Evaluating the use of fire to control shrub encroachment in global drylands: A synthesis based on ecosystem service perspective

Woody encroachment: social-ecological impacts and sustainable management

Woody plants are encroaching across terrestrial ecosystems globally, and this has dramatic effects on how these systems function and the livelihoods of producers who rely on the land to support livestock production. Consequently, the removal of woody plants is promoted widely in the belief that it will reinstate former grasslands or open savanna. Despite this popular management approach to encroachment, we still have a relatively poor understanding of the effects of removal on society, and of alternative management practices that could balance the competing needs of pastoral production, biodiversity conservation and cultural values. This information is essential for maintaining both ecological and societal benefits in encroached systems under predicted future climate changes. In this review, we provide a comprehensive synthesis of the social-ecological perspectives of woody encroachment based on recent studies and global meta-analyses by assessing the ecological impacts of encroachment and its effects on sustainable development goals (SDGs) when woody plants are retained and when they are removed. We propose a working definition of woody encroachment based on species- and community-level characteristics; such a definition is needed to evaluate accurately the effects of encroachment. We show that encroachment is a natural process of succession rather than a sign of degradation, with encroachment resulting in an overall 8% increase in ecosystem multifunctionality. Removing woody plants can increase herbaceous plant richness, biomass and cover, but at the expense of biocrust cover. The effectiveness of woody plant removal depends on plant identity, and where, when and how they are removed. Under current management practices, either removal or retention of woody plants can induce trade-offs among ecosystem services, with no management practice maximising all SDGs [e.g. SDG2 (end hunger), SDG13 (climate change), SDG 15 (combat desertification)]. Given that encroachment of woody plants is likely to increase under future predicted hotter and drier climates, alternative management options such as carbon farming and ecotourism could be effective land uses for areas affected by encroachment.

Experimentally testing animal responses to prescribed fire size and severity

Deserts are often highly biodiverse and provide important habitats for many threatened species. Fire is a dominant disturbance in deserts, and prescribed burning is increasingly being used by conservation managers and Indigenous peoples to mitigate the damaging effects of climate change, invasive plants, and land-use change. The size, severity, and patchiness of fires can affect how animals respond to fire. However, there are almost no studies examining such burn characteristics in desert environments, which precludes the use of such information in conservation planning. Using a before-after control-impact approach with 20 sampling sites, we studied the outcomes of 10 prescribed burns of varying size (5-267 ha), severity, and patchiness to identify which variables best predicted changes in small mammal and reptile species richness and abundance. Three of the 13 species showed a clear response to fire. Captures increased for 2 species (1 mammal, 1 reptile) and decreased for 1 species (a reptile) as the proportional area burned around traps increased. Two other mammal species showed weaker positive responses to fire. Total burn size and burn patchiness were not influential predictors for any species. Changes in capture rates occurred only at sites with the largest and most severe burns. No fire-related changes in capture rates were observed where fires were small and very patchy. Our results suggest that there may be thresholds of fire size or fire severity that trigger responses to fire, which has consequences for management programs underpinned by the patch mosaic burning paradigm. The prescribed burns we studied, which are typical in scale and intensity across many desert regions, facilitated the presence of some taxa and are unlikely to have widespread or persistent negative impacts on small mammal or reptile communities in this ecosystem provided that long unburned habitat harboring threatened species is protected.

Woody encroachment induced earlier and extended growing season in boreal wetland ecosystems

Woody plant encroachment (WPE), a widespread ecological phenomenon globally, has significant impacts on ecosystem structure and functions. However, little is known about how WPE affects phenology in wetland ecosystems of middle and high latitudes. Here, we investigated the regional-scale effects of WPE on the start (SOS), peak (POS), end (EOS), and length (GSL) of the growing season in boreal wetland ecosystems, and their underlying mechanisms, using remote sensing dataset during 2001-2016. Our results showed that WPE advanced the annual SOS and POS, while delaying EOS and extending GSL in boreal wetlands with these impacts increasing over time. When boreal wetland ecosystems were fully encroached by woody plants, the SOS and POS were advanced by 12.17 and 5.65 days, respectively, the EOS was postponed by 2.74 days, and the GSL was extended by 15.21 days. We also found that the impacts of WPE on wetland SOS were predominantly attributed to the increased degree of WPE (α), while climatic factors played a more significant role in controlling the POS and EOS responses to WPE. Climate change not only directly influenced phenological responses of wetlands to WPE but also exerted indirect effects by regulating soil moisture and α. Winter precipitation and spring temperature primarily determined the effects of WPE on SOS, while its impacts on POS were mainly controlled by winter precipitation, summer temperature, and precipitation, and the effects on EOS were mainly determined by winter precipitation, summer temperature, and autumn temperature. Our findings offer new insights into the understanding of the interaction between WPE and wetland ecosystems, emphasizing the significance of considering WPE effects to ensure accurate assessments of phenology changes.

Is Joint Management between Conservationists and Farmers Sustainable and Biodiversity-friendly? A Ten-year Study in Residual Grasslands of a Protected Area

In recent decades, there has been a discernible reduction in temperate and Mediterranean grasslands with consequences on the decline of biodiversity and landscape heterogeneity. When this decline is due to agricultural abandonment, a renewed joint management, combining bush clearing by conservationists and grazing by farmers, should favor the maintenance of grasslands, their protected habitats and species and forage production. Rainfall irregularity explains part of the variation of these parameters. To verify these hypotheses, we conduct a comprehensive, multi-scale, multi-taxa study over a ten-year period in a Mediterranean protected area. At the regional scale, experimental plots in which this joint management was implemented are representative of residual managed grasslands of the protected area. At the mesoscale, rainfall irregularity is the main factor explaining inter-annual differences in the biomass of open landscapes, while fauna depends on management, tree cover and trophic resources. At the local scale, in a representative experimental plot, clearing had an immediate negative impact on plant richness and bird and positive on forage. Over a decade, plant biodiversity increased while forage, specialist plants and bird maintained, despite the regrowth of bush. Drought had a negative impact on richness, plant and forage abundance and phenological asynchrony on butterflies. In conclusion, joint management has positive, neutral and negative impacts to be considered before implementing this strategy. This long-term monitoring study draws important lessons for designing a sustainable management of grasslands under abandonment and irregular climate, that should be applied in temperate and Mediterranean regions that are increasingly vulnerable to these trends.

Shrubs magnify soil phosphorus depletion in Tibetan meadows: Conclusions from C:N:P stoichiometry and deep soil profiles

Globally, the proliferation of shrubs within grasslands stimulates soil phosphorus (P) cycling and increases topsoil P storage beneath their canopies. However, little is known regarding the impact of shrub encroachment on subsoil P storage, and whether shrubs mediate changes in soil stoichiometry, like increasing P cycling. In grazed meadows on the Tibetan Plateau, soil and roots were sampled to 1 m depth in shrubby Hippophae rhamnoides ssp. sinensis groves and the surrounding grassy areas. Shrubs had higher P content than grasses, but lower C:P ratios in their leaves, litter, and roots. Similarly, shrubs had higher microbial P content than grasses, but lower microbial biomass C:P and N:P ratios in the soil. The larger microbial P stock in the 1 m of soil beneath shrubs responded to the larger root P stock there as well. Thus, both the plants and microbes acquired more P in shrubby areas than in grassy areas by accelerating P mineralization. The greater net production of available P in the topsoil and the synthesis of microbial P throughout the profile under shrubs increased the P solubility. Total P, inorganic P, and organic P stocks were lower under shrubs than under grasses in the top 1 m of soil. This decrease in soil P storage beneath shrubs is most likely attributable to P leaching due to higher P solubility, heavy rainfall, and larger soil gaps. Moreover, shrubs also had larger plant biomass P stock compared to grasses, and thus the depletion of P from the top 1 m of soil was further magnified via plant biomass removal. We concluded that shrubs increase P cycling to overcome the stoichiometric imbalance between their P requirement and the supply in the soil, and the fast P cycling under shrubs magnify P depletion within the rooted soil depth in alpine meadows.

Soil organic carbon in drylands: shrub encroachment and vegetation management effects dwarf those of livestock grazing

Dryland ecosystems occur worldwide and play a prominent, but potentially shifting, role in global biogeochemical cycling. Widespread woody plant proliferation, often associated with declines in palatable grasses, has jeopardized livestock production in drylands and prompted attempts to reduce woody cover by chemical or mechanical means. Woody encroachment also has the potential to significantly alter terrestrial carbon storage. However, little is known of the long-term biogeochemical consequences of woody encroachment in the broader context of its interaction with common dryland land uses, including "brush management" (woody plant clearing) and livestock grazing. Present assessments exhibit considerable variation in the consequences of these land use/land cover changes, with evidence that brush management may counteract sizeable impacts of shrub encroachment on soil biogeochemical pools. A challenge to assessing the net effects of brush management in shrub-encroached grasslands on soil organic carbon (SOC) and total nitrogen (N) pools is that land management practices are typically considered in isolation, when they are co-occurring phenomena. Furthermore, few studies have assessed spatial patterns in brush management and how these are affected in decades following treatment on sites with contrasting grazing histories. To address these uncertainties and interactions, we quantified the impacts of shrub encroachment and their subsequent mortality resulting from brush management (herbicide application) on SOC and N pools in a Sonoran Desert grassland where long-term grazing manipulations (>100 yr) co-occur with shrub encroachment and brush management. Pools of SOC and N associated with herbicided shrubs declined markedly over ~40 yr, offsetting 66% of the increases from shrub encroachment. However, spatial patterns in SOC induced by shrubs persisted over the decades following brush management. Century-long protection from grazing did little to change SOC and N pools. Accordingly, shrub encroachment and shrub mortality from brush management each far outweighed livestock grazing impacts. Consideration of the patterns of SOC and N through space (e.g., bole-to-dripline gradients), time (e.g., shrub age/size), land use (e.g., livestock grazing and brush management), and their interactions will position us to improve predictions of SOC and N responses to land use/land cover change, inform C-based management decisions, and objectively evaluate trade-offs with other ecosystem services.

Coordinating ecosystem service trade-offs to achieve win-win outcomes: A review of the approaches

Ecosystem service (ES) trade-offs have been broadly recognized and studied over the past decade. However, how to coordinate the relationships among ES trade-offs to achieve win-win outcomes remains a considerable challenge for decision makers. Here, we summarize the current approaches applied to minimize ES trade-offs for win-wins and analyze the trade-offs among different ESs and their drivers. Based on a systematic review of the literature from 2005 to 2018, we identified 170 potentially relevant articles, 47 of which were selected for the review, recording 70 actual or potential trade-offs. Analysis of these case studies showed that trade-off pairs between provisioning services and regulating services/biodiversity accounted for 80% of total pairs. Furthermore, more than half of the ES trade-offs were driven by land use/land cover changes. Harvest and resource demand, natural resource management, and policy instruments were also among the main drivers. Four approaches to coordinate ES trade-offs were identified, including ecosystem, landscape-scale, multi-objective optimization, and policy intervention (and other) approaches. Based on the above, we recommend a rigorous understanding of the roles of different stakeholders, spatial scales of management, trade-off dynamics, and integrated implementation of diverse approaches to coordinate ES trade-offs in order to better achieve win-win outcomes.