Transitions from grass to woody plant dominance, widely reported in arid systems, are typically attributed to changes in disturbance regimes in combination with abiotic feedbacks, whereas biotic ...mechanisms such as competition and facilitation are often overlooked. Yet, research in semi‐arid and subhumid savannas indicates that biotic interactions are important drivers in systems at risk for state transition. We sought to bridge this divide by experimentally manipulating grass‐on‐shrub and shrub‐on‐shrub interactions in early and late stages of grassland–shrubland state transition, respectively, and to assess the extent to which these interactions might influence arid land state transition dynamics.
Target Prosopis glandulosa shrubs had surrounding grasses or conspecific neighbours left intact or killed with foliar herbicide, and metrics of plant performance were monitored over multiple years for shrubs with and without grass or shrub neighbours.
Productivity of small shrubs was enhanced by grass removal in years with above‐average precipitation, a result not evident in larger shrubs or during dry years. Proxy evidence based on nearest neighbour metrics suggested shrub–shrub competition was at play, but our experimental manipulations revealed no such influence.
Competition from grasses appears to attenuate the rate at which shrubs achieve the size necessary to modify the physical environment in self‐reinforcing ways, but only during the early stages of shrub encroachment. Our results further suggest that at late stages of grassland‐to‐shrubland state transitions, shrub–shrub competition will not slow the rate of shrub expansion, and suggest that maximum shrub cover is regulated by something other than density‐dependent mechanisms. We conclude that grass effects on shrubs should be included in assessments of desert grassland state transition probabilities and rates, and that desertification models in arid ecosystems that traditionally focus on disturbance and abiotic feedbacks should be broadened to incorporate spatial and temporal variations in competitive effects.
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Summary
Drylands, which cover > 40% of Earth's terrestrial surface, are dominant drivers of global biogeochemical cycling and home to more than one third of the global human population. Climate ...projections predict warming, drought frequency and severity, and evaporative demand will increase in drylands at faster rates than global means. As a consequence of extreme temperatures and high biological dependency on limited water availability, drylands are predicted to be exceptionally sensitive to climate change and, indeed, significant climate impacts are already being observed. However, our understanding and ability to forecast climate change effects on dryland biogeochemistry and ecosystem functions lag behind many mesic systems. To improve our capacity to forecast ecosystem change, we propose focusing on the controls and consequences of two key characteristics affecting dryland biogeochemistry: (1) high spatial and temporal heterogeneity in environmental conditions and (2) generalized resource scarcity. In addition to climate change, drylands are experiencing accelerating land‐use change. Building our understanding of dryland biogeochemistry in both intact and disturbed systems will better equip us to address the interacting effects of climate change and landscape degradation. Responding to these challenges will require a diverse, globally distributed and interdisciplinary community of dryland experts united towards better understanding these vast and important ecosystems.
1. State-and-transition models (STMs) synthesize and communicate knowledge about the alternative states of an ecosystem and causes of state transitions. Data supported narrative descriptions within ...STMs are used to select or justify management actions. State transitions are characteristically heterogeneous in space and time, but spatial heterogeneity is seldom described in STMs, thereby limiting their utility. 2. We conducted a review that indicates how spatially explicit data can be used to improve STMs. We first identified three spatial scales at which spatial patterns and processes are manifest: patches, sites and landscapes. We then identified three classes of spatial processes that govern heterogeneity in state transitions at each scale and that can be considered in empirical studies, STM narratives and management interpretations. 3. First, spatial variations in land-use driver history (e.g. grazing use) can explain differences in the occurrence of state transitions within land areas that are otherwise uniform. Secondly, spatial dependence in response to drivers imposed by variations in soils, landforms and climate can explain how the likelihood of state transition varies along relatively static environmental gradients. Thirdly, state transition processes can be contagious, under control of vegetation-environment feedbacks, such that the spatiotemporal evolution of state transitions is predictable. 4. We suggest a strategy for considering each of the three spatial processes in the development of STM narratives. We illustrate how spatial data can be employed for describing early warning indicators of state transition, identifying areas that are most susceptible to state transitions, and designing and implementing monitoring schemes. 5. Synthesis and applications. State-and-transition models are increasingly important tools for guiding land-management activities. However, failure to adequately represent spatial processes in STMs can limit their ability to identify the initiation, risk and causes of state transitions and, therefore, the appropriate management responses. We suggest that multi-scaled studies targeted to different kinds of ecosystems can be used to uncover evidence of spatial processes. Such evidence should be included in STM narratives and can lead to novel interpretations of land change and improved management.
Neotropical xerophytic forest ecosystems evolved with fires that shaped their resilience to disturbance events. However, it is unknown whether forest resilience to fires persists under a new fire ...regime influenced by anthropogenic disturbance and climate change. We asked whether there was evidence for a fire severity threshold causing an abrupt transition from a forest to an alternative shrub thicket state in the presence of typical postfire management. We studied a heterogeneous wildfire event to assess medium‐term effects (11 years) of varying fire severity in a xerophytic Caldén forest in central Argentina. We conducted vegetation surveys in patches that were exposed to low (LFS), medium (MFS), and high (HFS) fire severities but had similar prefire woody canopy cover. Satellite images were used to quantify fire severity using a delta Normalized Burning Ratio (dNBR) and to map prefire canopy cover. Postfire total woody canopy cover was higher in low and medium than high severity patches, but the understory woody component was highest in HFS patches. The density of woody plants was over three times higher under HFS than MFS and LFS due to the contribution of small woody plants to the total density. Unlike LFS and MFS patches, the small plants in HFS patches were persistent, multistem shrubs that resulted from the resprouting of top‐killed Prosopis caldenia trees and, more importantly, from young shrubs that probably established after the wildfire. Our results suggest that the Caldén forest is resilient to fires of low to moderate severities but not to high‐severity fires. Fire severities with dNBR values > ~600 triggered an abrupt transition to a shrub thicket state. Postfire grazing and controlled‐fire treatments likely contributed to shrub dominance after high‐severity wildfire. Forest to shrub thicket transitions enable recurring high‐severity fire events. We propose that repeated fires combined with grazing can trap the system in a shrub thicket state. Further studies are needed to determine whether the relationships between fire and vegetation structure examined in this case study represent general mechanisms of irreversible state changes across the Caldenal forest region and whether analogous threshold relationships exist in other fire‐prone woodland ecosystems.
Theoretical models predict that drylands can cross critical thresholds, but experimental manipulations to evaluate them are non‐existent. We used a long‐term (13‐year) pulse‐perturbation experiment ...featuring heavy grazing and shrub removal to determine if critical thresholds and their determinants can be demonstrated in Chihuahuan Desert grasslands. We asked if cover values or patch‐size metrics could predict vegetation recovery, supporting their use as early‐warning indicators. We found that season of grazing, but not the presence of competing shrubs, mediated the severity of grazing impacts on dominant grasses. Recovery occurred at the same rate irrespective of grazing history, suggesting that critical thresholds were not crossed, even at low cover levels. Grass cover, but not patch size metrics, predicted variation in recovery rates. Some transition‐prone ecosystems are surprisingly resilient; management of grazing impacts and simple cover measurements can be used to avert undesired transitions and initiate restoration.
Rangelands encompass 30-40% of Earth's land surface and support 1 to 2 billion people. Their predominant use is extensive livestock production by pastoralists and ranchers. But rangelands are ...characterized by ecological, economic, and political marginality, and higher value, more intensive land uses are impinging on rangelands around the world. Earth Stewardship of rangelands must address both livestock management and the broader socioecological dynamics that promote land-use changes, fragmentation, and degradation. We identify specific gradients on which human-rangeland systems can be arrayed, including issues of variability, adaptation to disturbance, commercialization, land-use change, land-tenure security, and effective governance, and we illustrate the gradients' interactions and effects in sites worldwide. The result is a synthetic framework to help in understanding how rangeland Earth Stewardship can be achieved in the face of marginality, globalization, and climate change.
Cross-scale interactions refer to processes at one spatial or temporal scale interacting with processes at another scale to result in nonlinear dynamics with thresholds. These interactions change the ...pattern-process relationships across scales such that fine-scale processes can influence a broad spatial extent or a long time period, or broad-scale drivers can interact with fine-scale processes to determine system dynamics. Cross-scale interactions are increasing recognized as having important influences on ecosystem processes, yet they pose formidable challenges for understanding and forecasting ecosystem dynamics. In this introduction to the special feature, “Cross-scale interactions and pattern-process relationships”, we provide a synthetic framework for understanding the causes and consequences of cross-scale interactions. Our framework focuses on the importance of transfer processes and spatial heterogeneity at intermediate scales in linking fine- and broad-scale patterns and processes. Transfer processes and spatial heterogeneity can either amplify or attenuate system response to broad-scale drivers. Providing a framework to explain cross-scale interactions is an important step in improving our understanding and ability to predict the impacts of propagating events and to ameliorate these impacts through proactive measures.
Context
Fire is an important driver of ecological processes in semiarid systems and serves a vital role in shrub-grass interactions. In desert grasslands of the southwestern US, the loss of fire has ...been implicated as a primary cause of shrub encroachment. Where fires can currently be re-introduced given past state changes and recent restoration actions, however, is unknown and controversial.
Objectives
Our objective was to evaluate the interactive effects of climate, urban development, and topo-edaphic properties on fire distribution in the desert grassland region of the southwestern United States.
Methods
We characterized the spatial distribution of fire in the Chihuahuan Desert and Madrean Archipelago ecoregions and investigated the influence of soil properties and ecological site groups compared to other commonly used biophysical variables using multi-model inference.
Results
Soil-landscape properties significantly influenced the spatial distribution of fire ignitions. Fine-textured bottomland ecological site classes experienced more fires than expected in contrast to upland sites with coarse soil textures and high fragment content that experienced fewer fire ignitions than expected. Influences of mean annual precipitation, distance to road/rail, soil available water holding capacity (AWHC) and topographic variables varied between ecoregions and political jurisdictions and by fire season. AWHC explained more variability of fire ignitions in the Madrean Archipelago compared to the Chihuahuan Desert.
Conclusions
Understanding the spatiotemporal distribution of recent fires in desert grasslands is needed to manage fire and predict responses to climate change. The use of landscape units such as ecological sites presents an opportunity to improve predictions at management scales.
Abstract Rangelands and the supply chains connected to them are central to the agrifood systems of the Southwestern United States. Local ranchers are simultaneously arid lands managers, herd ...managers, and marketing managers. To stay in business, they must adapt to unpredictable forage resources and markets while conserving soils and vegetation resources for the long term. As climate warming and drying exacerbate the complexity and difficulty of day‐to‐day production, producers and policymakers are seeking alternatives to “business as usual.” The Long‐Term Agroecosystem Research (LTAR)‐Jornada team has developed a package of strategies to help producers adapt to the local and inter‐regional challenges. The package includes heritage cattle, precision ranching systems, and adaptive value chains. Five ranches across the Southwest have adopted different combinations and are partnering with LTAR and each other to measure their benefits and drawbacks in real‐world conditions. Opportunities for controlled experimentation differ among the ranches, so we use LTAR's indicator system to assess and compare results. Even as we invest in co‐producing knowledge about these three strategies, we recognize that progressive aridification and urbanization of Southwestern rangelands create challenges for which a single “silver bullet” of agricultural innovation is unlikely to provide durable solutions. We are learning from our customers and stakeholders about ways to adjust the development of new options.
Core Ideas Rangelands and their supply chains are central to the agrifood systems of the Southwestern United States. To stay in business, ranchers must cope with uncertainty in forage and markets yet supply ecosystem services. Long‐Term Agroecosystem Research (LTAR)‐Jornada studies heritage genetics, precision technologies, and adaptive value chains to support these goals. Formalized knowledge co‐production is underway among five ranches across the Southwest. LTAR's indicator system is used to assess and compare social‐ecological outcomes of the three‐part management package.
Plain Language Summary Rangelands and the supply chains connected to them are central to the agrifood systems of the Southwestern United States. To stay in business, ranchers must cope with uncertainty in forage and markets yet conserve ecosystem services. Long‐Term Agroecosystem Research (LTAR)‐Jornada studies heritage genetics, precision technologies, and adaptive value chains to support these goals. Formalized knowledge co‐production is underway among five ranches across the Southwest. LTAR's indicator system is used to assess and compare social‐ecological outcomes of the three‐part management “package.”
The social and ecological contexts for rangeland management are changing rapidly, prompting a reevaluation of science, management, and their relationship. We argue that progression from steady-state ...management to ecosystem management has served the rangeland profession well, but that further development toward resilience-based management is required to ensure that ecosystem services are sustained in an era of rapid change. Resilience-based management embraces the inevitability of change and emphasizes that management should seek to guide change to benefit society. The objectives of this forum are to: 1) justify the need for adopting resilience-based management, 2) identify the challenges that will be encountered in its development and implementation, and 3) highlight approaches to overcoming these challenges. Five grand challenges confronting the adoption of resilience-based management, based upon the insights of 56 rangeland researchers who have contributed to this special issue, were identified as: 1) development of knowledge systems to support resilience-based management, 2) improvement of ecological models supporting science and management, 3) protocols to assess and manage tradeoffs among ecosystem services, 4) use of social-ecological system models to integrate diverse knowledge sources, and 5) reorganization of institutions to support resilience-based management. Resolving the challenges presented here will require the creation of stronger partnerships between ecosystem managers, science organizations, management agencies, and policymakers at local, regional, and national scales. A realistic near-term goal for achieving such partnerships is to initiate and support collaborative landscape projects. The creation of multiscaled social learning institutions linked to evolving knowledge systems may be the best approach to guide adaptation and transformation in rangelands in the coming century.
Los contextos ecológicos y sociales para el manejo de pastizales están cambiando rápidamente, provocando una reevaluación de la ciencia, y los vínculos entre ellos. Aquí proponemos que el reciente cambio de enfoque de un estado constante a un modelo de manejo de ecosistema ha servido bien a la profesión de manejo de pastizales, pero es necesario un cambio adicional al manejo basado en resiliencia que garantice que los servicios de los pastizales continuaran beneficiando a la sociedad en una era de cambios rápidos. El manejo basado en la resiliencia enfatiza el manejo colaborativo y el aprendizaje social para guiar la adopción y transformación en sistemas ecológico-sociales. Los objetivos de este foro son 1) justificar le necesidad para adoptar manejo basado en la resiliencia, 2) identificar los desafíos que se interpondrán en el camino para su desarrollo e implementación y 3) destacar los enfoques para superar estos desafíos. Se identificaron cinco categorías de desafíos de un manejo basado en resiliencia, usando las ideas de 55 científicos especializados en el manejo de pastizales que han contribuido especialmente a este tema: a) desarrollo de sistemas de conocimiento para soportar los sistemas de manejo basado en resiliencia, b) mejoramiento de modelos ecológicos apoyados por la ciencia y manejo, c) protocolos para evaluar y manejar el intercambio entre los servicios del ecosistema, d) uso de sistemas de perspectivas socio ecológicas para lograr una mayor participación de los interesados, y e) reorganización de las instituciones para apoyar el manejo basado en resiliencia. Se ha hecho evidente que un mayor conocimiento científico por sí mismo es insuficiente para promover los modelos de manejo basado en resiliencia en los pastizales. Las soluciones de los retos que se presentan aquí requerirán de la creación de asociaciones más fuertes entre manejadores de ecosistemas, organizaciones científicas, agencias de manejo y los creadores de políticas a nivel local, regional y nacional. La creación de instituciones sociales de aprendizaje vinculadas a la evolución de los sistemas de conocimiento es nuestra esperanza para guiar la adaptación y transformación en pastizales en el presente siglo.
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