Shrub Removal Research Articles

Arctic soil water chemistry in dry and wet tundra subject to snow addition, summer warming and herbivory simulation

Multiple and rapid environmental changes in the Arctic have major consequences for the entire ecosystem. Soil water chemistry is one component with important implications for understanding climate feedbacks, plant growth, microbial turnover and net greenhouse gas emissions. Here we assess the contrasting growing season soil water chemistry in a Low arctic Greenlandic mesic tundra heath and a fen, which have been subjected to factorial treatments of summer warming using open top chambers (OTCs), snow addition using snow fences, which increase soil temperature in late winter, and shrub removal mimicking herbivory attack. Dissolved Organic Carbon (DOC) and plant nutrients, including NO3−, NH4+, PO42+ and total dissolved N were measured during multiple growing seasons (2013–2016) to quantify the treatment effects on nutrient availability in two dominating, but contrasting, vegetation types.Ambient nutrient concentrations in the mesic tundra heath decreased throughout the growing season and increased during senescence, while concentrations were highest during peak growing season in the fen. The content of NH4+ and DOC were highest in the fen, whereas NO3− was highest in the mesic tundra heath. The fen had no seasonal pattern.Summer warming in the mesic tundra heath did not change the availability of nutrients, but in combination with shrub removal, both NO3− and DOC concentrations increased, likely due to reduced plant uptake. Shrub removal alone increased NO3− in one growing season, and, combined with snow addition, increased DOC. Significant effects of shrub removal were mostly found in 2016. Snow addition combined with summer warming increased DOC and total N concentrations and highlights the potential loss of dissolved C from the ecosystem.In the fen, shrub removal alone and combined with summer warming decreased DOC. Snow addition alone and in combination with summer warming similarly decreased DOC.In the mesic tundra heath, shrub removal caused higher soil water contents in all years. In the dry and warm 2016, it meant <10% soil water content in controls and 15–20% in shrub removal plots during the peak growing season, which may have relieved soil moisture limitation on mineralization rates in the latter.We conclude that soil water chemistry is vegetation-specific, and that treatment effects are surprisingly limited when comparing multiple years with contrasting precipitation patterns. Herbivory may have larger impact in very dry, warm summers and, together with extreme weather events, exert similar or larger effects than four years of temperature manipulations. The effects of summer warming or increased winter snow depend on ecosystem type and moisture status of the soil. The combination of multi-year and multi-site studies therefore seem important for understanding future biogeochemical dynamics in Arctic landscapes.

Impact of plant functional group and species removals on soil and plant nitrogen and phosphorus across a retrogressive chronosequence

Abstract In the prolonged absence of catastrophic disturbance, ecosystem retrogression occurs, which is characterized by declining soil nitrogen (N) and phosphorus (P) availability, increasing plant and soil N to P ratios, and reduced plant biomass and productivity. It is, however, largely unknown as to how the effects of plant communities on soil nutrients change during retrogression or might contribute to declining nutrient availability as retrogression proceeds. We studied a well‐characterized system of 30 lake islands in northern Sweden that collectively represent a 5,000‐year post‐fire retrogressive chronosequence. For each island, we established an experiment that involved full factorial removal of three plant functional groups (tree roots, dwarf shrubs and mosses), and of three species of dwarf shrub (Vaccinium myrtillus, V. vitis‐idaea and Empetrum hermaphroditum). After 19 years, we took various measures of soil N and P availability, and measured foliar N and P for each dwarf shrub species, for each plot in the experiment. Although plant removal effects (and particularly removal of tree roots, shrubs and Vaccinium species) on below‐ground N and P measures sometimes changed during retrogression, this seldom happened in a way that explains the decline in nutrient availability and increase in N to P ratios that characterize ecosystem retrogression. The only exceptions were that the positive effects of tree roots on soil mineral N and P, and of V. myrtillus on soil mineral P, declined during retrogression. Plant removal effects on community‐level measures of shrub N and P varied greatly across the chronosequence, but these effects again did not align with the changes in soil nutrient availability or N to P ratios that characterize ecosystem retrogression. Synthesis. Our results suggest that retrogression, and associated changes in nutrient availability and soil N to P ratios, is driven mainly by longer‐term pedogenic processes as opposed to shorter‐term effects of plant communities on soil N and P availability. More generally, they illustrate the value of long‐term and large‐scale experimental manipulations of plant communities for showing how effects of biodiversity loss on ecosystem properties vary across contrasting ecosystems.

Does habitat disturbance promote geographical parthenogenesis in whiptail lizards?

Sexual and asexual organisms often vary in their distribution and abundances among habitats. These patterns of “geographical parthenogenesis” can shed light on ecological conditions underlying the evolution of sex. Habitat disturbance is hypothesized to be a mechanism that generates geographical parthenogenesis. Parthenogens are predicted to be more prevalent in disturbed habitats than sexuals due to the greater colonizing ability of parthenogens and the tendency of parthenogens to avoid competition with sexuals in undisturbed habitat. We tested whether habitat disturbance (i.e., a rapid state transition between vegetation communities) causes geographical parthenogenesis in whiptail lizards in the Chihuahuan Desert of southern New Mexico. Non-experimental approaches have shown the parthenogenetic Aspidoscelis uniparens commonly occurs in habitat with a history of vegetation disturbance from shrub removal, whereas the sexual A. marmorata occurs more often in undisturbed shrubland habitat. We used a field experiment replicated across 16 sites to test whether the parthenogen A. uniparens and sexual A. marmorata differ in their response to vegetation disturbance from shrub removal. The sites were distributed across a broad region of southern New Mexico, and we used a paired design with each site including a shrub-removal treatment and a control on 9-ha plots. Using a co-abundance model that accounts for imperfect detection, we found the parthenogen A. uniparens and sexual A. marmorata both responded positively to disturbance, but only when the congener was rare. Our results are inconsistent with the idea that parthenogens exploit disturbed habitat to avoid competition with sexuals. In our study system, A. uniparens often dominates older disturbed sites, especially two decades or more after shrub removal. Collectively, these results indicate geographical parthenogenesis emerges from biotic interactions in heterogeneous landscapes that include disturbed habitats used by sexual and asexual species alike.

Clearing vs. burning for restoring Pyrenean grasslands after shrub encroachment

Anthropogenic activities have modified vegetation on subalpine belts for a long time, lowering the treeline ecotone and influencing the landscape mainly through grazing and fire. The abandonment of these traditional land use practices during the last decades and global warming are contributing largely to the colonization of woody species in subalpine grasslands, causing irreversible changes in ecosystem functioning. A variety of management strategies requiring the use of fire and/or clear-cutting are carried out to prevent the expansion of highly encroaching shrubs and improve the conservation status of subhumid high-productive grasslands ecosystems. However, it is still poorly understood how different management strategies affect the recovery of subalpine grasslands, which is influenced concurrently by the vegetation community involved. In this study we used field experiments to test the impact of management treatments on soil properties and vegetation responses in subalpine Bromion erecti grassland communities colonized by the pyrophyte shrub Echinospartum horridum (Vahl) Rothm. on the southern Pyrenees. Vegetation was monitored for 5 years in E. horridum plots (two plots per treatment) where the vegetation was removed by prescribed fire (Burnt treatment) or by mechanical removal (Clear-cut treatment). Undisturbed E. horridum plots were used as a control (C-Erizón) for shrub removal treatments and a Bromion erecti grassland community regularly grazed (C-Grass) was used as a control for subalpine grassland. Clear-cut treatment of E. horridum community was more effective to control E. horridum colonization than Burnt treatment and contributed to a higher extent to recover original grassland vegetation, because E. horridum seedling performed worse (lower germination rates) and plant species were more similar to the original grassland (legumes, non-legume forbs, and grasses). Shrubs and sub-shrubs cover in Burnt areas increased faster than in Clear-cut areas during the 5 years following the treatment, although it did not reach the level of C-Erizón. Species richness and diversity increased in comparison with C-Erizón in both treatments, but they were significantly lower than those in the C-Grass. Network connectivity was larger in well preserved grasslands, i.e, C-Grass, than in any other treatments, mainly dominated by negative plant-plant association. Soils nutrients declined in Burnt sites 4 years after the fire treatment and no difference was observed between Clear-cut and C-Erizón sites, although they did not reach the soil fertility values of C-Grass in terms of organic matter and C/N ratio. This study showed that grazing favors plant diversity and community complexity in subalpine grasslands. It also demonstrated that clearing is a better strategy than burning to restore grasslands after shrub encroachment because burning entails deeper soil degradation and faster recovering of the pyrophyte shrub, E. horridum.

Dominant plant species influence nematode richness by moderating understory diversity and microbial assemblages

Belowground assemblages are tightly linked to the aboveground vegetation, and often differ between plant species and vegetation types due to direct and indirect influences. However, the relative contribution of these direct and indirect influences of plants on belowground organisms differs among taxa and remains poorly understood. We established a plant removal experiment to better understand the effects of dominant plant species on nematode assemblages in an alpine meadow on the Tibetan plateau. We quantified plant community diversity, soil microbial assemblage richness and edaphic properties to explore how a dominant shrub and graminoids affect nematode assemblage structure and community-weighted mean (CWM) biomass using structural equation modelling (SEM). We found contrasting responses of nematode richness and CWM biomass of nematodes to the removal of shrubs and graminoids, including an intriguing interaction between them. The latter indicates that interactions between shrubs and graminoids modify nematode communities rather than their independent effects. Our results further suggest that dominant plants predominantly indirectly influence nematode richness through changes in biotic variables, specifically understory diversity, and bacterial and fungal richness. However, the dominant plants had no direct and limited indirect effects on CWM nematode biomass, which instead increased with soil nitrogen content, and decreased with soil pH, carbon and ammonium content. Our results provide insight into the relative role of direct and indirect influences on belowground assemblages and highlight the importance of dominant plants in maintaining belowground diversity. By contrast, edaphic properties appear more important to belowground biomass, likely reflecting longer term influences of the plant community on resource availability and quality. Vegetation changes in grassland, such as reduced dominance of shrubs and graminoids, might therefore reduce soil biodiversity relatively rapidly but contribute to longer term impacts through changes in biogeochemical cycles.

Climate Effect on Ponderosa Pine Radial Growth Varies with Tree Density and Shrub Removal

With increasing temperatures and projected changes in moisture availability for the Mediterranean climate of northern California, empirical evidence of the long-term responses of forests to climate are important for managing these ecosystems. We can assess forest treatment strategies to improve climate resilience by examining past responses to climate for both managed and unmanaged plantations. Using an experimental, long-term density and shrub removal study of ponderosa pine (Pinus ponderosa Lawson &amp; C. Lawson) on a poor-quality site with low water-holding capacity and high runoff of the North Coastal mountain range in California, we examined the relationships between radial growth and climate for these trees over a common interval of 1977–2011. Resistance indices, defined here as the ratio between current year radial growth and the performance of the four previous years, were correlated to climatic variables during the same years. We found that all treatments’ radial growth benefited from seasonal spring moisture availability during the current growing year. Conversely, high spring and early summer temperatures had detrimental effects on growth. High-density treatments with manzanita understories were sensitive to summer droughts while lower densities and treatments with full shrub removal were not. The explanatory power of the climate regression models was generally more consistent for the same shrub treatments across the four different densities. The resistance indices for the lower density and complete shrub removal treatment groups were less dependent on previous years’ climatic conditions. We conclude that, for ponderosa pine plantations with significant manzanita encroachment, understory removal and heavy thinning treatments increase subsequent growth for remaining trees and decrease sensitivity to climate.

Vegetation responses and trade‐offs with soil‐related ecosystem services after shrub removal: A meta‐analysis

AbstractAimTo assess the sustainability of different shrub control practices (fire, mechanical, and chemical) based on their efficacy to control shrubs and their effects on multiple ecosystem service provisions, including possible trade‐off and/or synergy.MethodsUsing a meta‐analysis approach, this study synthesized results from global shrub removal experiments. Log response ratio (lnR) between the outcome of shrub removal and that of the untreated control was used to estimate proportional changes in soil and vegetation properties resulting from each shrub control practice.ResultsWhen forage provisioning is the only service considered, shrub removal could achieve this desirable outcome as indicated by increasing herbaceous biomass. However, observable decreases in litter, biological crust cover, and soil nutrients, as well as increases in bare soil indicated long‐term potential trade‐offs with other ecosystem services (e.g., erosion control service and nutrient cycling); the degree was influenced by different shrub control methods. Synergistic properties were probably limited to a short‐term boost of herb productivity resulting from short‐term increase in herb biomass and diversity as well as nutrient availability.ConclusionHuman‐induced drivers manifested in shrub control practices changed vegetation response. However, management also changed non‐targeted processes, generating potential reduction in several regulating ecosystem services. Continuous monitoring to assess landscape conditions should therefore become the key for adaptive management. Sustainable forage production should focus on strategies to maintain multiple ecosystem services because consideration of those services can lead to long‐term protection of the landscape and provide a broader range of environmental benefits.

Nest box selection and reproduction of European Rollers in Central Italy: a 7-year study

BackgroundChanges and increased mechanisation of agricultural practices have influenced the biodiversity composition of farmland habitats and caused a decline of bird communities in many European countries. The removal of shrubs and large trees rich in natural cavities, has also led to a drastic decrease in nest site availability for cavity-nesting bird species. Nest-boxes are a common conservation tool used to improve nest-site availability, and have helped to reverse declines in many endangered bird populations. Nonetheless to maximize the results of such interventions it is crucial to know where nest-boxes should be sited. The objective of this study was to investigate the effectiveness of the nest-box program for the European Roller (Coracias garrulus) population of Lazio region (Central Italy). More specifically, we focused on what landscape features were preferred (or avoided) in the process of nest box selection and how they influenced population's breeding parameters. Particular attention was paid to identifying potential limitations and to provide management recommendations for future interventions.MethodsUsing data from 70 nest boxes sited on power lines monitored over a 7-year period (representing 140 breeding attempts), we developed probability functions to evaluate if nest box location, in terms of distance from habitat resources and habitat composition and structure, had an effect on nest box occupancy and on the main reproductive parameters.ResultsNest boxes were more likely to be occupied if they were located near arable fields and in areas characterized by a higher amount of incoming solar radiation. Higher fledging success was associated with fallow fields and with a moderate/low habitat structural complexity. Higher breeding success was associated with solar radiation and with greater distance from urban areas.ConclusionsOur results highlight the importance of specific habitat variables in influencing nest occupancy, and show which drivers primarily affect species’ reproduction and persistence over time. Siting nest boxes in habitats where occupancy rate and fledging success is higher, such as in arable and fallow fields and on south-facing slopes where solar radiation is maximised, may help to extend the suitable habitat for rollers and facilitate its local expansion.

One-hundred years after shrub encroachment: Policy directions towards sustainable rangeland-use

In many shrub-encroached lands, livestock grazing is the dominant land-use type which shapes stakeholders’ perspective on single ecosystem service provision (i.e., forage production). Although recent ecological studies suggested otherwise, recognition of multiple ecosystem services from shrublands is rarely translated into policy, likely due to the lack of robust scientific evidence on trade-offs between ecosystem services following shrub removal. Based on meta-analysis from global publications, we found that while shrub removal increased forage provision, such effect was generally short-lived (˜5 years). At the same time, shrub removal also increased bare soil, decreased soil nutrients and soil organic carbon, which potentially reduced erosion control and nutrient cycling service. These trade-offs tended to be more prominent with increasing disturbance intensity (i.e., higher shrub removal frequencies or switching from single to multiple shrub removal methods). To encourage adaptation to the provisioning of multiple ecosystem services from shrublands, we provided a framework on how to value such landscapes, including the estimated monetary values that could be generated from maintaining them. Since there will be a time-lag until most monetary values appear, initial incentives may be necessary to encourage the adoption of conservation practices, in addition to efforts to increase society awareness (e.g., eco-labelling, alternative food networks or social media) on multiple ecological benefits from shrublands and investment to provide capacity building for pastoralists.

Invasive shrub removal benefits native plants in an eastern deciduous forest of North America

AbstractIn eastern deciduous forests of North America, invasive shrubs are increasing in richness and abundance at the expense of native species across taxa. Invasive shrubs create an understory that is more dense than both recent and historical preinvasion conditions. Interest in invasive shrub removal to restore native habitat is growing, but our understanding of natural regeneration following treatment of a diverse invasive shrub community is lagging. Using an invasive shrub removal experiment, we provide insight into the effect of repeated removal of a suite of 18 invasive shrub species dominated by border privet (Ligustrum obtusifolium Siebold &amp; Zucc.). In 2009, invasive shrubs were removed from five 20-m-diameter treatment plots, each with a paired control plot. Seven years later, we find an increase in plant diversity, native understory species abundance, and overstory tree species regeneration for individuals under a meter in height. For plants 1 to 4 m in height, the removal treatment has a positive effect on understory woody species, but there has been no change in regenerating overstory trees. A lack of overstory tree regeneration to greater heights is not surprising, given the time frame and the closed-canopy conditions. However, other factors, such as white-tailed deer (Odocoileus virginianus Zimmermann) browse, could be serving as an impediment to taller tree regeneration in the forest understory. An ambient sampling approach in unmanaged, invaded, and uninvaded forest has been used in other studies to estimate the potential impacts of invasive shrub species to native plant communities. However, in this study the ambient sampling approach underestimated the impacts of invasive shrubs compared to their experimental removal. Overall, invasive shrub removal increased plant diversity and allowed passive natural regeneration of native plants that exceeded native cover in the unmanaged, ambient forest under minimal invasive shrub abundance.

Coastal Prairie Recovery in Response to Shrub Removal Method and Degree of Shrub Encroachment

Shrub encroachment into grasslands is a worldwide phenomenon with no signs of abating and numerous ecological consequences. In South Texas, honey mesquite (Prosopis glandulosa Torr.) and huisache (Vachellia farnesiana [L.] Wight & Arn.) are two shrubs encroaching into coastal prairies, reducing cover of the dominant native grass, gulf cordgrass (Spartina spartinae [Trin.] Merr. ex Hitchc.), and decreasing habitat for the endangered Aplomado Falcon (Falco femoralis), which requires grasslands or savannas for survival. To determine the best management approach for deterring shrub encroachment and restoring native grasslands, the US Fish and Wildlife Service used several shrub removal techniques within coastal prairies of the Bahía Grande Wetland Complex of the Laguna Atascosa National Wildlife Refuge, a core site for Aplomado Falcon reintroductions. Here, we assess native grass recovery over a 2-yr period in response to these shrub removal methods (mechanical plus prescribed fire and/or herbicide treatments) and degree of shrub encroachment before treatment. In general, areas with high levels of shrub encroachment before treatment had the highest amount of bare ground and lowest grass cover immediately following an initial mechanical treatment; this legacy effect persisted throughout the study irrespective of shrub removal treatment. Regardless of degree of shrub encroachment before treatment, grasses in areas treated with either mechanical or mechanical followed by herbicide methods recovered the slowest, likely due to residual woody material that hindered seed germination. Herbicide treatment following mechanical removal or mechanical removal plus fire effectively hindered shrub regrowth. Overall, mechanical treatment followed by prescribed fire and then herbicide application most effectively promoted grass recovery while hindering shrub regrowth. These findings suggest that grassland recovery following shrub encroachment into South Texas coastal prairies may be promoted through the application of shrub removal methods that combine mechanical, fire, and herbicide treatments.

The ratio of Gram-positive to Gram-negative bacterial PLFA markers as an indicator of carbon availability in organic soils

Despite recent progress in understanding soil microbial responses to carbon (C) limitation, the functional shifts in microbial community structure associated with decreasing soil C availability and changes in organic matter chemistry remain poorly known. It has been proposed that Gram-negative (GN) bacteria use more plant-derived C sources that are relatively labile, while Gram-positive (GP) bacteria use C sources derived from soil organic matter that are more recalcitrant. Because these two groups may differ in how they influence the fate of different C forms in soils, it is important to understand how they vary across ecosystems that differ in their vegetation cover and ecosystem productivity or across environmental gradients. In this study, we used a 19-year plant functional group removal experiment across a long term post-fire chronosequence to assess how microbial community structure (assessed using phospholipids fatty acids; PLFAs) and the association of bacterial functional groups (specifically, the GP:GN ratio) responded to changes in organic matter chemistry (measured via nuclear magnetic resonance; NMR). We found that the GP:GN ratio increased upon removal of shrubs and tree roots and with decreasing ecosystem productivity along the chronosequence, thus showing the greater dependence of GN than GP bacteria on more labile plant-derived C. Overall, GN bacteria were associated with simple C compounds (alkyls) whereas GP bacteria were more strongly associated with more complex C forms (carbonyls). Therefore, we conclude that the GP:GN ratio has potential as a useful indicator of the relative C availability for soil bacterial communities in organic soils, and can be used as a coarse indicator of energy limitation in natural ecosystems.

Short-Term Vegetation Responses to Invasive Shrub Control Techniques for Amur Honeysuckle (Lonicera maackii [Rupr.] Herder)

Invasive shrubs in forest understories threaten biodiversity and forest regeneration in the eastern United States. Controlling these extensive monotypic shrub thickets is a protracted process that slows the restoration of degraded forest land. Invasive shrub removal can be accelerated by using forestry mulching heads, but evidence from the western United States indicates that mulching heads can promote exotic species establishment and mulch deposition can reduce native plant species abundance. We compared the effectiveness of the mulching head and the “cut-stump” method for controlling the invasive shrub Amur honeysuckle (Lonicera maackii), as well as their impacts on native plant community recovery, in mixed-hardwood forests of Indiana. After two growing seasons, mulching head treatment resulted in greater L. maackii regrowth and regeneration. The recovery of native plant abundance and diversity following shrub removal did not differ between the two methods. However, mulch deposition was associated with increased abundance of garlic mustard (Alliaria petiolata), an invasive forb. Increasing mulching head treatment depth reduced L. maackii regrowth, but additional study is needed to determine how it affects plant community responses. The mulching head is a promising technique for invasive shrub control and investigating tradeoffs between reducing landscape-scale propagule pressure and increased local establishment will further inform its utility.

Plant community response to Artemisia rothrockii (sagebrush) encroachment and removal along an arid elevational gradient

Questions Shrub expansion into alpine ecosystems worldwide raises important questions regarding the influence of shrub encroachment on alpine species diversity. The stress gradient hypothesis (SGH) predicts interactions will be competitive when resources are plentiful and the environment is benign, but that facilitative interactions will dominate when conditions are stressful. We asked how Artemisia rothrockii (sagebrush) encroachment in an arid mountain range is affecting alpine plant species there and how the plant community responds to the experimental removal of sagebrush at three sites along an elevational gradient. Location The White Mountains, California, USA (37°30′N, 118°10′W). Methods A shrub removal experiment was established at three elevations (2,900, 3,100 and 3,750 m) to evaluate how sagebrush interacts with alpine and sub-alpine plant communities. Results The study sites experienced a strong drought over the 4 yrs of the experiment and plant cover declined overall. However, in the sagebrush removal treatment, cover of co-occurring species increased at both the high-elevation and low-elevation sites, with no differences observed at the mid-elevation site. Conclusions We observed the greatest inhibitory effects of sagebrush at high and low elevations, where plants experience the largest temperature and moisture stress, respectively, and no evidence of facilitation anywhere along the elevational gradient. These results demonstrate that while sagebrush has important influences on herbaceous species composition in the White Mountains, they are inconsistent with the classic predictions of the SGH.

Herbaceous-layer diversity and tree seedling recruitment are enhanced following Rhododendron maximum shrub removal

Forest ecosystems dominated by Tsuga canadensis are undergoing fundamental changes in function and composition from infestations by hemlock woolly adelgid (Adelges tsugae). We proposed that the first step to restoring southern Appalachian riparian forests following T. canadensis mortality would be eliminating the evergreen shrub, Rhododendron maximum. We hypothesized that removing R. maximum would increase light transmittance, soil moisture and temperature; and subsequently, enhance herbaceous-layer diversity and promote tree seedling recruitment and survival. We tested these hypotheses at two locations, (CWT, Coweeta Hydrologic Laboratory; WOC, White Oak Creek) in the Nantahala Mountain Range of western North Carolina, both with heavy T. canadensis mortality and a dense R. maximum subcanopy. The treatments were designed to remove only soil O-horizon (FF), remove only R. maximum (CR), remove R. maximum and soil O-horizon (CFFR), and untreated, reference (REF). We installed permanent plots across treatments and locations and measured light transmittance (Qi/Qo), soil water content (θ), herbaceous-layer cover and diversity (Shannon’s index (H′cover) and species richness), and tree seedling recruitment.As expected, cutting the R. maximum subcanopy (CR and CFFR) immediately increased Qi/Qo in the spring months across locations, and it was sustained through the first growing season. θ was generally high across plots, averaging 26% during the growing season, and didn’t vary over time. By the second growing season (2017) after treatments, herbaceous-layer cover and diversity increased on CR and CFFR. Herbaceous-layer cover was significantly related to Qi/Qo (r2 = 0.22, p < 0.001) and θ (r2 = 0.13, p = 0.009), while diversity was only related to Qi/Qo (H′cover, r2 = 0.14, p < 0.001; species richness, r2 = 0.21, p < 0.001). Tree seedling density was related to Qi/Qo (r2 = 0.10, p = 0.001) and θ (r2 = 0.26, p < 0.001). Tree seedling density was low before treatment (1.4 ± 0.3 seedlings m−2) and increased by 10-fold in CR and CFFR two growing seasons after treatment. In CR, species with the highest density ranked Betula spp. > Acer rubrum > Quercus coccinea > Liriodendron tulipifera > Q. rubra. In CFFR, tree seedling recruitment ranked Betula spp. > A. rubrum > L. tulipifera. These vegetation responses have important implications for potential recovery of riparian forests following T. canadensis mortality.

Soil chemistry and microbial community functional responses to invasive shrub removal in mixed hardwood forests

Invasive woody plant species are a threat to biodiversity and ecosystem function in forests of the eastern U.S., due in part to their effects on soil properties and nutrient cycling. Controlling invasive shrubs can benefit the ecosystem at multiple scales, but these species tend to resprout when cut, and post-cutting flushes of root exudates have been linked to accelerated decomposition of soil organic matter (SOM), a rhizosphere priming effect. We removed the invasive shrub Lonicera maackii (Rupr.) Herder (Amur honeysuckle) from forested sites in central Indiana, USA, using cut-stump and forestry mulching head treatments. For two growing seasons after the initial removal treatments, we compared the soil chemistry and microbial community function of bulk and honeysuckle rhizosphere soils in shrub removal areas to those in invaded reference areas. Microbial activity measured using multiple substrate induced respiration (MSIR) in bulk soils was generally lower in removal areas than reference areas the first year, coinciding with relatively elevated responses in the rhizosphere soils of resprouting shrubs. Elevated SOM and organic C in rhizosphere soils of resprouting shrubs suggested a flush of rhizodeposits from cutting and regrowth. However, bulk soil chemistry responses to shrub removal did not show any evidence of a rhizosphere priming effect, but instead reflected the reduced effects of honeysuckle on throughfall precipitation chemistry—higher ammonium and lower magnesium. Nonetheless, changes in bulk soil chemistry between years were driven by the chemical characteristics of rhizosphere soil associated with resprouting honeysuckle. This study is the first to document the potential for invasive shrub control to affect soil properties through rhizodeposition by the target species.

Grass-Shrub Competition in Arid Lands: An Overlooked Driver in Grassland–Shrubland State Transition?

Traditional models of state transition in arid lands emphasize changes in disturbance regimes and abiotic feedbacks that promote the degradation of grassland into shrubland, whereas biotic interactions like competition and facilitation are often overlooked. Here, we conducted an experiment to determine whether shrubs have a positive, neutral, or negative effect on grasses and if these interactions may play a role in grassland–shrubland state transition. Prosopis glandulosa shrub neighbors within 5 m of Bouteloua eriopoda grass patches were left intact (controls) or killed with foliar herbicide, and metrics of grass performance were evaluated over 5 years. We saw no evidence of shrub facilitation of grasses. Instead, grass ANPP responded positively to shrub removal in all years, but more so in years with above-average rainfall. Grass allocation to vegetative reproduction and grass patch size also increased when shrub neighbors were removed. These results demonstrate that biotic interference by shrubs upon grasses reinforce and magnify grazing- and drought-induced abiotic feedbacks during grassland–shrubland transitions. Shrub effects on grass should therefore be considered a key process in desert grassland state transitions.

Growth and nutrients content of Atriplex canescens across a soil electric conductivity gradient

Atriplex canescens canopy cover, biomass production, and nutrient content were evaluated after four years of livestock grazing exclusion and mechanical shrub removal, except A. canescensin an arid rangeland with a slightly saline soil and with a long history of heavy livestock grazing in the southern Chihuahuan Desert, Mexico (24° N). Twenty 3 × 3 m paddocks were established in terrains with three levels of electric conductivity (EC): &lt;1.4 dS/m (n=7), 1.4–1.6 dS/m (n=7) and &gt;1.6 dS/m. Atriplex canescens canopy cover was higher (49.5%; p&lt;0.01) on paddocks with soil EC &gt;1.6 dS/m than paddocks with soil EC &lt;1.4 and 1.4–1.6 dS/m (32.1 and 22.9%, respectively). Above-ground biomass did not differ between paddocks with soils with EC of &lt;1.4 and 1.4–1.6 dS/m (1309 ± 535 and 1372 ± 180 kg DM/ha), but biomass increased 2.7 times (p&lt;0.01) when soil EC was greater than 1.6 dS/m. The soil EC had no effect on the levels of crude protein (range 13.6 to 14.3%), neutral detergent fiber (range 56.5 to 57.7%) and ash (range 14.5 to 16.4%). In vitro dry matter digestibility (IVDMD) of the foliage of A. canescens was not affected by of soil EC level (range 60.4 to 62.2%). It was concluded that in an arid rangeland with slight saline soil, the increase in salinity favors canopy cover and biomass production of A. canescens without altering nutrient content and IVDMD of this fodder shrub.

Post Hoc Assessment of Stand Structure Across European Wood-Pastures: Implications for Land Use Policy

Europe’s woodland and savanna rangelands, often part of silvopastoral systems known as wood-pastures, are deteriorating because of abandonment that leads to return to a forested state or lack of tree regeneration from overgrazing or tree and shrub removal. Despite numerous local studies, there has been no broader survey of the stand structure of European wood-pastures showing which systems are at risk of losing their semiopen character. This overview aims to 1) show some of the differences and similarities in wood-pastures from landscapes across Europe and 2) identify which of these wood-pastures are at risk of losing their semiopen character. We collated a dataset of 13 693 trees from 390 plots in wood-pastures from eight different European regions (western Estonia, eastern Greece, northern Germany, Hungary, northern Italy, southern Portugal, central Romania, and southern Sweden), including tree diameters at breast height, tree density, management type, and tree species composition. On the basis of their structural characteristics, we classified wood-pastures using principal component analysis (PCA) and cluster analysis. The PCA showed a gradient from dense wood-pastures with high levels of regeneration (e.g., in Estonia) to sparse wood-pastures with large trees but a lack of regeneration (e.g., in Romania). Along this gradient, we identified three main groups of wood-pastures: 1) sparse wood-pastures with mostly big trees; 2) dense wood-pastures composed of small trees, and 3) wood-pastures containing a wide range of tree ages. Our results show a large structural gradient in European wood-pastures, as well as regeneration problems varying in their severity, highlighting the importance of social-ecological context for wood-pasture conditions. To maintain the ecological and cultural integrity of European wood-pastures, we suggest 1) more comprehensively considering them in European policies such as the Common Agricultural Policy and EU Habitats Directive, while 2) taking into account their structural characteristics and social-ecological backgrounds.

Shrub range expansion alters diversity and distribution of soil fungal communities across an alpine elevation gradient.

Global climate and land use change are altering plant and soil microbial communities worldwide, particularly in arctic and alpine biomes where warming is accelerated. The widespread expansion of woody shrubs into historically herbaceous alpine plant zones is likely to interact with climate to affect soil microbial community structure and function; however, our understanding of alpine soil ecology remains limited. This study aimed to (i) determine whether the diversity and community composition of soil fungi vary across elevation gradients and to (ii) assess the impact of woody shrub expansion on these patterns. In the White Mountains of California, sagebrush (Artemisia rothrockii) shrubs have been expanding upwards into alpine areas since 1960. In this study, we combined observational field data with a manipulative shrub removal experiment along an elevation transect of alpine shrub expansion. We utilized next-generation sequencing of the ITS1 region for fungi and joint distribution modelling to tease apart effects of the environment and intracommunity interactions on soil fungi. We found that soil fungal diversity declines and community composition changes with increasing elevation. Both abiotic factors (primarily soil moisture and soil organic C) and woody sagebrush range expansion had significant effects on these patterns. However, fungal diversity and relative abundance had high spatial variation, overwhelming the predictive power of vegetation type, elevation and abiotic soil conditions at the landscape scale. Finally, we observed positive and negative associations among fungal taxa which may be important in structuring community responses to global change.