Increasing densities of Leucosidea sericea have minimal effects on grazing capacity and soil characteristics of a high-altitude communal rangeland at Vuvu, South Africa

Increasing densities of woody plants, known as woody plant encroachment, is a phenomenon affecting savannas and grasslands in many parts of the world. Yet, these ecosystems sustain a significant proportion of the human population through the provision of ecosystem services, such as forage for livestock and wildlife production. While low to medium altitude rangelands are encroached by many species of woody plants, high altitude rangelands in southern Africa show increasing densities of Leucosidea sericea, a woody shrub or small to medium-sized tree. Influences of this species on rangeland dynamics are unknown. This study aimed to determine the influence of L. sericea on rangeland functioning in the Vuvu communal area in the Eastern Cape, South Africa. Effects of L. sericea on plant species diversity and composition, rangeland condition and grazing capacity were measured in sites of variable densities of the species in topographical locations designated as plains, upland and stream sites, using a point-to-tuft method along 50-m long transects. Soil samples were collected to a depth of 5 cm from plains, streams, and upland sites, and analysed for organic carbon, nitrogen, phosphorus, magnesium, calcium, and pH. Plant species richness and abundance were similar among topographical locations, which was reflected by the similar Shannon-Weiner (H') diversity indices among sites. Topographical locations differed significantly in species composition. The plains sites had a higher grazing capacity than stream sites, which had a grazing capacity similar to that of upland sites. Values of soil physicochemical properties were similar among the sites. Overall, soils were acidic (range in pH: 4.4-4.6) and had low amounts of organic carbon and total nitrogen. These findings suggest that L. sericea is not the primary cause of rangeland degradation as all sites were in poor condition as shown by the low grazing capacity, poor rangeland condition and depauperate species richness and diversity. Therefore, rangeland management should shift towards restoration strategies aimed to revitalise the rangeland.

Where have all the flowers gone? A systematic evaluation of factors driving native terrestrial plant decline in North America

Prior to the arrival of Europeans in North America, forest and grasslands individually covered a 3rd of the conterminous United States; however, following the colonial and pioneer periods, respectively, these land cover categories were reduced to 70% and 50% of their original prominence. The dominant driving force for native land conversion was agriculture, which expanded exponentially from the Atlantic to the Pacific, comprising over half the total land area of America at its peak in 1950. However, farmland area has subsequently declined by 25%, so what has been driving native plant declines north of the 30th latitudinal parallel over the past 75 years? Analysis of recovery plans issued by the U.S. Fish and Wildlife Service indicates that of the over 900 plant species "listed" as threatened and endangered the primary driver of decline was invasive species, followed by habitat alteration, and development, which collectively accounted for 93.2% of the primary drivers for listed species. In Canada, these three drivers of decline were the primary drivers for 81% of listed species. Comparatively, herbicides were identified as the primary or secondary driver in 13 out of 1124 cases (1.2%). Given that agricultural land area is contracting in the U.S. and Canada, there appears to be a misconception that agrochemicals are the seminal cause of native plant decline. Here, we explore the individual contribution of drivers relative to the historical events of North America to provide context and perspective as well as focus and prioritize conservation efforts accordingly.

Responses of streamflow to forest expansion in a typical subhumid watershed under future climate conditions

Water availability in the subhumid region is highly vulnerable to frequent droughts. Water scarcity in this region has become a limiting factor for ecosystem health, human livelihood, and regional economic development. A notable pattern of land cover change in the subhumid region of the United States is the increasing forest area due to afforestation/reforestation and woody plant encroachment (WPE). Given the distinct hydrological processes and runoff generation between forests and grasslands, it is important to evaluate the impacts of forest expansion on water resources, especially under future climate conditions. In this study, we focused on a typical subhumid watershed in the United States - the Little River Watershed (LRW). Utilizing SWAT + simulations, we projected streamflow dynamics at the end of the 21st century in two climate scenarios (RCP45 and RCP85) and eleven forest expansion scenarios. In comparison to the period of 2000-2019, future climate change during 2080-2099 will increase streamflow in the Little River by 5.1% in the RCP45 but reduce streamflow significantly by 30.1% in the RCP85. Additionally, our simulations revealed a linear decline in streamflow with increasing forest coverage. If all grasslands in LRW were converted into forests, it would lead to an additional 41% reduction in streamflow. Of significant concern is Lake Thunderbird, the primary reservoir supplying drinking water to the Oklahoma City metropolitan area. Our simulation showed that if all grasslands were replaced by forests, Lake Thunderbird during 2080-2099 would experience an average of 8.6 years in the RCP45 and 9.4 years in the RCP85 with water inflow amount lower than that during the extreme drought event in 2011/2012. These findings hold crucial implications for the formulation of policies related to afforestation/reforestation and WPE management in subhumid regions, which is essential to ensuring the sustainability of water resources.

Trade-off of ecosystem productivity and water use related to afforestation in southcentral USA under climate change

The increase of tree canopy cover due to woody plant encroachment and tree plantations modifies both carbon and water dynamics. The tradeoffs between ecosystem net primary productivity (NPP) and water use with increasing tree cover in different climate conditions, particularly under future climate scenarios, are not well understood. Within the climate transition zone of the southern Great Plains, USA, we used the Soil and Water Assessment Tool+ (SWAT+) to investigate the combined impacts of increasing tree cover and climate change on carbon and water dynamics in three watersheds representing semiarid, subhumid, and humid climates. Model simulations incorporated two land use modifications (Baseline: existing tree cover; Forest +: increasing evergreen tree cover), in conjunction with two climate change projections (the RCP45 and the RCP85), spanning two time periods (historic: 1991-2020; future: 2070-2099). With climate change, the subhumid and humid watersheds exhibited a greater increase in evapotranspiration (ET) and a corresponding reduction in runoff compared to the semi-arid watershed, while the semi-arid and subhumid watersheds encountered pronounced losses in water availability for streams (>200 mm/year) due to increasing tree cover and climate change. With every 1 % increase in tree cover, both NPP and water use efficiency were projected to increase in all three watersheds under both climate change scenarios, with the subhumid watershed demonstrating the largest increases (>0.16 Mg/ha/year and 170 %, respectively). Increasing tree cover within grasslands, either through woody plant expansion or afforestation, boosts ecosystem NPP, particularly in subhumid regions. Nevertheless, this comes with a notable decrease in water resources, a concern made worse by future climate change. While afforestation offers the potential for greater NPP, it also brings heightened water scarcity concerns, highlighting the importance of tailoring carbon sequestration strategies within specific regions to mitigate unintended repercussions on water availability.

Spatiotemporal variability and key factors of evergreen forest encroachment in the southern Great Plains

Woody plant encroachment has been long observed in the southern Great Plains (SGP) of the United States. However, our understanding of its spatiotemporal variability, which is the basis for informed and targeted management strategy, is still poor. This study investigates the encroachment of evergreen forest, which is the most important encroachment component in the SGP. A validated evergreen forest map of the SGP (30 m resolution, for the time period 2015 to 2017) from our previous study was utilized (referred to as evergreen_base). Sample plots of evergreen forest (as of 2017) were collected across the study area, based on which a threshold of winter season (January and February) mean normalized difference vegetation index (NDVI_winter) was derived for each of the 5 sub-regions, using Landsat 7 surface reflectance data from 2015 to 2017. Then a NDVI_winter layer was created for each year within the four time periods of 1985-1989, 1995-1999, 2005-2009, and 2015-2017, with winter season surface reflectance data from Landsat 4, 5, and 7. By applying the sub-region specific NDVI_winter thresholds to the annual NDVI_winter layers and the evergreen_base, a SGP evergreen forest map was generated for each of those years. The annual evergreen forest maps within each time period were composited into one. According to the resulting four composite evergreen forest maps, mean annual encroachment rate (km²/year) was calculated at sub-region and ecoregion scales, over each of the three temporal stages 1990-1999, 2000-2009, and 2010-2017, respectively. To understand the spatiotemporal variability of the encroachment, the encroachment rate at each temporal stage was related to the corresponding initial evergreen forest area, mean annual precipitation (MAP), and mean annual burned area (MABA) through linear regression and pairwise comparison. Results suggest that most of the ecoregions have seen a slowing trend of evergreen forest encroachment since 1990. The temporal trend of encroachment rate tends to be consistent with that of MAP, but opposite to that of MABA. The spatial variability of the encroachment rate among ecoregions can be largely (>68%) explained by initial evergreen forest area but shows no significant relationship with MAP or MABA. These findings provide pertinent guidance for the combat of woody plant encroachment in the SGP under the context of climate change.

Response of Surface Runoff and Sediment to the Conversion of a Marginal Grassland to a Switchgrass (Panicum virgatum) Bioenergy Feedstock System

The land systems between the humid and arid zones around the globe are critical to agricultural production and are characterized by a strong integration of the land use and water dynamics. In the southern Great Plains (SGP) of the United States, lakes and farm ponds are essential components in the land systems, and they provide unique habitats for wildlife, and critical water resources for irrigation and municipal water supplies. The conversion of the marginal grasslands to switchgrass (Panicum virgatum) biofuel feedstock for energy production has been proposed in the region. However, we have limited experimental data to assess the impact of this potential land-use change on the surface runoff, which is the primary water source for surface impoundments. Here, we report the results from a paired experimental watershed study that compared the runoff and sediment responses that were related to the conversion of prairie to a low-input biomass production system. The results show no significant change in the relationship between the event-based runoff and the precipitation. There was a substantial increase in the sediment yield (328%) during the conversion phase that was associated with the switchgrass establishment (i.e., the site preparation, herbicide application, and switchgrass planting). Once the switchgrass was established, the sediment yield was 21% lower than the nonconverted watershed. Our site-specific observations suggest that switchgrass biofuel production systems will have a minimum impact on the existing land and water systems. It may potentially serve as an environmentally friendly and economically viable alternative land use for slowing woody encroachment on marginal lands in the SGP.

A Process-Oriented Model of Decision-Making toward Landscape-Scale Prescribed Fire Implementation in the Southern Great Plains, USA

In this investigation, we developed a model of the psychological drivers of landowners' decisions to implement prescribed fire on their properties. The Southern Great Plains in the USA evolved with fire and prescribed fire is an important management tool aimed at maintaining and enhancing ecological and economic resilience in the region. The conceptualized model is reflective of a decision-making paradigm that considers decision making to be a process inclusive of a variety of factors and their inter-relationships to arrive at judgments on whether or not to utilize prescribed fire. The approach considered a spectrum of inputs, obstacles, and their associations to capture the complexity of decision making that is often lost when modeling single factors in dynamic social-ecological settings. Further, we considered the decision to use prescribed fire as a multifactor process that incorporates not only individual barriers to fire implementation but inter-barrier associations and other inputs (e.g., sociodemographic variables). Path analysis revealed five statistically significant relationships within the hypothesized model. For prescribed fire decision making, women tended to be more analytical whereas men were more inclined to rely on heuristics. Additionally, those who indicated owning their property for non-consumptive recreation-related reasons were also more inclined to rely upon heuristics. Texans reported more experience with prescribed fire as did respondents who indicated owning property for livestock product. Alternately, those owning their property for an investment and non-consumptive recreation opportunities reported less experience with prescribed fire. Last, ownership for crop and livestock production was positively associated with past wildfire experience. Findings have implications for three issue areas: (1) the provision of an evolved conceptualization through which prescribed fire implementation decisions can be examined, (2) enhancing the approach of prescribed fire outreach to a changing landowner population, and (3) improving the content and delivery of prescribed fire education efforts.

Overcoming an "irreversible" threshold: A 15-year fire experiment

A key pursuit in contemporary ecology is to differentiate regime shifts that are truly irreversible from those that are hysteretic. Many ecological regime shifts have been labeled as irreversible without exploring the full range of variability in stabilizing feedbacks that have the potential to drive an ecological regime shift back towards a desirable ecological regime. Removing fire from grasslands can drive a regime shift to juniper woodlands that cannot be reversed using typical fire frequency and intensity thresholds, and has thus been considered irreversible. This study uses a unique, long-term experimental fire landscape co-dominated by grassland and closed-canopy juniper woodland to determine whether extreme fire can shift a juniper woodland regime back to grassland dominance using aboveground herbaceous biomass as an indicator of regime identity. We use a space-for-time substitute to quantify herbaceous biomass following extreme fire in juniper woodland up to 15 years post-fire and compare these with (i) 15 years of adjacent grassland recovery post-fire, (ii) unburned closed-canopy juniper woodland reference sites and (iii) unburned grassland reference sites. Our results show grassland dominance rapidly emerges following fires that operate above typical fire intensity thresholds, indicating that grassland-juniper woodlands regimes are hysteretic rather than irreversible. One year following fire, total herbaceous biomass in burned juniper stands was comparable to grasslands sites, having increased from 5 ± 3 g m^-2 to 142 ± 42 g m^-2 (+2785 ± 812 percent). Herbaceous dominance in juniper stands continued to persist 15-years after initial treatment, reaching a maximum of 337 ± 42 g m^-2 eight years post-fire. In juniper encroached grasslands, fires that operate above typical fire intensity thresholds can provide an effective method to reverse juniper woodland regime shifts. This has major implications for regions where juniper encroachment threatens rancher-based economies and grassland biodiversity and provides an example of how to operationalize resilience theory to disentangle irreversible thresholds from hysteretic system behavior.

Vulnerability of rangeland beef cattle production to climate-induced NPP fluctuations in the US Great Plains

The vulnerability of rangeland beef cattle production to increasing climate variability in the US Great Plains has received minimal attention in spite of potentially adverse socioeconomic and ecological consequences. Vulnerability was assessed as the frequency and magnitude of years in which net primary production (NPP) deviated >±25% from mean values, to represent major forage surplus and deficit years, for a historic reference period (1981-2010), mid-century (2041-2065), and late-century (2075-2099) periods. NPP was simulated by MC2, a dynamic global vegetation model, driven by five climate projections for representative concentration pathway (RCP) 4.5 and 8.5. Historically, 4-4.7 years per decade showed either NPP surpluses or deficits. The future number of extreme years increased to 5.4-6.4 and 5.9-6.9 per decade for RCP 4.5 and 8.5, respectively, which represents an increase of 33%-56% and 38%-73%, respectively. Future simulations exhibited increases in surplus years to between 3 and 5 years in the Northern Plains and 3-3.5 in the Southern Plains. The number of deficit years remained near historic values of 2 in the Northern Plains, but increased in the Southern Plains from 2.5 to 3.3 per decade. Historically, NPP in extreme surplus and deficit years both deviated 40% from mean NPP in all three regions. The magnitude of deficit years increased by 6%-17% in future simulations for all three regions, while the magnitude of surplus years decreased 16% in the Northern Plains and increased 16% in the Southern Plains. The Southern Plains was the only region to exhibit an increase in the magnitude of both surplus and deficit years. Unprecedented future variability of NPP may surpass the existing adaptive capacity of beef producers and adversely impact the economic viability of rangeland cattle production and ecological sustainability of rangeland resources.

Modeling rangelands as spatially-explicit complex adaptive systems

Rangelands cover one third of the earth's land area, provide livelihoods for one billion persons, and most have been degraded by overgrazing of domestic livestock. Recent debate about best management practices often has centered on comparison of continuous grazing and rotational grazing. Resolution to this debate may lie in viewing rangelands as complex adaptive systems. We describe a spatially-structured, individual-based model of rangelands that embodies this perspective, and simulate forage dynamics and cattle production under semi-arid rangeland conditions typical of the southern Great Plains of the USA employing both continuous and rotational grazing. Relative "success" of simulated grazing schemes depended primarily on the evaluation metric used (e.g., rangeland ecological condition, sale weight of cattle, secondary production efficiency) and the particular manage scheme employed, and neither continuous nor rotational grazing schemes were uniformly more successful. Our results demonstrate that solution of the grazing systems debate is unlikely to be found in a single group of grazing schemes, but, rather, in adaptive management of feedbacks among system components. The present work provides an example of how modeling rangelands as complex adaptive systems can aid in the evaluation of management schemes.

The ecological and economic determinants of eastern redcedar (Juniperus virginiana) encroachment in grassland and forested ecosystems: A case study from Oklahoma

Frequent fires were used as a management tool to maintain prairies, savannas, and woodlands in the southern Great Plains of the United States. However, fire exclusion beginning in the mid-1900s allowed for the establishment and growth of fire-intolerant species such as eastern redcedar (Juniperus virginiana: ERC) beyond their naturally occurring habitats. Apart from the reduction in burning, wide soil and climate adaptability, and seed dispersal by birds have facilitated the expansion of ERC in the southern Great Plains. The encroachment of ERC has caused heavy ecological and economic losses to Oklahoma and thus has been a major management concern for the past few decades. This study utilized count data modeling to analyze USDA Forest Service's (USFS) Forest Inventory and Analysis (FIA) data to investigate the relationship between available ecological and economic factors and the abundance of ERC in grassland and forested ecosystems of Oklahoma. The results suggested that low site productivity, high basal area, dense canopy, and silt loam soil texture significantly increase the abundance of ERC on a given site. The results also indicated that the rate of ERC encroachment is 3.3% higher in the softwood and 2.0% higher in the miscellaneous forests, compared to the hardwood forests. However, the economic variables of the study such as ownership type, adoption of active management, and proximity to a metropolitan area did not show a significant relationship to the abundance of ERC.

A Three-Dimensional Assessment of Soil δ13C in a Subtropical Savanna: Implications for Vegetation Change and Soil Carbon Dynamics

Tree/shrub encroachment into drylands is a geographically widespread vegetation change that often modifies soil organic carbon (SOC) storage and dynamics, and represents an important yet uncertain aspect of the global carbon (C) cycle. We quantified spatial patterns of soil δ13C to 1.2 m depth in a subtropical savanna to evaluate the magnitude and timing of woody encroachment, and its impacts on SOC dynamics. Woody encroachment dramatically altered soil δ13C spatial patterns throughout the profile; values were lowest in the interiors of woody patches, increased towards the peripheries of those patches, and reached highest values in the surrounding grasslands. Soil δ13C and 14C revealed this landscape was once dominated by C4 grasses. However, a rapid vegetation change occurred during the past 100–200 years, characterized by (1) the formation and expansion of woody patches across this landscape, and (2) increased C3 forb abundance within remnant grasslands. Tree/shrub encroachment has substantially increased SOC and the proportion of new SOC derived from C3 plants in the SOC pool. These findings support the emerging perspective that vegetation in many dryland ecosystems is undergoing dramatic and rapid increases in SOC storage, with implications for the C cycle at regional and global scales.