Woody Encroachment Research Articles

Watershed and fire severity are stronger determinants of soil chemistry and microbiomes than within-watershed woody encroachment in a tallgrass prairie system.

Fire can impact terrestrial ecosystems by changing abiotic and biotic conditions. Short fire intervals maintain grasslands and communities adapted to frequent, low-severity fires. Shrub encroachment that follows longer fire intervals accumulates fuel and can increase fire severity. This patchily distributed biomass creates mosaics of burn severities in the landscape-pyrodiversity. Afforded by a scheduled burn of a watershed protected from fires for 27 years, we investigated effects of woody encroachment and burn severity on soil chemistry and soil-inhabiting bacteria and fungi. We compared soils before and after fire within the fire-protected, shrub-encroached watershed and soils in an adjacent, annually burned and non-encroached watershed. Organic matter and nutrients accumulated in the fire-protected watershed but responded less to woody encroachment within the encroached watershed. Bioavailable nitrogen and phosphorus and fungal and bacterial communities responded to high-severity burn regardless of encroachment. Low-severity fire effects on soil nutrients differed, increased bacterial but decreased fungal diversity and effects of woody encroachment within the encroached watershed were minimal. High-severity burns in the fire-protected watershed led to a novel soil system state distinct from non-encroached and encroached soil systems. We conclude that severe fires may open grassland restoration opportunities to manipulate soil chemistry and microbial communities in shrub-encroached habitats.

Could CO2-induced changes to C4 grass flammability aggravate savanna woody encroachment?

Fire plays an important role in maintaining the savanna tree-grass balance by limiting the recruitment of heat-sensitive tree seedlings. However, fire behaviour may change under increasing CO2 concentrations, due to altered flammability of the grassy layer. Here, we determined the effect of predicted future CO2 concentrations, and how it interacts with water-availability, on grass flammability and traits influential to flammability, and uncovered the physiological mechanisms underpinning these responses. Using the widespread C4 savanna grass, Themeda triandra, as a model, we found that improved water-use efficiency under elevated CO2 (800 ppm) resulted in a larger (greater aboveground biomass), but wetter (higher moisture content) grass fuel load, that cured at a slower rate under drought conditions. These changes were associated with increased time to ignition, reduced flaming times and reduced predicted rate of spread. We modelled the effect of altered grass flammability on fire behaviour at a national level (South Africa), finding large-scale reductions in fire spread under elevated CO2, mitigating the converse effects of predicted increases in aridity, and marginal increases in fireline intensity. CO2-induced reductions in fire frequency, spread or intensity could have serious implications for savanna vegetation dynamics, possibly exacerbating the woody encroachment already seen in these ecosystems across the world.

Dominant woody plants alter soil microbial community composition during succession

Belowground assemblages are closely related to the aboveground vegetation and edaphic properties, which are also driven by dominant plants due to direct and indirect influences. However, the effects of dominant woody plants on the belowground organisms along successional gradients remain poorly understood. Plant and soil samples were collected from an initial herbaceous stage (i.e. alpine meadows) and four stages dominated by woody species, beneath and between patches of the dominant woody plants, to assess the effects of dominant woody plants on the succession of microbial communities along a secondary successional gradient. We quantified herbaceous, edaphic, bacterial, and fungal dissimilarities between stages to explore how dominant woody plants affect bacterial and fungal dissimilarities between stages using structural equation modeling. We found that dominant woody plants generally increase the succession of microbial communities in early stages, but decrease it in late stages. Our results further suggest that the herbaceous dissimilarity between stages plays more important roles than the edaphic one in mediating the effect of dominant woody plants on both bacterial and fungal dissimilarities between stages. Our results provide insight into the relative role of direct and indirect influences on microbial dissimilarity between stages and highlight the importance of dominant woody plants in driving microbial succession. As woody encroachment increases in alpine meadows, the dominant woody plants may have strong consequences on the dynamic of microbial communities, thereby affecting ecosystem functioning.

No evidence of positive feedback between litter deposition and seedling growth rate in Neotropical savannas

PurposePlant-soil feedbacks are important drivers of ecosystem dynamics and have been hypothesized to affect woody encroachment in savannas. Woody encroachment is expected to increases savanna soil fertility through deposition of organic matter, favoring further establishment of woody individuals. In this context, we tested if litter input promotes forest seedling growth in dystrophic savanna soils, and if this was accompanied by an increase in microbial activity.MethodsIn a glasshouse experiment, we planted woody seedlings of three forest species in savanna soils either mixed or not (control) with litter from closely related savanna or forest species (10 species). We evaluated the growth of the woody seedlings as well as the response of the soil microbiota activity and biomass to litter addition.ResultsLitter addition had either no effect or negative effects on seedling growth, and different seedling species responded differently to litter addition. However, we did find microbial activity to increase in response to litter addition, especially through the input of litter rich in phosphorus (P) and carbon (C).ConclusionsOur results indicate that litter input does not favor woody seedlings growth in savanna soil. Instead, litter input showed a potential to hinder seedling growth, especially of fast-growing species. Furthermore, litter input consistently increased soil microbiota activity, mainly through the input of P and C, highlighting the importance of energy and P in the nutrient dynamics of Cerrado. Thus, our results did not support the hypothesis that litter deposition triggers a positive feedback with woody encroachment via increased seedling growth.

Divergent Patterns and Spatial Heterogeneity of Soil Nutrients in a Complex and Dynamic Savanna Landscape

AbstractMany grassy ecosystems around the world are experiencing woody encroachment. These woody encroachers often cause nutrient enrichment in the plant‐soil environment, potentially facilitating their growth and reproduction. However, studies of encroachment effects on nutrient distributions have been confined to a few major elements (e.g.,N, and P) and limited in spatial extent. We analyzed 19 elements in dominant plants and in georeferenced soils across a subtropical savanna landscape experiencing woody encroachment to quantify their spatial patterns and elucidate drivers responsible for these patterns. We found divergent patterns of spatial heterogeneity of these elements in surface soils across this complex landscape. Nutrient accumulation and redistribution underneath woody canopies occurred in a subset of elements (i.e., N, P, S, Ca, Cu, and Sr). Though some of these elements are not necessarily growth‐limiting, they do occur in higher concentrations in woody compared to herbaceous plants. Distributions of the other elements were closely related to spatial variation in soil pH, clay content, and slope rather than to woody encroachment. Our nuanced spatial sampling approach and analysis reveal significant variation in nutrient distributions in response to woody encroachment, and illustrate the role of landscape patterns in mediating ecosystem processes. These changes in the concentrations and distributions of key essential nutrients broaden our understanding of the biogeochemical consequences of woody encroachment, and provide new insights regarding the significance of long‐term vegetation dynamics in dryland ecosystems.

Climate‐mediated shifts toward bunchgrass dominance 40 years after grazing cessation in Saguaro National Park, Arizona, USA

AbstractQuestionsVegetation transects were surveyed in Saguaro National Park, Arizona, in 1976 and 2007 as part of a grazing study. We resurveyed the same transects in 2018 and accessed fire and climate data in order to examine trends in plant community structure. We asked the question: has the plant community shifted in species’ density, canopy cover and diversity in the past 40 years since grazing cessation? And if so, do these shifts favor a specific species, or plant functional guild? We also asked if climate and time since fire influenced plant community structure?LocationSaguaro National Park East is located in the Sonoran Desert and Madrean Sky Island ecoregion interface in Tucson, Arizona, United States.MethodsPlots (10) were 20 m × 50 m, with 21 20‐m transects within each plot used to gather plant canopy cover and density of perennial species with the line‐intercept method. Data were aggregated by plot and analyzed using generalized linear mixed‐effects modeling and non‐metric multi‐dimensional scaling.ResultsOverall canopy cover, density, and Shannon diversity increased significantly between 1976 and 2018. Ferns, forbs, graminoids, subshrubs and vines increased significantly in cover and density between 1976 and 2018. Wildfire recency was significantly negatively correlated with forb density, and density or cover of five dominant species (mostly woody). Shifts in plant community structure between survey years was correlated with increased temperature, precipitation, and vapor pressure differential.ConclusionAfter 40 years of grazing cessation, significant increases in herbaceous cover, and a lack of significant increase in woody encroachment are signs of recovery. Increases in temperature may be responsible for accelerating shifts to warm‐season species‐dominant communities. Fire was unexpectedly not a significant factor in plant community structure; however, park management will likely need to address increased wildfire risk given the increase in herbaceous cover and density.

Recovery of herb‐layer vegetation and soil properties after pile burning in a Midwestern oak woodland

Thinning and removal of woody vegetation is the first step in restoring ecosystem structure to systems altered by woody encroachment. However, pile burning—a common method of eliminating woody residue at restoration sites—can promote the establishment of exotic species and adversely impact soils and native vegetation via extreme soil heating. Despite its widespread use, pile burning effects remain poorly understood in oak woodlands compared to coniferous forests. We examined how pile burning influenced soil properties and herb‐layer recovery in a Midwestern, U.S. oak woodland undergoing restoration via exotic shrub removal, tree thinning, and prescribed burning. We quantified soil properties and passive vegetation recovery inside and adjacent to pile burn scars after 3 years, and tested whether native seed additions in year 1 increased native cover and reduced exotic cover in year 3. Bare ground cover, soil pH, and concentrations of P and Ca remained elevated 3 years after pile burning. In contrast, native cover, native richness, and floristic quality recovered to levels similar to adjacent control locations. Pile burning did not promote exotic invasions, increase inorganic forms of N, or alter patterns of the native cover of most growth forms. However, Carex species failed to reestablish in burn scars. Compared to passive recovery, seed additions increased native cover in burn scars and reduced exotic cover in adjacent control locations. Our results indicate that burn scars can naturally recover in oak woodland, but native seed additions may accelerate this process and improve restoration outcomes.

Land degradation in South Africa: Justice and climate change in tension

Abstract Land degradation is a global problem impacting biodiversity and livelihoods, with profound effects on resource‐based livelihoods. As such, it impedes progress towards sustainable development goals (SDGs) and overcoming climate‐related poverty. Interrelated biophysical and social factors are driving further land degradation, and, internationally, there is a range of policies and initiatives designed to address these. In this paper, we argue that analysis of land degradation must encompass three key dimensions: firstly, that the causes are both physical and social; secondly, that they are shaped by historically unjust land tenure and resource allocations; and thirdly, that outcomes are the result of entwined processes at the global, national and local levels. To do this, we modify an analytical framework derived from structuration theory and populate it with illustrative material from the case of rangeland management in the Eastern Cape, South Africa. In this way, we show how understanding land degradation requires an analytical approach that is simultaneously bio‐social, historically informed and multiscalar. Land degradation caused by woody encroachment is a major bio‐social issue for the rangelands of South Africa, exacerbated by intersecting factors including climate change, historical land tenure policies and post‐apartheid reforms. However, contemporary land use policies in South Africa designed to redress historic land injustices and enhance rural livelihoods are not directly connected with those which prioritise the protection of ecosystems and biodiversity, or climate mitigation. Finally, South African policymakers face the challenge of reconciling political commitments to improve the lives of local populations whilst meeting international targets to address degradation, carbon emissions and SDGs. Whilst the South African case is unique, many countries face the simultaneous challenges of trying to prevent ecological degradation whilst mitigating historical patterns of unjust access to land and natural resources. More broadly, this talks of the global challenge of reconciling goals of poverty alleviation with climate mitigation. A free Plain Language Summary can be found within the Supporting Information of this article.

What and Why: South Dakota Rangeland Livestock Producers’ Usage of Parasiticides

In many mesic grasslands, such as the central Great Plains in North America, frequent fire is a key regulator of ecological processes. Long periods of infrequent fire facilitate the conversion of herbaceous-dominated grassland to woody-dominated shrubland or woodland. At the Konza Prairie Biological Station in northeast Kansas, one infrequently burned portion of the landscape has undergone transformation from grassland to woodland after nearly 30 yr without fire. In Spring 2017, a prescribed burn was implemented to assess fire effectiveness on woody plant mortality. A postfire census of 3 000+ individual woody plants identified the distribution of species by size (height), topographic position, and slope on the landscape. Mortality and canopy fire damage were calculated for each individual. In lowland locations with near-continuous shrub cover (30.7% of the landscape), woody plants were unaffected by fire. However, in upland and slope locations, where shrubs and trees were sparser, survival probability varied by topographic position and species. In these locations 68% of all woody individuals experienced 90% or greater fire damage to the canopy, with 56% of these individuals exhibiting new canopy regrowth within 2 mo after the fire. The two most abundant woody shrubs, Cornus drummondii and Rhus aromatica, showed high survival at all height classes and landscape positions. The two abundant tree species, Gleditsia triacanthos and Juniperus virginiana, showed increased survival probability with tree height that varied by landscape position. Survival of J. virginiana also varied according to proximity and size of neighboring clonal shrubs, providing a mechanism for persistence of this fire-sensitive tree species even at small height classes. The probability survival curves developed here are useful for managers assessing when to prescribe fire to maximize mortality for J. virginiana and provide insight relevant for broader ecological understanding of woody encroachment within grasslands throughout the world.

Changes in Soil Carbon Sequestration during Woody Plant Encroachment in Arid Ecosystems

Carbon sequestration is one of the most important and highly recommended measures for mitigating climate change. Soil organic carbon (SOC) has potential to sequester the largest amount of carbon (C) for the longest time period in the midst of the organic C sinks in terrestrial ecosystems of the earth. In recent years, apprehension of the role of soils as sink for carbon on a wide-ranging scale has become dynamic. From last 150 years, encroachment of trees and shrubs into grasslands and the ‘thicketization’ of savannas have been reported and is a global phenomenon. One possibly beneficial effect could be that the shrub and tree-dominated ecosystems will sequester more carbon and will be a buffer for elevated atmospheric carbon dioxide (CO2) levels. The question of what is impact of woody encroachment on soil carbon balance of an ecosystem has proved difficult to answer, and the results remain debatable. The magnitude and pattern of changes in the SOC with woody encroachment are exceedingly abstruse and varies from significant increases, to significant decreases to no net change in SOC. Impact of wood plant encroachment on carbon sequestration is discussed in this paper considering various studies with different results so it will lead to better understanding of the complex phenomenon. SOC sequestration is effective greenhouse gas mitigation strategy and a vital ecosystem service. Increasing SOC may helpful to mitigate negative effects of growing concentration of CO2 in atmosphere and may be advantageous in decelerating or reversal in global climate change rate.

Changes in Soil Carbon Sequestration during Woody Plant Encroachment in Arid Ecosystems

Carbon sequestration is one of the most important and highly recommended measures for mitigating climate change. Soil organic carbon (SOC) has potential to sequester the largest amount of carbon (C) for the longest time period in the midst of the organic C sinks in terrestrial ecosystems of the earth. In recent years, apprehension of the role of soils as sink for carbon on a wide-ranging scale has become dynamic. From last 150 years, encroachment of trees and shrubs into grasslands and the ‘thicketization’ of savannas have been reported and is a global phenomenon. One possibly beneficial effect could be that the shrub and tree-dominated ecosystems will sequester more carbon and will be a buffer for elevated atmospheric carbon dioxide (CO2) levels. The question of what is impact of woody encroachment on soil carbon balance of an ecosystem has proved difficult to answer, and the results remain debatable. The magnitude and pattern of changes in the SOC with woody encroachment are exceedingly abstruse and varies from significant increases, to significant decreases to no net change in SOC. Impact of wood plant encroachment on carbon sequestration is discussed in this paper considering various studies with different results so it will lead to better understanding of the complex phenomenon. SOC sequestration is effective greenhouse gas mitigation strategy and a vital ecosystem service. Increasing SOC may helpful to mitigate negative effects of growing concentration of CO2 in atmosphere and may be advantageous in decelerating or reversal in global climate change rate.

Variations in soil aggregation, microbial community structure and soil organic matter cycling associated to long-term afforestation and woody encroachment in a Mediterranean alpine ecotone

Afforestation and subsequent expansion of trees on former grasslands may significantly impact the structure and activity of the soil microbial community, altering soil aggregation and affect its potential to store and cycle organic matter (OM). We investigated OM dynamics in aggregate-size topsoil samples collected along a Mediterranean alpine ecotone consisting of three vegetation types (grassland/shrubland, mixed shrubland-pine, and pine forest) in central Spain. Analytical determinations of soil organic carbon (SOC), total nitrogen (TN), particulate OM (POM), mineral-associated OM (MaOM), and the stable isotopic composition of carbon were conducted in each of the four aggregate-size fractions considered. Additionally, the structure of the microbial community (assessed as PLFA abundance), and the β-glucosidase and β-glucosaminidase activities were determined in bulk soil samples. More than half of the soil mass was contained within small macroaggregates regardless of vegetation type. SOC and TN values increased with decreasing aggregate-size classes across all vegetation types. The stability of microaggregates was negatively affected by the expansion of woody vegetation, which resulted in tree-dominated stands showing comparatively lower SOC and TN values in the smaller aggregate-size classes. On the other hand, these vegetation dynamics promoted soil macro-aggregation. While SOC contents did not show significant differences between land covers, vegetation shifts induced changes in the soil microbial community. Soil δ13C values, the abundance of gram-positive bacteria and β-glucosidase activity were significantly higher in grasslands/shrublands than in forests, while significantly higher fungi/bacteria ratio was observed in forests. Small macroaggregates appear to play a key role in the stabilisation of relatively unprocessed OM across all vegetation types, as suggested by their significantly higher concentrations of POM. However, this fraction represents the most labile pool of OM, and as such, it is the most exposed to mineralisation. We conclude that the afforestation and potential vegetation shifts experienced in Mediterranean alpine grasslands lead to distinct changes in soil microbial communities, aggregation and soil OM dynamics, which given the strong temperature sensitivity to decomposition commonly reported in cold environments, suggests that soil OM in these high-elevation ecosystems may become highly vulnerable to environmental change.

Ecological Disturbance Through Patch‐Burn Grazing Influences Lesser Prairie‐Chicken Space Use

ABSTRACTAcross portions of the western Great Plains in North America, natural fire has been removed from grassland ecosystems, decreasing vegetation heterogeneity and allowing woody encroachment. The loss of fire has implications for grassland species requiring diverse vegetation patches and structure or patches that have limited occurrence in the absence of fire. The lesser prairie‐chicken (Tympanuchus pallidicinctus) is a declining species of prairie‐grouse that requires heterogeneous grasslands throughout its life history and fire has been removed from much of its occupied range. Patch‐burn grazing is a management strategy that re‐establishes the fire‐grazing interaction to a grassland system, increasing heterogeneity in vegetation structure and composition. We evaluated the effects of patch‐burn grazing on lesser prairie‐chicken space use, habitat features, and vegetation selection during a 4‐year field study from 2014–2017. Female lesser prairie‐chickens selected 1‐ and 2‐year post‐fire patches during the lekking season, ≥4‐year post‐fire patches during the nesting season, and year‐of‐fire and 1‐year post‐fire patches during post‐nesting and nonbreeding seasons. Vegetation selection during the lekking season was not similar to available vegetation in selected patches, suggesting that lesser prairie‐chickens cue in on other factors during the lekking season. During the nesting season, females selected nest sites with greater visual obstruction, which was available in ≥4‐year post‐fire patches; during the post‐nesting season, females selected sites with 15–25% bare ground, which was available in the year‐of‐fire, 1‐year post‐fire, and 2‐year post‐fire patches; and during the nonbreeding season they selected sites with lower visual obstruction, available in the year‐of‐fire and 1‐year post‐fire patches. Because lesser prairie‐chickens selected all available time‐since‐fire patches during their life history, patch‐burn grazing may be a viable management tool to restore and maintain lesser prairie‐chicken habitat on the landscape. © 2021 The Wildlife Society.

Woody encroachment happens via intensification, not extensification, of species ranges in an African savanna.

Widespread woody encroachment is a prominent concern for savanna systems as it is often accompanied by losses in productivity and biodiversity. Extensive ecosystem-level work has advanced our understanding of its causes and consequences. However, there is still debate over whether local management can override regional and global drivers of woody encroachment, and it remains largely unknown how encroachment influences woody community assemblages. Here, we examined species-level changes in woody plant distributions and size structure from the late 1980s to the late 2000s based on spatially intensive ground-based surveys across Kruger National Park, South Africa. This study region spans broad gradients in rainfall, soil texture, fire frequency, elephant density, and other topographic variables. Species-level changes in frequency of occurrence and size class proportion reflected widespread woody encroachment primarily by Dichrostachys cinerea and Combretum apiculatum, and a loss of large trees mostly of Sclerocarya birrea and Acacia nigrescens. Environmental variables determining woody species distributions across Kruger varied among species but did not change substantially between two sampling times, indicating that woody encroachers were thickening within their existing ranges. Overall, more areas across Kruger were found to have an increased number of common woody species through time, which indicated an increase in stem density. These areas were generally associated with decreasing fire frequency and rainfall but increasing elephant density. Our results suggest that woody encroachment is a widespread but highly variable trend across landscapes in Kruger National Park and potentially reflects an erosion of local heterogeneity in woody community assemblages. Many savanna managers, including in Kruger, aim to manage for heterogeneity in order to promote biodiversity, where homogenization of vegetation structure counters this specific goal. Increasing fire frequency has some potential as a local intervention. However, many common species increased in commonness even under near-constant disturbance conditions, which likely limits the potential for managing woody encroachment in the face of drivers beyond the scope of local control. Regular field sampling coupled with targeted fire management will enable more accurate monitoring of the rate of encroachment intensification.

Season of prescribed fire determines grassland restoration outcomes after fire exclusion and overgrazing

AbstractFire exclusion and mismanaged grazing are globally important drivers of environmental change in mesic C4 grasslands and savannas. Although interest is growing in prescribed fire for grassland restoration, we have little long‐term experimental evidence of the influence of burn season on the recovery of herbaceous plant communities, encroachment by trees and shrubs, and invasion by exotic grasses. We conducted a prescribed fire experiment (seven burns between 2001 and 2019) in historically fire‐excluded and overgrazed grasslands of central Texas. Sites were assigned to one of four experimental treatments: summer burns (warm season, lightning season), fall burns (early cool season), winter burns (late cool season), or unburned (fire exclusion). To assess restoration outcomes of the experiment, in 2019, we identified old‐growth grasslands to serve as reference sites. Herbaceous‐layer plant communities in all experimental sites were compositionally and functionally distinct from old‐growth grasslands, with little recovery of perennial C4 grasses and long‐lived forbs. Unburned sites were characterized by several species of tree, shrub, and vine; summer sites were characterized by certain C3 grasses and forbs; and fall and winter sites were intermediate in composition to the unburned and summer sites. Despite compositional differences, all treatments had comparable plot‐level plant species richness (range 89–95 species/1000 m2). At the local‐scale, summer sites (23 species/m2) and old‐growth grasslands (20 species/m2) supported greater richness than unburned sites (15 species/m2), but did not differ significantly from fall or winter sites. Among fire treatments, summer and winter burns most consistently produced the vegetation structure of old‐growth grasslands (e.g., mean woody canopy cover of 9%). But whereas winter burns promoted the invasive grass Bothriochloa ischaemum by maintaining areas with low canopy cover, summer burns simultaneously limited woody encroachment and controlled B. ischaemum invasion. Our results support a growing body of literature that shows that prescribed fire alone, without the introduction of plant propagules, cannot necessarily restore old‐growth grassland community composition. Nonetheless, this long‐term experiment demonstrates that prescribed burns implemented in the summer can benefit restoration by preventing woody encroachment while also controlling an invasive grass. We suggest that fire season deserves greater attention in grassland restoration planning and ecological research.

Facilitation by isolated trees triggers woody encroachment and a biome shift at the savanna–forest transition

Abstract Woody encroachment into grassy biomes is a global phenomenon, often resulting in a nearly complete turnover of species, with savanna specialists being replaced by forest‐adapted species. Understanding the mechanisms involved in this change is important for devising strategies for managing savannas. We examined how isolated trees favour woody encroachment and species turnover by overcoming dispersal limitation and environmental filtering. In a savanna released from fire in south‐eastern Brazil (Cerrado), we sampled woody plants establishing under 40 tree canopies and in paired treeless plots. These trees comprised eight species selected for habitat preference (savanna or forest) and dispersal syndrome (bird dispersed or not). We recorded dimensions of each tree, dispersal syndrome and habitat preference of recruits, and quantified the physical environment within each plot, aiming at a mechanistic understanding of woody encroachment. We found clear evidence that isolated trees cause nucleation and drive changes in functional composition of savanna. Effectiveness as nucleator differed among species, but was unrelated to their functional guilds (habitat preference or dispersal syndrome). The density of saplings in nuclei was partially explained by soil moisture (+), daily temperature amplitude (−) and sum of bases (−). Our results indicate that isolated trees act first as perches, strongly favouring bird‐dispersed species. They then act as nurse trees, considerably changing the environment in favour of forest‐adapted recruits. In the long term, as the nuclei expand and merge, savanna specialists tend to disappear and the savanna turns into a low‐diversity forest. Synthesis and applications. Fire suppression has allowed the nucleation process and consequently the woody encroachment and fast replacement of savanna specialists by forest species in the Cerrado. By elucidating the mechanisms behind woody encroachment, we recommend using prescribed fires to burn forest seedlings and to reduce tree canopy size wherever the management goal is to maintain the typical savanna structure and composition.

Woody encroachment of grasslands: Near-surface thermal implications assessed through the lens of an astronomical event.

Temperature has long been understood as a fundamental condition that influences ecological patterns and processes. Heterogeneity in landscapes that is structured by ultimate (climate) and proximate (vegetation, topography, disturbance events, and land use) forces serve to shape thermal patterns across multiple spatio‐temporal scales. Thermal landscapes of grasslands are likely shifting as woody encroachment fragments these ecosystems and studies quantifying thermal fragmentation in grassland systems resulting from woody encroachment are lacking. We utilized the August 21st, 2017, solar eclipse to mimic a rapid sunrise/sunset event across a landscape characterized as a grassland to experimentally manipulate levels of solar radiation in the system. We then quantified changes in near‐surface temperatures resulting from changes in solar radiation levels during the eclipse. Temperatures were monitored across three grassland pastures in central Oklahoma that were characterized by different densities (low, medium, and high) of Juniperus virginiana to understand the impact of woody encroachment on diurnal temperature patterns and thermal heterogeneity in a grassland's thermal landscape. The largest temperature range across sites that occurred during the eclipse was in the mixed grass vegetation. Similarly, the largest change in thermal heterogeneity occurred in the grassland with the lowest amount of woody encroachment. Thermal heterogeneity was lowest in the highly encroached grassland, which also experienced the lowest overall change in thermal heterogeneity during the eclipse. Time series models suggested that solar radiation was the most influential factor in predicting changes in thermal heterogeneity as opposed to ambient temperature alone. These results suggest that highly encroached grasslands may experience lower diurnal variability of temperatures at the cost of a decrease in the overall thermal heterogeneity of that landscape. It appears that fine‐scale spatio‐temporal thermal variation is largely driven by solar radiation, which can be influenced by vegetation heterogeneity inherent within a landscape.

Landscape Heterogeneity and Woody Encroachment Decrease Mesocarnivore Scavenging in a Savanna Agroecosystem

Increased agricultural intensification and extensive woody plant encroachment are having widespread effects on the functioning of grass-dominated systems at multiple spatial scales. Yet there is little understanding of how the provisioning of biodiversity-based ecosystem services might be altered by these ongoing changes. One fundamental ecosystem service that is decreasing globally, especially in human-altered landscapes, is scavenging that regulates disease processes, alters species distributions, and influences nutrient cycling. Accordingly, our goal was to understand how facultative scavenging, particularly that of mesocarnivores, was affected by landscape heterogeneity and woody encroachment in tropical-grassy savannas within an agricultural landscape mosaic. We baited (using chicken carcasses) plots across a gradient of land cover heterogeneity in areas with an open and closed canopy and subsequently measured scavenging rates. We found that scavenging efficiency of mesocarnivores and other small vertebrates was dependent on environmental variation at multiple spatial scales within our savanna agroecosystem. Mesocarnivores removed more bait when the overstory canopy at the plot (i.e., exact location of bait station) was more closed; in contrast, mesocarnivore scavenging was less efficient when patches (50 × 50 m area around the bait station) within the site had a higher density of shrubs. At the landscape scale, increased land cover fragmentation resulted in decreased amounts of scavenging by mesocarnivores. This study demonstrates that a relatively transformed agroecosystem can support the provision of important ecosystem services and offer an important buffer against loss of ecosystem services. Our results suggest that targeted woody encroachment control, protection of large trees, and management or mitigation of extreme levels of fragmentation can help maintain ecosystem service provision and biodiversity.

Correction to: Ecosystem sulfur accumulation following woody encroachment drives a more open S-cycle in a Subtropical Savanna

In the original version the name of the second author was misspelled. The correct name is Ayumi Hyodo.

Are field crops refuge for woody invaders? Rainfall, crop type and management shaped tree invasion in croplands

Woody encroachment represents a global concern which is mostly studied in natural and semi-natural grasslands and savannas. Woody invasion is a growing phenomenon in agricultural landscapes, but it remains elusive how different crop types and their management could shape woody invasion in croplands. Understanding the mechanisms involved in woody establishment in crops may contribute to design strategies to reduce the magnitude and consequences of invasion. We examined to what extent current agricultural practices affect early establishment of the invasive tree Gleditsia triacanthos (honey locust) in maize and soybean crops, during a wet and a dry growing season. In Pampean croplands (Argentina), we selected zero tillage fields of both crops and controlled management (with/without agricultural practices) and crop presence (with/without soybean or maize plants) during two growing seasons. After maize and soybean harvest, we examined survival, growth, and starch reserves of G. triacanthos seedlings. Results showed that G. triacanthos survival decreased from 76% to 34% in the dry year with respect to the wet year. Crop presence enhanced tree survival, likely by protecting seedlings from herbicide and desiccation, but reduced seedling growth. Seedlings were smaller in soybean than in maize, and larger when crop was removed, independent of crop type. Even so, woody survival was on average 61% and 49% after maize and soybean harvest, respectively. Interestingly, seedling starch reserves were modulated by crop type and management, and were positively associated with G. triacanthos survival. In conclusion, we found a hierarchy of controls acting on G. triacanthos establishment in cropping systems, with growing season rainfall variability, crop type and the applied management modifying tree survival and growth. Overall, our study highlights that, under current management, annually cropped fields may represent a refuge for woody plant invaders and a potential front of woody expansion.