Woody Plant Expansion Research Articles

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.

Diet Mixing and Supplementation Present an Opportunity to Increase the Use of Encroaching Woody Plants by Goats.

Along with the woody plant expansion that is predicted to continue at the expense of the grassy layer is the increasing societal demand for animal protein and livestock products. Unless concerted efforts by land users, ecologists, and animal scientists are made to increase the utilization of trees and shrubs as forage, it will be impossible to meet future demand for meat and meat products. We conducted two short-term pen experiments to determine the effects of (1) supplementation with polyethylene glycol (PEG-a polymer purported to bind and neutralize the negative effects of tannins), a high-protein source (soybean meal), and a high-energy source (yellow maize grain) and (2) diet mixing (single-species vs. multispecies diets) on the intake of condensed tannin-rich woody plants (i.e., Searsia lancea, S. pyroides, and Euclea crispa) by goats. While all three forage species were used in the diet mixing experiment (Exp. 2), only E. crispa was used in the supplementation experiment (Exp. 1). Supplementing goats with energy- and protein-rich sources significantly increased the intake of E. crispa (p < 0.05), 713.4 g ± 13.5 and 760 g ± 28.9, respectively, whereas those on the control diet maintained their intake at 540.32 g ± 11.2. Although PEG tended to increase the consumption of E. crispa by goats, the observed increase was not significant (p > 0.05) from that observed in other treatments. In the diet mixing experiment, goats offered a combination of all three forage species attained substantially higher dry matter intakes compared to the goats offered these species individually (p < 0.05). While longer-term field experiments are needed in the African savannas, we postulate from the current results that management strategies that provide animals with (1) a variety of species in the diet vs. monocultures and (2) a combination of nutrient-rich and tannin-rich species may improve the ability of goats to consume chemically defended woody plants.

Elevated CO2 counteracts effects of water stress on woody rangeland-encroaching species.

The ubiquity of woody plant expansion across many rangelands globally has led to the hypothesis that the global rise in atmospheric carbon dioxide concentration ([CO2]) is a global driver facilitating C3 woody plant expansion. Increasing [CO2] also influences precipitation patterns seasonally and across the landscape, which often results in the prevalence of drought in rangelands. To test the potential for [CO2] to facilitate woody plant growth, we conducted a greenhouse study for 150days to measure CO2 effects on juveniles from four woody species (C. drummondii, R. glabra, G. triacanthos and J. osteosperma) that are actively expanding into rangelands of North America. We assessed chronic water-stress (nested within CO2 treatments) and its interaction with elevated [CO2] (800ppm) on plant growth physiology for 84days. We measured leaf-level gas exchange, tissue-specific starch concentrations and biomass. We found that elevated [CO2] increased photosynthetic rates, intrinsic water-use efficiencies, and leaf starch concentrations in all woody species but at different rates and concentrations. Elevated [CO2] increased leaf starch levels for C. drummondii, G. triacanthos, J. osteosperma, and R. glabra by 90%, 39%, 68%, and 41% respectively. We also observed that elevated [CO2] ameliorated the physiological effects of chronic water stress for all our juvenile woody species within this study. Elevated [CO2] diminished the impact of water stress on the juvenile plants, potentially alleviating an abiotic limitation to woody plant establishment in rangelands, thus facilitating the expansion of woody plants in the future.

Invasive and native woody plant encroachment: Definitions and debates

In this short opinion piece, we discuss the appropriate use of the term ‘invasion’ for woody plant expansion and refer to the various ways in which the term is being used in the literature. We point out the present confusion and make suggestions for the use of a more appropriate term (i.e., ‘woody plant encroachment’). We continue with an overview of the various definitions of ‘woody plant encroachment in the literature, we mention associated alternative terms, and we explain the circumstances in which each of these are used. With this piece, we hope to provide more clarity on the use of correct terminology related to woody plant expansion research.

Large contribution of woody plant expansion to recent vegetative greening of the Northern Great Plains

AbstractAimExtensive portions of high‐latitude grasslands worldwide have recently experienced increased vegetative productivity (i.e., greening) and have undergone a rapid transition towards woody plant dominance via the process of woody plant expansion (WPE). This raises the underlying question: To what degree are WPE and greening spatiotemporally linked? Given that these vegetative changes are predicted to continue, we seek to understand how recent changes in vegetation extent and productivity have interacted under recent climate change and anthropogenic disturbance to provide insights surrounding the future trajectory of temperate grasslands broadly.LocationNorthern Great Plains (NGP), North America.TaxonWoody plants.MethodsGreening was measured as the significant increase in three metrics between 2000 and 2019: leaf area index (LAI), annual maximum normalized difference vegetative index (NDVI) and annual mean NDVI. WPE was measured as the significant proportional increase in percent tree cover change between 2000 and 2019 in grasslands. We then examined these variables across a host of 26 potential driving variables.ResultsWe show that average proportional greening increased by 0.2–1.3% year−1 (depending on metric), and proportional WPE increased by 6.9% year−1 since 2000 across the NGP. Both changes are largely driven by the absence of wildfire and changing climate. Furthermore, WPE was spatially coherent and positively associated with a large component of recent greening, as revealed by their coupling across 34.1%–40.6% of grassland area and as evidenced by the 9.7%–19.7% of the variability in greening explained by WPE.Main conclusionsWPE and greening are spatiotemporally coupled across large portions of the NGP. Under continued climate change and wildfire suppression, WPE and greening are likely to continue across large swathes of grasslands globally. Furthermore, our results show that using a single greening metric may be insufficient to capture the large‐scale vegetative changes such as the expansion of woody vegetation.

Abandonment of Silvopastoral Practices Affects the Use of Habitats by the European Hare (Lepus europaeus)

Silvopasture, a traditional agroforestry practice, combines the presence of trees, shrubs, herbage, and livestock in time and space to provide multiple ecosystem services that contribute to human well-being. However, the abandonment of traditional agroforestry practices across Europe has led to substantial changes in vegetation characteristics, mainly due to woody plant expansion and, as a consequence, changes in wildlife that rely on open habitats. This study examines the effects of a 20-year abandonment of silvopastoral practices (i.e., livestock grazing and fuelwood harvesting) in a typical agroforestry Mediterranean landscape (kermes oak shrubland, natural grassland, and olive groves) on European hare (Lepus europaeus) habitat use. We estimated tree, shrub, and herb cover using a densitometer and hare habitat use using pellet counts within 2004-m2 rectangular plots in 2002, 2011, and 2021. Hare pellet density in olive groves was significantly lower in 2021 compared to 2002, while the opposite trend was found in grassland for the same period. Woody plant cover expanded from 2002 to 2021. We suggest that the woody plant encroachment that followed the abandonment of traditional silvopastoral practices in the area is the main driver behind the reported decline in hare use of the habitat, as it became less open and therefore less favorable for the species. Maintaining a mosaic of open and closed habitats at the landscape level, which was once provided by silvopastures, is vital for the conservation of this species.

Tracking spatial regimes as an early warning for a species of conservation concern.

In this era of global environmental change and rapid regime shifts, managing core areas that species require to survive and persist is a grand challenge for conservation. Wildlife monitoring data are often limited or local in scale. The emerging ability to map and track spatial regimes (i.e., the spatial manifestation of state transitions) using advanced geospatial vegetation data has the potential to provide earlier warnings of habitat loss because many species of conservation concern strongly avoid spatial regime boundaries. Using 23 yr of data for the lek locations of Greater Prairie-Chicken (Tympanuchus cupido; GPC) in a remnant grassland ecosystem, we demonstrate how mapping changes in the boundaries between grassland and woodland spatial regimes provide a spatially explicit early warning signal for habitat loss for an iconic and vulnerable grassland-obligate known to be highly sensitive to woody plant encroachment. We tested whether a newly proposed metric for the quantification of spatial regimes captured well-known responses of GPC to woody plant expansion into grasslands. Resource selection functions showed that the grass:woody spatial regime boundary strength explained the probability of 80% of relative lek occurrence, and GPC strongly avoided grass:woody spatial regime boundaries at broad scales. Both findings are consistent with well-known expectations derived from GPC ecology. These results provide strong evidence for vegetation-derived delineations of spatial regimes to serve as generalized signals of early warning for state transitions that have major consequences to biodiversity conservation. Mapping spatial regime boundaries over time provided interpretable early warnings of habitat loss. Woody plant regimes displaced grassland regimes starting from the edges of the study area and constricting inward. Correspondingly, the relative probability of lek occurrence constricted in space. Similarly, the temporal trajectory of spatial regime boundary strength increased over time and moved closer to the observed limit of GPC lek site usage relative to grass:woody boundary strength. These novel spatial metrics allow managers to rapidly screen for early warning signals of spatial regime shifts and adapt management practices to defend and grow habitat cores at broad scales.

Effects of fire seasonality and intensity on resprouting woody plants in prairie‐forest communities

Woody plant expansion is one of the greatest contemporary threats to fire‐dependent ecosystems. Reducing woody plant prevalence is often a primary objective of prescribed burns, yet little attention has been given to understanding the efficacy of burning to reduce their abundance. Fire intensity characteristics and plant phenology/physiology, which are sometimes presented as competing hypotheses, influence how woody plants respond to a fire event. Little work has been done in the prairie‐forest region of the upper Midwest to understand how fire characteristics interact with woody species phenology and/or physiology. Using a controlled field experiment, we examined effects of timing (seasonality) and intensity (temperature and duration) of fires on top‐kill and resprouting of three invasive woody plants in this region (common buckthorn,Rhamnus cathartica; bush honeysuckles,Loniceraspp.; and a native species, northern pin oakQuercus ellipsoidalis). Honeysuckles and pin oak burned in the spring dormant period, a common practice in the region, resulted in low levels of top‐kill and high levels of resprouting. Burning during the late growing season yielded highest levels of top‐kill and lowest levels of resprouting for honeysuckles and pin oaks. However, there was no apparent effect of season or fire intensity treatment for buckthorn stems. Under all treatment combinations, buckthorn was easily top‐killed but resprouted prolifically. Collectively, most prescribed burning in the Midwest appears to be conducted during the least effective season (early growing season), when top‐kill is reduced and/or resprouting most pronounced. Our results indicate that fire use could be better prescribed in this region for controlling woody plants.

Freezing temperatures restrict woody plant recruitment and restoration efforts in abandoned montane pastures

Tropical montane forests are being lost at an alarming rate but harbor some of the globe’s most unique biodiversity. The Hawaiian archipelago is a prime example of the importance of high elevation forests to species conservation and persistence as they serve as the last refugia for Hawaiian birds. Yet these forests have been converted to invasive dominated pastures, and efforts to restore them have been met with limited success. Unsuccessful forest restoration may be due to freezing temperatures acting as a demographic bottleneck by killing seedlings recruiting into pastures. We determined freezing tolerances of eight common native woody plants at a high-elevation forest on Hawaiʻi Island and compared these freezing tolerances to two years of site-specific winter temperatures and 17 years of regional temperature records. Low temperature extremes were more severe and common in pastures than under nearby 30-year-old canopy trees. Freezing temperatures over two years were severe enough to damage leaf tissues of six of eight species tested. Those species that displayed the greatest freezing tolerance were also those found naturally recruiting into open pastures. Temperature trends over the past 17 years show monthly minimum temperatures are not increasing as predicted by climate change. Persistent severe freezing events may limit seedling recruitment in the pasture, slowing native woody plant expansion into these abandoned pastures. The species-level differences in freezing tolerance show that current management actions are using species that are at high risk to freezing damage outside of the forest canopy and that alternative species may warrant consideration.

Critical transition to woody plant dominance through microclimate feedbacks in North American coastal ecosystems.

Climate warming is facilitating the expansion of many cold-sensitive woody species in woodland-grassland ecotones worldwide. Recent research has demonstrated that this range expansion can be further enhanced by positive vegetation-microclimate feedbacks whereby woody canopies induce local nocturnal warming, which reduces freeze-induced damage and favors the establishment of woody plants. However, this local positive feedback can be counteracted by biotic drivers such as browsing and the associated consumption of shrub biomass. The joint effects of large-scale climate warming and local-scale microclimate feedbacks on woody vegetation dynamics in these ecotones remain poorly understood. Here, we used a combination of experimental and modeling approaches to investigate the effects of woody cover on microclimate and the consequent implications on ecological stability in North American coastal ecosystems. We found greater browsing pressure and significant warming (~2°C) beneath shrub canopies compared to adjacent grasslands, which reduces shrub seedlings' exposure to cold damage. Cold sensitivity is evidenced by the significant decline in xylem hydraulic conductivity in shrub seedlings when temperatures dropped below -2°C. Despite the negative browsing-vegetation feedback, a small increase in minimum temperature can induce critical transitions from grass to woody plant dominance. Our framework also predicts the threshold temperature of -7°C for mangrove-salt marsh ecotones on the Atlantic coast of Florida. Above this reference temperature a critical transition may occur from salt marsh to mangrove vegetation, in agreement with empirical studies. Thus, the interaction between ongoing global warming trends and microclimate feedbacks may significantly alter woody vegetation dynamics and ecological stability in coastal ecosystems where woody plant expansion is primarily constrained by extreme low temperature events.

Effect of woody plant expansion on decomposition of fine root mixtures in a grass-dominated temperate wetland

Little is known about the effect of woody plant expansion on decomposition of root mixtures in grass-dominant temperate wetlands. Here, we collected fine roots (< 1 mm) of Deyeuxia angustifolia (grass) and Betula fruticosa (shrub) from a freshwater wetland in the Sanjiang Plain, Northeast China, and decomposition rates of their monocultures and mixtures (with 2:1 and 1:2 mass ratios) were determined by measuring carbon (C) mineralization and mass loss with a 365 days incubation experiment. Meanwhile, litter mixing effects were assessed by the differences in observed decomposition rates of root mixtures and predicted values based on the performance of component species decomposing alone. Betula fruticosa fine roots had higher N, total polyphenol, and condensed tannin concentrations than D. angustifolia fine roots. By the end of incubation, B. fruticosa fine roots generally had greater C mineralization rate and mass loss than the other three root treatments. Notably, both C mineralization rate and mass loss were lower than the expected values in the B. fruticosa-dominant mixture, whereas there were no significant differences between observed and expected values in the D. angustifolia-dominant mixture. Our findings imply that the increase in woody plant abundance will exert influences on fine root decomposition rates through altered substrate quality and litter mixing effects in grass-dominated temperate wetlands.

Alnus sibirica encroachment promotes dissolved organic carbon biodegradation in a boreal peatland

Symbiotic dinitrogen (N2)-fixing trees have been expanding to boreal peatlands, yet its influence on dissolved organic carbon (DOC) biodegradation is unclear. Here, we measured DOC, ammonium‑nitrogen (NH4+-N), nitrate‑nitrogen (NO3−-N), dissolved inorganic nitrogen (DIN), and dissolved total nitrogen (DTN) concentrations, specific ultraviolet absorbance at 254 nm (SUVA254), and humification index in the extracts obtained from peats in the 0–10 cm, 10–20 cm, and 20–40 cm depths in the open peatlands and Alnus sibirica islands in a boreal peatland, Northeast China. Afterwards, the peat extracts were used to assess the effect of N2-fixing woody plant expansion on DOC biodegradation with a 42-day incubation experiment. The expansion of A. sibirica significantly increased NH4+-N, NO3−-N, DIN, and DTN concentrations, but did not produce a significant effect on SUVA254 and humification index in the extracts in each depth. Following A. sibirica expansion, DOC biodegradation was enhanced by 24.5%, 15.4%, and 38.3% at 0–10 cm, 10–20 cm, and 20–40 cm depths, respectively. Furthermore, DOC biodegradation was significantly and negatively correlated with DOC:DIN and DOC:DTN ratios, but exhibited no significant relationship with SUVA254 and humification index. This implied that improved N availability and associated shifts in C:N stoichiometry determined the increase in DOC biodegradation following A. sibirica expansion. Our findings suggest that N2-fixing tree encroachment promotes microbial decomposition of DOC through improved N availability in boreal peatlands, which may cause organic C loss from soils in these C-enriched ecosystems.

Contrasting responses of soil nematode communities to native and non-native woody plant expansion.

Woody plant expansion into grasslands is widespread, driven by both successions to dominance by native woody species or invasion by non-native woody species. These shifts from grass- to woody-dominated systems also have profound effects on both above- and belowground communities and ecosystem processes. Woody-plant expansion should also alter the functional composition of the soil biota, including that of nematodes, which are major drivers of soil food-web structure and belowground processes, but such belowground impacts are poorly understood. We determined whether succession by a widespread native (Kunzea ericoides) and invasion by a non-native woody species (Pinus nigra) into tussock grasslands affect the composition of nematode functional guilds and the structure of nematode-based food webs. Although increasing dominance by woody species in both systems altered the functional guild composition of the nematode community, we found contrasting responses of nematode functional guilds to the different dominant plant species. Specifically, nematode communities reflected conditions of resource enrichment with increasing K. ericoides tree cover, whereas communities became structurally simplified and dominated by stress-tolerant nematode families with increasing P. nigra tree cover. Because nematodes regulate both bacterial- and fungal-dominated food webs in soils, these shifts could in turn alter multiple ecosystem processes belowground such as nutrient cycling. Incorporating species' functional traits into the assessment of habitat-change impacts on communities can greatly improve our understanding of species responses to environmental changes and their consequences in ecosystems.

Assessing the Roles of Fire Frequency and Precipitation in Determining Woody Plant Expansion in Central U.S. Grasslands

AbstractWoody plant expansion into grasslands and savannas is occurring and accelerating worldwide and often impacts ecosystem processes. Understanding and predicting the environmental and ecological impacts of encroachment has led to a variety of methodologies for assessing its onset, transition, and stability, generally relying on dynamical systems approaches. Here we continue this general line of investigation to facilitate the understanding of the roles of precipitation frequency and intensity and fire frequency on the conversion of grasslands to woody‐dominated systems focusing on the central United States. A low‐dimensional model with stochastic precipitation and fire disturbance is introduced to examine the complex interactions between precipitation and fire as mechanisms that may suppress or facilitate increases in woody cover. By using Lyapunov exponents, we are able to ascertain the relative control exerted on woody encroachment through these mechanisms. Our results indicate that precipitation frequency is a more important control on woody encroachment than the intensity of individual precipitation events. Fire, however, exerts a much more dominant impact on the limitation of encroachment over the range of precipitation variability considered here. These results indicate that fire management may be an effective strategy to slow the onset of woody species into grasslands. While climate change might predict a reduced potential for woody encroachment in the near future, these results indicate a reduction in woody fraction may be unlikely when considering anthropogenic fire suppression.

Slash Application Reduces Soil Erosion in Steep-Sloped Piñon-Juniper Woodlands

Mitigating runoff and associated erosion is a fundamental challenge for sustainable management of rangelands. Hillslope runoff and erosion are strongly influenced by ground cover; thus, a strategic management option exists to increase cover with slash from woody plant removal activities, particularly on lands experiencing woody plant expansion. Most studies assessing slash effects on runoff and erosion have been limited to moderate slopes; however, substantial portions of rangelands exist on steeper slopes where the effectiveness of slash application is less clear. On a steep (30% ± 5%) slope that had been encroached by piñon and juniper trees, we evaluated the effectiveness of slash in reducing runoff and erosion using a portable rainfall simulator (100-yr return period events). Although total runoff did not differ across slash levels, there was marginal evidence of a difference associated with vegetation cover. Sediment yield for plots with low vegetation cover (< 13% cover) was 3.4 times greater than those with high cover, while plots with slash present (≥ 30% cover) experienced 5.4 times less sediment yield than plots without slash. These results extend findings from moderate to steep slopes, highlighting the potential efficacy of slash application for reducing erosion in steep-sloped rangelands.

Understory Plant Community Responses to Fuel-Reduction Treatments and Seeding in an Upland Piñon-Juniper Woodland

Woody plant expansion and infilling into nonwooded rangeland ecosystems have been observed worldwide. Such expansion may lead to declines in herbaceous understory plant communities and increased fuel loads in rangelands. Under the US National Fire Plan, fuel-reduction treatments have been implemented over vast expanses of western forest types to reduce the risk of catastrophic wildfire and restore historical ecosystem structure, function, and diversity. The benefits of fuel-reduction may, however, also carry inherent ecological risk such as promoting non-native species colonization. Here, we compare understory plant community responses to three commonly used fuel-reduction treatments with seeding applications in an upland piñon (Pinus edulis Engelm.)–juniper (Juniperus osteosperma [Torr.] Little) woodland on the Colorado Plateau: 1) mechanical mastication, 2) lop and slash piled then burned (pile burn), and 3) lop and scatter followed by a broadcast burn (broadcast burn). Data were collected pretreatment (2009) and one (2010), two (2011), and six (2015) growing seasons post treatment. We found while understory perennial herbaceous plant cover remained low 1 and 2 yr post treatment, it increased by > 700% in all fuel-reduction treatment plots six growing seasons post treatment. Furthermore, while we observed minor increases in invasive annual grass, Bromus tectorum L. (cheatgrass), colonization in 2010 and 2011, there were substantial increases in B. tectorum cover by 2015. B. tectorum cover varied among treatments with the greatest cover in the unseeded mastication plot at nearly 30%. Seeding applications did not increase overall seed mix species cover but enhanced seed mix species richness and, thus, may have increased resistance to B. tectorum invasion in seeded treatment plots. Our findings offer valuable insights to the ecological consequences of fuel-reduction activities in piñon-juniper woodlands through comparison of common fuel-reduction treatments and seeding applications and highlight differences in understory plant community responses to treatments across short to longer time scales.

Mapping Tree Canopy Cover in Support of Proactive Prairie Grouse Conservation in Western North America

Invasive woody plant expansion is a primary threat driving fragmentation and loss of sagebrush (Artemisia spp.) and prairie habitats across the central and western United States. Expansion of native woody plants, including conifer (primarily Juniperus spp.) and mesquite (Prosopis spp.), over the past century is primarily attributable to wildfire suppression, historic periods of intensive livestock grazing, and changes in climate. To guide successful conservation programs aimed at reducing top-down stressors, we mapped invasive woody plants at regional scales to evaluate landscape level impacts, target restoration actions, and monitor restoration outcomes. Our overarching goal was to produce seamless regional products across sociopolitical boundaries with resolution fine enough to depict the spatial extent and degree of woody plant invasion relevant to greater sage-grouse (Centrocercus urophasianus) and lesser prairie-chicken (Tympanuchus pallidicinctus) conservation efforts. We mapped tree canopy cover at 1-m spatial resolution across an 11-state region (508 265 km2). Greater than 90% of occupied lesser prairie-chicken habitat was largely treeless for conifers (<1% canopy cover), whereas > 67% was treeless for mesquite. Conifers in the higher canopy cover classes (16−50% and >50% canopy cover) were scarce (<2% and 1% canopy cover), as was mesquite (<5% and 1% canopy cover). Occupied habitat by sage-grouse was more variable but also had a relatively large proportion of treeless areas (x− = 71, SE = 5%). Low to moderate levels of conifer cover (1−20%) were fewer (x− = 23, SE = 5%) as were areas in the highest cover class (>50%; x− = 6, SE = 2%). Mapping indicated that a high proportion of invading woody plants are at a low to intermediate level. Canopy cover maps for conifer and mesquite resulting from this study provide the first and most geographically complete, high-resolution assessment of woody plant cover as a top-down threat to western sage-steppe and prairie ecosystems.

Trees improve forage quality and abundance in South American subtropical grasslands

Woody plant expansion into rangelands has raised widespread concerns about the potential impacts on livestock production. However, the way in which trees influence the structure, composition and dynamics of herbaceous communities may vary widely depending on local conditions. We studied the effects of trees on the sub-humid grasslands of Uruguay, in southeastern South America, comparing the abundance, diversity and nutrient composition of the herbaceous plants growing under the canopy of isolated trees with those growing at adjacent open places. We analyzed the vegetation patterns at increasing distances from the edge of riparian forests, where tree cover is highest, into the open grasslands. We did not find significant differences between the total biomass of the herbaceous layer growing under and outside tree canopies, but the relative abundance of C3 grasses doubled under trees. Nitrogen content of grasses growing under tree canopies was significantly higher than in adjacent open grasslands, whereas no significant differences were found in P or fiber content. Our results suggest that scattered trees in subtropical grasslands can increase the abundance of high quality forage and contribute to improve the provisioning services of these rangelands.

Influence of resource availability onJuniperus virginianaexpansion in a forest–prairie ecotone

AbstractWoody plant expansion into grasslands and savannas is a global concern. Rapid expansion ofJuniperus virginiana, a tree native to North America, has profound ecological consequences. We used transplantedJ. virginianaseedlings to investigate the role of resource availability onJ. virginianaexpansion following the removal of fire, the factor historically limiting range expansion of this fire‐intolerant species. We evaluatedJ. virginianaseedling survival and seedling growth, two important phases in woody plant expansion, relative to two belowground resource factors, plant‐available soil water (soil clay content, an index of plant‐available soil water) and plant‐available nitrogen (PAN), and an aboveground factor, photosynthetic active radiation (PAR). In three plant communities associated with an oak forest–tallgrass prairie ecotone, we transplanted 2‐yr‐oldJ. virginianaseedlings in a systematic grid design and measuredJ. virginianaseedling survival and growth 8, 20, and 30 months following the transplant. We also measured soil clay content, PAN, and PAR in 1‐m2quadrats centered on each transplanted seedling. We employed path analysis at two spatial scales (144 and 2916 m2) to compare the role of resource factors in seedling growth and survival.Juniperus virginianaseedling survival was about 10% greater in tallgrass prairie and upland oak forest than in the old field, and seedling growth in tallgrass prairie exceeded the two other communities by about a factor of five. Tallgrass prairie in our study area is clearly more vulnerable to expansion ofJ. virginianathan the other two plant communities. Survival and growth were controlled largely by available light (PAR) and secondarily by plant‐available soil water, indexed by soil clay content. In all three vegetation types and at both fine and coarse scales, soil clay content also exerted an indirect effect, possibly by mitigating the influence of light. Results of this study suggest that soil distribution maps and associated ecological site designations might be useful for directingJ. virginianamanagement in the oak forest–tallgrass prairie ecotone by identifying the potential hotspots of invasion.

Woody expansion facilitates liana expansion and affects physical structure in temperate coastal communities

AbstractCoastal ecosystems in general, and barrier islands in particular, offer unique opportunities to observe ecological patterns and processes in spatially and temporally compressed settings. Harsh abiotic conditions and frequent disturbances limit the number of resident species, and highly dynamic geomorphology may result in a space‐for‐time substitution as young shoreward communities give way to more developed later‐successional interior communities. We investigated relationships between lianas and woody plants at barrier island sites in Virginia (Hog Island) and North Carolina (Duck FRF), to evaluate whether liana proliferation accelerates or delays succession from the dominant woody community, Morella shrub thicket, to the historic climax community, maritime forest. Using aerial imagery, Lidar data, community surveys, and edaphic sampling, we found a correlative relationship between abiotic environmental variables and woody community structure, and also between woody community distribution and liana success. Environmental variables demonstrated little predictive ability for liana distribution, but there was evidence of an association between lianas and the prominent woody species at Hog Island, though patchiness of the community and abiotic homogeneity prevented these associations from establishing at Duck FRF. We suggest that trends of woody plant expansion, compounded with current and predicted global change effects on growth and success of lianas, may contribute to a delay or prevention of further successional development at these and other temperate coastal sites.