Woody Encroachment Research Articles

Managing fires in a woody encroachment context: Fine fuel load does not change across fire seasons in a Guinean savanna (West Africa)

Fine surface fuels play a key role in driving fire spread, and therefore play an important role in wildfire management in savannas. In protected areas of the Guinean savannas (humid savannas of West Africa), despite prescribed early-dry season (EDS) or mid-dry season fire (MDS), woody encroachment is increasingly occurring. Recently, N'Dri et al. (2022) showed that late-dry season (LDS) fire applied at least three successive years alternating with a maximum of two successives EDS or MDS fires could reduce woody encroachment in protected areas of the regions. However, the relationship between time of burning (TOB) and fine fuel load (standing grass and fallen leaves of grasses and trees) remains uncertain. In a 10-year experimental field study (2013–2023), we monitored fine fuel load under annual fires set in three different TOB within the dry season: EDS, MDS and LDS. We also explored how fuel loads were influenced by fireline intensity and vice-versa, in addition to grass growth dynamic during one year after each fire. Results showed that grass grows faster in height after fires, reaching the highest peak (∼2 m) in November regardless of the TOB. Overall, the TOB within the dry season did not affect significantly the total fine fuel load even after 10 years (1.22 ± 0.12, 1 ± 0.09 and 0.96 ± 0.09 kg m−2 respectively for EDS, MDS and LDS fires). Nevertheless, in EDS plots, the greatest biomass of grass and litter was generally observed in alternate years. This is most likely due to a high decomposition rate and termite activity which played a rôle in maintaining basic levels. Overall, fireline intensity did not influence the subsequent fine fuel load recorded in November and fine fuel load recorded just before fires did not influence fireline intensity. Given the results of the current study, the best procedure in setting successive fires to reduce woody encroachment in Guinean savannas, remains that of N'Dri et al. (2022).

Woody Encroachment Modifies Subsurface Structure and Hydrological Function

ABSTRACTWoody encroachment—the expansion of woody shrubs into grasslands—is a widely documented phenomenon with global significance for the water cycle. However, its effects on watershed hydrology, including streamflow and groundwater recharge, remain poorly understood. A key challenge is the limited understanding of how changes to root abundance, size and distribution across soil depths influence infiltration and preferential flow. We hypothesised that woody shrubs would increase and deepen coarse‐root abundance and effective soil porosity, thus promoting deeper soil water infiltration and increasing soil water flow velocities. To test this hypothesis, we conducted a study at the Konza Prairie Biological Station in Kansas, where roughleaf dogwood (Cornus drummondii) is the predominant woody shrub encroaching into native tallgrass prairie. We quantified the distribution of coarse and fine roots and leveraged soil moisture time series and electrical resistivity imaging to analyse soil water flow beneath shrubs and grasses. We observed a greater fraction of coarse roots beneath shrubs compared to grasses, which was concurrent with greater saturated hydraulic conductivity and effective porosity. Half‐hourly rainfall and soil moisture data show that the average soil water flow through macropores was 135% greater beneath shrubs than grasses at the deepest B horizon, consistent with greater saturated hydraulic conductivity. Soil‐moisture time series and electrical resistivity imaging also indicated that large rainfall events and greater antecedent wetness promoted more flow in the deeper layers beneath shrubs than beneath grasses. These findings suggest that woody encroachment alters soil hydrologic processes with cascading consequences for ecohydrological processes, including increased vertical connectivity and potential groundwater recharge.

Warming and Rising CO2 Concentrations Drive Global Woody Encroachment from 2001 to 2020

Warming and Rising CO2 Concentrations Drive Global Woody Encroachment from 2001 to 2020

SooSim, a landscape model for assessing mire habitat degradation and restoration

Open mires constitute a characteristic part of boreal natural landscapes, which is under various cumulative anthropogenic pressures. As a response, remaining mires are increasingly protected, and their degradation is addressed by ecological restoration (mostly drainage closure). To evaluate alternative environmental policies, there is a necessity for high-resolution landscape simulation models to assess future dynamics of mires under different management scenarios. We present such a model, SooSim, its R-script, and derivation and validation of its key parameters. SooSim iterates mire types and woody encroachment dynamics within 25 × 25 m grid at 1-year intervals. Management interventions (restoration; ditch renovation) are sequentially introduced based on priority rules in locations delineated prior to simulation. We applied SooSim to three management scenarios, compared with natural succession, until 2050 in Estonia. The ‘current’ (2022) database comprised >3.8 M mire pixels and > 7 M peatland-forest pixels (sparse-cover ones considered for mire restoration). The model parameterization, based on Lidar data, revealed rapid ongoing woody encroachment across all mire types, with significant positive feedback. The simulations revealed that, even in scenarios with intensive restoration (2500 ha annually), open mire conditions are reduced by >10 % until 2050, while few mire types lose >1 % in area. Ditch renovations mostly reduced restoration perspectives in currently forested peatlands. Thus, SooSim explicitly depicts a decision-making dilemma where mire restoration is time-sensitive but also uncertain. To address this and related land-use dilemmas in the environmental policy, landscape models such as SooSim have further importance as visualization tools to explain complex processes to a wide range of stakeholders.

Global-scale water security and desertification management amidst climate change.

Deserts and semi-arid environments are habitats torare species, rich cultural heritage, and essential ecological processes. Approximately 46% of the world's surface area is covered by drylands (arid, semi-arid, and dry sub-humid areas), where 3 billion people live and unfortunately witness water insecurity and desertification implications. In this context, the present study argued that reduced dryland ecosystem services and decreased ecosystem health have resulted from the individual and compounding impacts of desertification, water scarcity, and climate change. At 1.5°C, 2°C, and 3°C of global warming, under the shared socio-economic pathway SSP2, the number of people living in drylands who will be affected by various effects on water, energy, and land sectors is projected to reach 951 million, 1152 million, and 1285 million, respectively. Due to combinations of land use change, rainfall variations, fire suppression, and CO2 fertilization, as well as unsustainable management, widespread woody encroachment has occurred in many shrublands and savannas in Africa, Australia, North America, and South America. This hasaltered biodiversity and reduces ecosystem services, such as water availability and grazing potential. The north side of the Mediterranean, southern Africa, and North and South America are projected to have the most semiarid expansion. Contrarily, drylands are expected to shrinkin India, northern China, eastern equatorial Africa, and the southern Sahara. Growing research evidence highlights the adoption of policy frameworks deriving the solutions from soil land management (SLM), indigenous and local knowledge (ILK), early warning systems coupled with adaptation and mitigation responses, and targets of sustainable development goals (SDGs).

Post-fire temporal dynamics of plant-pollinator communities in a tropical savanna.

Fire is a major ecological and evolutionary factor promoting biodiversity and maintaining functioning of naturally fire-prone ecosystems. In tropical savannas, plant communities show a set of fire-adapted traits and both flowering and pollination services have the potential to rapidly regenerate after fire, but fire-suppression policies may disrupt this adaptability following potential woody encroachment. Understanding the effects of fire on plant-pollinator interactions are required to advance conservation of biodiversity and ecosystem functioning. We evaluated the dynamics of plant community assemblage, flower availability, composition of flower functional traits associated with attractiveness to pollinators, and activity and diversity of insect pollinator guilds over ten post-fire stand ages along a 14-year chronosequence in a naturally burned region in the Cerrado, a megadiverse savanna in Brazil. We expect to find a high resilience of plant-pollinator communities and a steady decline in the successional recovery as time-since-fire proceeds. Along the post-fire chronosequence, vegetation was dominated by subshrubs with tubular, white, and nectar flowers arranged in inflorescences, while bees were the predominant pollinators. Plant assemblage and flower number showed an initial significant increase but monotonically declined after 7-9years after fire. Accordingly, pollinator richness and abundance significantly reached highest peaks in interim periods and a steady decline over time. In contrast, the frequency of community-wide plant-life form, flower functional traits, and pollinator diversity remained unaltered over the post-fire chronosequence. We added compelling evidence of a high post-fire resilience of plant-pollinator communities and further understanding of how fire-suppression policies may affect pollination in the Cerrado.

Unexpected hydrologic response to ecosystem state change in tallgrass prairie

In grasslands around the world, climate change is occurring in tandem with woody encroachment (the spread of woody vegetation in grass-dominated ecosystems). State transitions from grassland to shrub/woodland have been identified aboveground via changes in species cover and composition, but the hydrological impact of these transitions is not well understood. Shifts from grass- to woody-dominance have the potential to impact evapotranspiration, soil moisture dynamics, and groundwater recharge. Therefore, it is possible that the consequences of aboveground vegetation change may be observable in the hydrological system. We leveraged long-term hydrological records from a tallgrass prairie site in northeastern Kansas, USA, to examine how concurrent changes in climate and land-cover have altered hydrological dynamics over the last century. Stream discharge has declined in recent decades despite > 100 years of climate wetting. The relationship between incoming precipitation and streamflow has weakened over the last 40 years, suggesting that shifts in the physical landscape are altering patterns of hydrological connectivity. Long-term isotope records show a divergence in the isotopic composition of precipitation (no change in δ18O) and stream/groundwater (decreasing δ18O) over the last decade. These results suggest that woody encroachment is accelerating the hydrological cycle, potentially by decreasing groundwater recharge (via increased evapotranspiration) and/or increasing infiltration rates (via creation of macropores). Holistically, these changes illustrate the interdependence of above- and below-ground processes in the local hydrological cycle, and the cascading long-term consequences (decades to centuries) for critical zone function once woody encroachment has occurred.

Thinning relationships of woody encroachers in a US southwestern shrubland

Woody plant encroachment degrades the economic and environmental potential of drylands by altering processes such as nutrient fluxes, ecohydrology, and ecosystem services. Though past research has investigated the encroachment process, relatively little is known about post-encroachment shrub community ecology. To better quantify dynamics within post-woody encroachment shrub communities, we combined USGS lidar height data and multispectral imagery to estimate shrub density, shrub height, shrub cover, and shrub volume across the Jornada Basin Long Term Ecological Research (JRN-LTER) site in southern New Mexico, USA. Structural estimates were analyzed in search of telltale signs of community competition, specifically, the presence of thinning relationships. Results demonstrated density-dependent thinning relationships in shrub communities of creosote and mesquite, indicating a large role for competition in arid shrub communities even at relatively low shrub densities and cover (∼35% cover). In addition, shrub volume estimates better modeled the expected thinning dynamics of shrub communities than shrub canopy cover measurements. Overall, our results indicate the utility of lidar data in extending two-dimensional descriptions of woody vegetation structure (i.e., woody canopy cover) into the critical third dimension (i.e., woody plant volume), as well as the relative importance of competition and demographic bottlenecks to vegetation structure in drylands.

Resilience, remoteness and war shape the land cover dynamics in one of the world's largest miombo woodlands

The highlands of southeast Angola are one of the world's largest intact formations of miombo woodland. Recent interest from conservation groups is increasing the possibility of a new protected area in this conflict-afflicted, remote region, contributing to the “30 × 30” target of the Global Biodiversity Framework. With the potential for a new protected area, it is important to quantify the extent and change of natural and anthropogenic land covers in the region, not least because of the close dependence of livelihoods on natural resources in the miombo. We developed a 1990–2020 land cover time series, analysing deforestation, canopy opening, canopy closure, and vegetation regrowth after disturbance. Regional woodland extent has remained roughly constant despite frequent transitions between dense and open woodlands. Canopy opening peaked post-civil war, potentially related to the resettlement of displaced people. Over 30 years, 61 % ± 2 % of canopy opening was offset by subsequent canopy closure, which peaked a decade after the war ended, indicating the resilience of miombo systems. A woodland resource-use frontier, consisting of deforestation and canopy opening, is evident in the north-west of the area, likely driven by urban demand for agricultural products, charcoal, timber and other wood-derived goods. A distinct “core” of dynamic woodland occupies 52 % of the study region, where there is no evidence that shifting cultivation and local livelihoods are a net cause of land cover change. We do not find evidence for extensive net woody encroachment, only 2 % of the study region is being encroached by woody vegetation. This canopy closure is associated with remoteness from anthropogenic pressures and biophysical drivers that facilitate woody vegetation growth. Policymakers and conservation managers can use these data to aid in locating and prioritising interventions to sustainably produce agricultural and wood fuel products to meet increasing urban demand. Additionally, supporting conditions for maintaining both biophysical processes and livelihoods in remote areas is crucial to achieving 30 × 30 equitably.

Grassland woody plant management rapidly changes woody vegetation persistence and abiotic habitat conditions but not herbaceous community composition

Abstract Grasslands are among the most imperilled ecosystems worldwide, and many have experienced degradation due to the loss of historical disturbance regimes and subsequent woody encroachment. Management practitioners often use physical and chemical management interventions in combination with fire to counter encroachment, altering aboveground structure and belowground function, respectively. This may disrupt the feedbacks that perpetuate encroachment and restore the herbaceous community. We use a large‐scale field experiment to assess the initial effects of different management interventions on woody vegetation persistence, abiotic habitat conditions, and herbaceous community composition. We evaluate these effects across seven sites spanning a natural soil moisture gradient to capture one aspect of environmental heterogeneity with which managers regularly contend. We found that chemical intervention, both with and without the addition of physical intervention, was most effective at reducing woody plant cover and abundance, and a second application reduced woody plant abundance by more than one application alone. We also found that any management intervention increased light availability and air temperature and decreased soil moisture, with the combination of physical and chemical interventions having the greatest effects. Finally, none of the management interventions affected herbaceous richness and functional group cover within the study period, indicating delayed or nonexistent effects on herbaceous community composition. Synthesis and application. Our findings suggest that management should focus on chemical intervention for the greatest effects on woody plant persistence and abiotic habitat conditions. Changes to herbaceous community composition may occur in the long term and seem likely since short‐term effects of management were successful in altering processes related to encroachment feedbacks.

Do we need post-tree thinning management? Prescribed fire and goat browsing to control woody encroacher species in an Ethiopian savanna

Worldwide, bush encroachment threatens rangeland ecosystem services, including plant biodiversity and forage for livestock. Various control methods for encroaching woody species and restoring herbaceous vegetation exist but have rarely been explored experimentally. We assessed the impact of post-tree thinning management on tree mortality, the herbaceous community, and overall rangeland condition in Borana, an Ethiopian savanna ecosystem. At two 1.4 ha areas of encroached mono-specific Vachellia drepanolobium (whistling thorn) stands, we set up twenty-four 20 × 10 m experimental plots with four post-tree-thinning treatments (goat browsing only (1), prescribed fire (2), fire and goat browsing (3), and control (4) (i.e., no management after tree cutting), with three replications in a complete block design. Over two growing periods, we monitored resulting tree mortality, coppicing, seedling mortality and recruitment, as well as herbaceous layer attributes (diversity, biomass) and overall rangeland condition. All three post-tree thinning management scenarios significantly enhanced tree mortalities, reduced seedling recruitment and increased the abundance of the dominant desirable grass species. Prescribed fire and fire and goat-browsing treatments resulted in significantly greater grass and forb species richness, forb diversity, and biomass, as well as the overall rangeland condition compared to goat browsing only and the control treatment. However, grass species diversity did not respond to treatments. Post-tree management significantly increased tree mortality, reduced seedling recruitment, and increased the abundance of desirable grass species. Our findings strongly suggest that post-thinning management, particularly prescribed fire or a combination of fire and browsing, is highly effective in suppressing woody encroachment and improving biomass and overall rangeland condition.

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.

Post-Burn, Post-Flood Effects In A Degraded Grassland, Lake Texoma, Bryan County, Oklahoma

Plant communities change over time, sometimes leading to an increase or decrease in biological diversity. Often, absence of active management of a site leads to its degradation including loss of native species and invasion by non-native weeds. Lake Texoma, Texas and Oklahoma, represents an area where extensive landscape change has happened over the course of almost a century. The Denison Dam was completed in 1938, forming the lake, which over time has altered conditions in the forested and formerly-grazed locations surrounding it. The location studied in this paper is a 186-ha tract of land situated between Johnson Creek and the Roosevelt Bridge in Bryan County, Oklahoma. In summer 2000, a species list was compiled for a grassland located at the lake site as part of a larger study. This grassland comprised ~10% of the total site area. Following two major floods and an extended drought, the site was resampled in 2018. Results indicated it had suffered a serious decline in species richness and an increase in abundance of invasive or encroaching species. Species richness was reduced by approximately 50% between 2000 and 2018. Fewer transects were sampled in 2018 because of woody encroachment on the original site. In spring 2021, following an extensive prescribed burn, the site was resampled to see if burning led to any reduction in undesirable species. The most frequent species in 2000 included Panicum philadelphicum, Lespedeza virginica, Rudbeckia hirta and Ambrosia psilostachya and in 2018 they were Lespedeza cuneata, Ambrosia psilostachya, and Dichanthelium oligosanthes. It is possible that the invasive Lespedeza cuneata (sericea lespedeza) spread after a 2007 flood because of some combination of reduced competition and transport of seed in floodwater. In 2021, the most frequent species were the same as in 2018, showing little effect of the burn. However, the Shannon diversity and evenness in both early and late summer sampling periods after the burn were higher than those for the 2018 data, suggesting that the burn may have had some effect. To attempt to restore the site to more “native” conditions would probably require some combination of regular burning, flash grazing, and possibly herbicide use. Once sericea lespedeza establishes, it is very difficult to eradicate from a location.

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.

The effect of woody encroachment on taxonomic and functional diversity and soil properties in Cerrado wetlands

The encroachment of woody species has been affecting wetlands in the Cerrado biome, Brazil. This process has resulted in changes in plant diversity, soil physicochemical properties, and water availability. This study assessed changes in the taxonomic and functional diversity of plants and soil in Veredas wetlands in process of reduction in water availability and woody plant encroachment. Over a 14-year interval, we quantified taxonomic and plant functional diversity, soil physicochemical properties, groundwater depth, and its relationship with historical soil moisture using remote sensing in veredas. We investigated the effect of increased groundwater depth, woody vegetation cover, and changes in soil properties on species cover and richness. We observed a five to seven-fold increase in woody vegetation cover, accompanied by an increased in the richness of woody species from 21.4 % to 28.9 %. Nevertheless, this was counterbalanced by a 15.8 % to 35.2 % reduction in overall species richness and taxonomic diversity. At the same time, functional diversity decreased, leading to a community with more acquisitive traits. Groundwater depth increased from 20 to 60 cm. Soil properties changed, especially organic matter content, which increased two to 14 times. Changes in species richness and cover were related to increased organic matter and groundwater depth. The surveyed veredas exhibited considerable changes in plant species richness, soil, and hydrological properties, as well as woody vegetation cover, over 14 years. The veredas formed distinct functional composition groups between sampling times. However, only one vereda exhibited a reduction in functional richness, and neither showed temporal functional divergence. Woody encroachments are leading these veredas to an alternative state with a reduced herbaceous diversity, structurally denser, and with more resource-acquisitive plant traits in the community. The Woody encroachment reducing soil water availability, may impact ecosystem services, particularly water provision and biodiversity loss in the Cerrado region.

Reconstructing thicket clump formation using association rules analysis

AbstractAimVegetation change such as woody encroachment is characterised by changing species interactions, and processes such as competition and facilitation may be inferred from patterns of association between juveniles and mature individuals of different species. Our aim was to apply and evaluate association rules analysis (ARA), a rule‐based data‐mining technique more commonly known as market basket analysis, as a novel tool to examine the associations between woody species in different demographic stages along a thicket encroachment gradient.LocationThe research was conducted in a subtropical thicket–savanna mosaic (730 mm mean annual rainfall) in the Eastern Cape, South Africa.MethodsWe used a space‐for‐time substitution approach and sampled woody plants in different size classes at sites representing early, intermediate and late stages of encroachment. All individuals were recorded as occurring singly or in unique clumps. We used ARA to determine which associations between species in different size classes were common overall, and more common than expected, at each of the three encroachment stages.ResultsThe most important association rules indicated that Vachellia karroo recruited singly and in large numbers in open grassland and, once mature, provided nucleation sites for a small suite of species dominated by Scutia myrtina. In the later stages, multiple diverse associations were found in increasingly large clumps.ConclusionsOur sampling approach and ARA proved useful for characterising common species‐size class associations, illuminating changing species interactions and recruitment patterns along a thicket clump formation sequence. In studies of vegetation change, ARA can complement multivariate analyses of species composition to reveal specific associations, and it can provide a less laborious alternative to point‐pattern analysis for elucidating spatial associations.

Glacier retreat triggers changes in biodiversity and plant–pollinator interaction diversity

AbstractDue to global warming, the worldwide retreat of glaciers is causing changes in species diversity, community composition, and species interactions. However, the impact of glacier retreat on interaction diversity and ecological networks remains poorly understood. An integrative understanding of network dynamics may inform conservation actions that support biodiversity and ecosystem functioning after glacier extinction. Here, we address how glacier retreat affects the frequency, diversity, and complexity of plant–pollinator interactions, both directly and indirectly through biodiversity change. We surveyed flower visitors (pollinators) and analyzed pollination networks across a gradient of 170 years of glacier retreat (Mont Miné glacier, Valais, Switzerland) which ranges from patchy grasslands to closed forests. We reported a strong impact of glacier retreat on both plant and pollinator communities. Notably, the diversity of plant–pollinator interactions was sharply affected by glacier retreat: interaction diversity increased few years after glacier retreat, but it ultimately decreased in late stages dominated by forests. In contrast, we found that plant–pollinator network complexity did not change with glacier retreat. Our results indicate that the development of plant–pollinator networks is a two-phases process. In the first phase, glacier retreat makes space to plant colonization. This initial increase in plant diversity drives the increase in pollinator and interaction diversity. The second phase is characterized by turnover as woody species encroaches and dominates the community, decreasing the diversity of plant species in ultimate instance. The local decrease of plant diversity leads to a local decrease in pollinator and interaction diversity. Slowing down woody encroachment and enhancing flower diversity, which is initially supported by the glacial landscape, may be key strategies for halting the erosion of ecological networks while increasing biodiversity and ecosystem functioning. Our research thus can help resolve the overarching question of how to conserve ecosystems once glaciers are extinct, pointing toward a composite role of both habitat structure and biological functions.

Combined effects of fire and drought are not sufficient to slow shrub encroachment in tallgrass prairie.

Woody encroachment-the spread of woody vegetation in open ecosystems-is a common threat to grasslands worldwide. Reversing encroachment can be exceedingly difficult once shrubs become established, particularly clonal species that resprout following disturbance. Single stressors are unlikely to reverse woody encroachment, but using multiple stressors in tandem could be successful in slowing or reversing encroachment. We explored whether increasing fire frequency in conjunction with multi-year drought could reduce growth and survival of encroaching shrubs in a tallgrass prairie in northeastern Kansas, USA. Passive rainout shelters (~ 50% rainfall reduction) were constructed over mature clonal shrubs (Cornus drummondii) and co-existing C4 grasses in two fire treatments (1-year and 4-year burn frequency). Leaf- and whole-plant level physiological responses to drought and fire frequency were monitored in shrubs and grasses from 2019 to 2022. Shrub biomass and stem density following fire were unaffected by five years of consecutive drought treatment, regardless of fire frequency. The drought treatment had more negative effects on grass leaf water potential and photosynthetic rates compared to shrubs. Shrub photosynthetic rates were remarkably stable across each growing season. Overall, we found that five consecutive years of moderate drought in combination with fire was not sufficient to reduce biomass production or stem density in an encroaching clonal shrub (C. drummondii). These results suggest that moderate but chronic press-drought events do not sufficiently stress encroaching clonal shrubs to negatively impact their resilience following fire events, even when fire frequency is high.

Evaluating methods for measuring the leaf area index of encroaching shrubs in grasslands: From leaves to optical methods, 3-D scanning, and airborne observation

Leaf area index (LAI) is a key variable describing ecosystem structure and influencing the exchange of carbon, water, and energy. LAI is often evaluated with indirect methods. However, the accuracy of indirect measurements can vary with canopy structure and is not always generalizable across ecosystems. Previous research has characterized the accuracy of indirect methods for woody plants in forest ecosystems, but it is not well established for woody plants in open ecosystems—despite having large differences in canopy structure. In this study, we compared direct LAI measurements in clonal grassland shrub canopies to three indirect methods: a ceptometer, a handheld 3D scanner (processed using EXScanPro-3.6 software), and NEON's LAI product obtained from airborne hyperspectral imaging (derived from SAVI). To our knowledge, this is the first study to assess the accuracy of leaf area data in woody plants using a handheld 3D scanner and one of few studies assessing the accuracy of the National Ecological Observation Network's (NEON) Airborne Observation Platform—our source of airborne hyperspectral imaging. Data were collected in tallgrass prairie undergoing woody encroachment and three treatments: no herbivore disturbance, bison present, and simulated browsing. We found that direct LAI measurements of control and grazed C. drummondii canopies averaged ∼8.0. The ceptometer accurately estimated LAI in non-browsed canopies but overestimated LAI of browsed canopies by 38 %. One-sided leaf area of ramets measured with a handheld 3D scanner was strongly related to direct measurements (r2=0.86), but underestimated leaf area at greater values. LAI estimated from airborne spectral data underestimated LAI by 55 %. We conclude that a ceptometer was adequate for measuring LAI in dense shrub canopies when browsing was not present, the handheld 3D scanner was adequate for measuring leaf area of individual ramets, and the airborne spectral data was not suitable for estimating LAI of dense, grassland shrub canopies.

The long shadow of woody encroachment: An integrated approach to modeling grassland songbird habitat.

Animals must track resources over relatively fine spatial and temporal scales, particularly in disturbance-mediated systems like grasslands. Grassland birds respond to habitat heterogeneity by dispersing among sites within and between years, yet we know little about how they make post-dispersal settlement decisions. Many methods exist to quantify the resource selection of mobile taxa, but the habitat data used in these models are frequently not collected at the same location or time that individuals were present. This spatiotemporal misalignment may lead to incorrect interpretations and adverse conservation outcomes, particularly in dynamic systems. To investigate the extent to which spatially and temporally dynamic vegetation conditions and topography drive grassland bird settlement decisions, we integrated multiple data sources from our study site to predict slope, vegetation height, and multiple metrics of vegetation cover at any point in space and time within the temporal and spatial scope of our study. We paired these predictions with avian mark-resight data for 8 years at the Konza Prairie Biological Station in NE Kansas to evaluate territory selection for Grasshopper Sparrows (Ammodramus savannarum), Dickcissels (Spiza americana), and Eastern Meadowlarks (Sturnella magna). Each species selected different types and amounts of herbaceous vegetation cover, but all three species preferred relatively flat areas with less than 6% shrub cover and less than 1% tree cover. We evaluated several scenarios of woody vegetation removal and found that, with a targeted approach, the simulated removal of just one isolated tree in the uplands created up to 14 ha of grassland bird habitat. This study supports growing evidence that small amounts of woody encroachment can fragment landscapes, augmenting conservation threats to grassland systems. Conversely, these results demonstrate that drastic increases in bird habitat area could be achieved through relatively efficient management interventions. The results and approaches reported pave the way for more efficient conservation efforts in grasslands and other systems through spatiotemporal alignment of habitat with animal behaviors and simulated impacts of management interventions.