiotic (i.e., seasonality) factors, how they assemble in a changing landscape, and which taxa may be particularly responsive to shrub encroachment or even facilitating this transformation. We assessed microbial communities using soil surface samples across five distinct seasonal periods in a grassland-to-shrubland gradient in the Jornada Experimental Range in the Chihuahuan Desert through the use of phospholipid fatty-acid analysis and DNA metabarcoding techniques. Our findings reveal that bacterial and fungal biomass are significantly influenced by seasonal changes, with strong correlations to humidity and temperature fluctuations. We also found that fungal community assembly and diversity were highly impacted by vegetation whereas seasons were more impactful on bacteria. Our results support the idea that microbes may be playing a crucial role in facilitating the grassland-to-shrubland transition. Overall, our study highlights the complex interactions between microbial communities and biotic and abiotic factors in dryland systems. These findings are essential for understanding the future of dryland ecosystems undergoing shrub encroachment and provide a critical foundation for guiding restoration efforts, particularly those looking to incorporate microbial-mediated solutions. 

Abstract Ephemerally flooded playas are common in the southwestern United States and globally in drylands. Often formed in closed basins, playas are depressions which inundate infrequently from local precipitation and streamflow produced near the playa or from upland areas. Few studies have quantified the hydrologic connectivity between upland catchments and playas using observations. Here, we used rain gauge‐corrected precipitation from weather radar and water level measurements in 18 playas of the Chihuahuan Desert to identify precipitation thresholds leading to playa inundation over a 6.4‐year period. Geospatial data sets on topography, soil properties, and vegetation cover were employed to determine the controls on inundation. Only 9.4% of all precipitation events above 1 mm led to inundation, with 69.8% of all inundations occurring during the North American monsoon (NAM, July‐September). Mean and standard deviations (Std) of runoff ratios at all playas were 2.74 ± 4.08% and 3.29 ± 5.19% for annual and NAM periods. At the annual scale, playa inundation occurred when mean precipitation thresholds of 18.3 ± 7.5 mm (event total) and 12.0 ± 4.5 mm/hr (60‐min intensity) were exceeded. Across all playas, inundation occurrence and volume were related most strongly to precipitation metrics and catchment area, with secondary controls of soil and terrain properties. The explanatory power of the derived regressions describing the inundation response across the playas were significantly improved when considering their geological origin. As a result, the inundation response classification system could be applied to ephemeral playas in other arid and semiarid landscapes. 

Abstract Herbivores can be drivers of ecosystem change by triggering and reinforcing vegetation transitions. Such processes may be prevalent in drylands with low productivity where herbivore abundances are linked to climate‐driven resource pulses. In the Chihuahuan Desert, ecosystems are being transformed from black grama (Bouteloua eriopoda) grasslands to honey mesquite (Neltuma[formerlyProsopis]glandulosa) shrublands. Domestic livestock, exotic African oryx (Oryx gazella), and native rodents and lagomorphs have all been implicated as drivers of these transitions through multiple mechanisms affecting different plant life stages. Across shrub encroachment gradients, we paired a long‐term (21 years) herbivore exclusion experiment focused on established perennial grasses with field trials measuring herbivory risk for perennial grass seedlings. We evaluated the roles of cattle, oryx, and native herbivores in reducing grass cover, and tested whether herbivore effects on grass cover and seedling mortality varied among ecosystem states (grassland, ecotone, and shrubland). Cattle and African oryx did not contribute strongly to vegetation dynamics. However, long‐term exclusion of rodents and lagomorphs led to two‐to‐threefold increases in perennial grass cover compared to control plots (with open access to all herbivores) in shrub‐encroached states where mesquite shrubs provided these herbivores with cover from predators. Likewise, herbivory of perennial grass seedlings was highest in the shrub‐encroached states and was driven by rodents. Our results indicate that native rodents and lagomorphs exert strong control over perennial grass dynamics, creating positive feedbacks mediated by changes in habitat structure that can reinforce grassland–shrubland transitions in drylands. 

ABSTRACT MotivationHere, we make available a second version of the BioTIME database, which compiles records of abundance estimates for species in sample events of ecological assemblages through time. The updated version expands version 1.0 of the database by doubling the number of studies and includes substantial additional curation to the taxonomic accuracy of the records, as well as the metadata. Moreover, we now provide an R package (BioTIMEr) to facilitate use of the database. Main Types of Variables IncludedThe database is composed of one main data table containing the abundance records and 11 metadata tables. The data are organised in a hierarchy of scales where 11,989,233 records are nested in 1,603,067 sample events, from 553,253 sampling locations, which are nested in 708 studies. A study is defined as a sampling methodology applied to an assemblage for a minimum of 2 years. Spatial Location and GrainSampling locations in BioTIME are distributed across the planet, including marine, terrestrial and freshwater realms. Spatial grain size and extent vary across studies depending on sampling methodology. We recommend gridding of sampling locations into areas of consistent size. Time Period and GrainThe earliest time series in BioTIME start in 1874, and the most recent records are from 2023. Temporal grain and duration vary across studies. We recommend doing sample‐level rarefaction to ensure consistent sampling effort through time before calculating any diversity metric. Major Taxa and Level of MeasurementThe database includes any eukaryotic taxa, with a combined total of 56,400 taxa. Software Formatcsv and. SQL. 

ABSTRACT Though limiting resources differ among systems, water is limiting in most arid and many mesic systems, potentially allowing for direct measurement of competition by measurement of water uptake. Sapflow measurements provide a direct measure of water movement through plant stems, but, to our knowledge, sapflow has never been used to study density dependence and competition at large (regional or global) scales. Here, we examine a global database of sapflow measurements, the SapFluxNet database, for signs of density‐dependent competition for water. We find that plant‐level water uptake decreases with increasing competition from neighbours (specifically, neighbourhood basal area). Further analysis demonstrates that global‐scale variability in annual sapflow can be largely explained (R² = 0.522) by the combination of average vapour pressure and neighbourhood summed basal area. This analysis provides a rare quantification of plant competition for a limiting resource inferred directly via measurements of resource acquisition (i.e., sapflow). 

Abstract Patches of plants in drylands can arise when larger plants facilitate recruitment via ‘nurse‐plant’ effects. Simultaneously, plant–soil feedback processes can increase soil nutrients beneath shrubs leading to spatial patterns known as ‘islands of fertility’ that may also promote plant performance. These phenomena shape dryland plant communities; however, their comparative relative influences on plant density and diversity remain poorly quantified.Using reciprocal soil transplants among pairs of unvegetated interspaces and neighbouring shrubs in combination with cover treatments—open interspace, shaded interspace, intact shrub canopy and trimmed shrub canopy—this study aimed to disentangle the influences of shading from soil properties in native plant recruitment under the foundation species of shrubsLarrea tridentata andNeltuma glandulosain the Chihuahuan Desert, NM, USA.Seedling density varied significantly across years in response to interannual climate variability and was consistently higher underL. tridentatathanN. glandulosa. Cover treatments, soil provenance, shrub species and spatial factors (study blocks) significantly affected seedling density, species richness and community structure. Shading from intact shrubs or shade clothes had a stronger effect on density and richness than soil provenance or shrub species. Seedling density and richness were also higher in soil from beneath shrubs regardless of shrub species, location (native or transplanted locations) or time (study year) since initial translocation.The density of seedlings emerging from shaded interspace soil was initially similar to that observed under intact shrubs indicating the importance of shading by nurse plants for recruitment. However, the effect of shade‐cloth canopy surrogates on seedling density in interspaces switched from positive to negative by the third year of the study, likely due to seed bank depletion.Synthesis. The significant influence of shading highlights the role of environmental factors in shaping plant communities of this dryland system, while the evidence of seed depletion combined with the influence of plot location suggests a concurrent influence of stochastic community assembly processes. Understanding how these processes interact to shape spatial and temporal patterns of vegetation community assembly and potentially reinforce alternate ecological states remains an important focus in dryland ecosystems. 

Abstract Islands of fertility, patches of locally enhanced soil conditions, play a key role in increasing productivity in dryland regions. The fertile island effect (FIE) influences a range of variables including nutrient availability, soil moisture and microbial activity. While most examinations of the FIE focus on islands created by perennial plants at local scales, the effect may vary across spatial scales and under cover types including shrubs, grasses and biological soil crusts (biocrusts). This study explored differences in the FIE between soil depths across landforms and patch types for biogeochemical factors (nutrient availability) and biotic properties (microbial community structure, extracellular enzymatic activity). The FIE differed across landforms and soil depths, suggesting that soil geomorphology may play a major role in predicting soil fertility. Additionally, the FIE of enzymatic activity and available nutrients varied by patch type consistently across landforms, suggesting patch-scale processes influencing nutrient availability and acquisition are independent of landscape-scale differences. We show that biocrusts can have an FIE similar to that of shrubs and grasses, an underexplored control of variability and productivity in drylands. These findings necessitate further work to improve our understanding of how ecosystem processes vary across scales to influence patterns of productivity and soil fertility. 

Abstract Agroecosystems, which include row crops, pasture, and grass and shrub grazing lands, are sensitive to changes in management, weather, and genetics. To better understand how these systems are responding to changes, we need to improve monitoring and modeling carbon and water dynamics. Vegetation Indices (VIs) are commonly used to estimate gross primary productivity (GPP) and evapotranspiration (ET), but these empirical relationships are often location and crop specific. There is a need to evaluate if VIs can be effective and, more general, predictors of ecosystem processes through time and across different agroecosystems. Near‐surface photographic (red‐green‐blue) images from PhenoCam can be used to calculate the VI green chromatic coordinate (G_CC) and offer a pathway to improve understanding of field‐scale relationships between VIs and GPP and ET. We synthesized observations spanning 76 site‐years across 15 agroecosystem sites with PhenoCam G_CCand GPP or ET estimates from eddy covariance (EC) to quantify interannual variability (IAV) in the relationship between GPP and ET and G_CCacross. We uncovered a high degree of variability in the strength and slopes of the G_CC∼ GPP and ET relationships (R² = 0.1 ‐ 0.9) within and across production systems. Overall, G_CCis a better predictor of GPP than ET (R² = 0.64 and 0.54, respectively), performing best in croplands (R² = 0.91). Shrub‐dominated systems exhibit the lowest predictive power of G_CCfor GPP and ET but have less IAV in slope. We propose that PhenoCam estimates of G_CCcould provide an alternative approach for predictions of ecosystem processes. 

Abstract Land degradation is reducing biodiversity and crop yields, and exacerbating the impacts of climate change, throughout the world. Monitoring land degradation is required to determine the effectiveness of land management and restoration practices, and to track progress toward reaching land degradation neutrality (LDN). It is also needed to target investments where they are most needed, and will have the greatest impact. The most useful indicators of land degradation vary among soils and climates. The United Nations Convention to Combat Desertification (UNCCD) selected three widely accepted land degradation indicators for LDN: land cover, net primary production (NPP) and soil carbon stocks. In addition to non-universal relevance, the use of these indicators has been limited by data availability, especially for carbon. This article presents an alternative monitoring framework based on the definition and ranking ofstatesin a degradation hierarchy. Unique classifications can be defined for different regions and even different landscapes allowing, for example, perennial cropland to be ranked above a highly degraded grassland. The article concludes with an invitation to discuss the potential value of this approach and how it could be practically implemented at landscape to global scales. The ultimate objective is to support decision-making information at the local levels at which land degradation is addressed through improved management and restoration while providing the information necessary for reporting on progress toward meeting goals.