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106 results for “encroachment”
Not all trees can make a forest: tree species composition and competition control forest encroachment in a tropical savanna
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Data from: Precipitation and nitrogen enrichment impact carbon exchange and stability: From antagonism to synergy with increasing shrub encroachment
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Data from: To burn or not to burn: comparing re-introducing fire with cutting an encroaching conifer for conservation of an imperiled shrub-steppe
Woody vegetation has increased on rangelands worldwide for the past 100-200 years, often because of reduced fire frequency. However, there is a general aversion to re-introducing fire and therefore, fire-surrogates are often used in its place to reverse woody plant encroachment. Determining the conservation effectiveness of re-introducing fire compared to fire-surrogates over different time scales is needed to improve conservation efforts. We evaluated the conservation effectiveness of re-introducing fire with a fire-surrogate (cutting) applied over the last ~30 years to control juniper (Juniperus occidentalis Hook.) encroachment on 77 sagebrush-steppe sites. Critical to conservation of this imperiled ecosystem is to limit juniper, not encourage exotic annual grasses, and promote sagebrush dominance of the overstory. Re-introducing fire was more effective than cutting at reducing juniper abundance and extending the period of time that juniper was not dominating the plant community. Sagebrush was reduced more with burning than cutting. Sagebrush, however, was predicted to be a substantial component of the overstory longer in burned than cut areas because of more effective juniper control. Variation in exotic annual grass cover was explained by environmental variables and perennial grass abundance, but not treatment, with annual grasses being problematic on hotter and drier sites with less perennial grass. This suggests that ecological memory varies along an environmental gradient. Re-introducing fire was more effective than cutting at conserving sagebrush-steppe encroached by juniper over extended time-frames; however, cutting was more effective for short-term conservation. This suggests fire and fire-surrogates both have critical roles in conservation of imperiled ecosystems.
Soil organic carbon in drylands: shrub encroachment and vegetation management effects dwarf those of livestock grazing
Dryland ecosystems occur worldwide and play a prominent, but potentially shifting, role in global biogeochemical cycling. Widespread woody plant proliferation, often associated with declines in palatable grasses, has jeopardized livestock production in drylands and prompted attempts to reduce woody cover by chemical or mechanical means. Woody encroachment also has the potential to significantly alter terrestrial carbon storage. However, little is known of the long-term biogeochemical consequences of woody encroachment in the broader context of its interaction with common dryland land uses, including "brush management" (woody plant clearing) and livestock grazing. Present assessments exhibit considerable variation in the consequences of these land use/land cover changes, with evidence that brush management may counteract sizeable impacts of shrub encroachment on soil biogeochemical pools. A challenge to assessing the net effects of brush management in shrub-encroached grasslands on soil organic carbon (SOC) and total nitrogen (N) pools is that land management practices are typically considered in isolation, when they are co-occurring phenomena. Furthermore, few studies have assessed spatial patterns in brush management and how these are affected in decades following treatment on sites with contrasting grazing histories. To address these uncertainties and interactions, we quantified the impacts of shrub encroachment and their subsequent mortality resulting from brush management (herbicide application) on SOC and N pools in a Sonoran Desert grassland where long-term grazing manipulations (>100 y) co-occur with shrub encroachment and brush management. Pools of SOC and N associated with herbicided shrubs declined markedly over ~40 years, offsetting 66% of the increases from shrub encroachment. However, spatial patterns in SOC induced by shrubs persisted over the decades following brush management. Century-long protection from grazing did little to change SOC and N pools. Accordingly, shrub encroachment and shrub mortality from brush management each far outweighed livestock grazing impacts. Consideration of the patterns of SOC and N through space (e.g., bole-to-dripline gradients), time (e.g., shrub age/size), land use (e.g., livestock grazing and brush management) and their interactions will position us to improve predictions of SOC and N responses to land use/land cover change, inform C-based management decisions, and objectively evaluate trade-offs with other ecosystem services.
Data from: Climate and landscape drive the pace and pattern of conifer encroachment into subalpine meadows
Mountain meadows have high biodiversity and help regulate stream water release following the snowmelt pulse. However, many meadows are experiencing woody plant encroachment, threatening these ecosystem services. While there have been field surveys of individual meadows and remote sensing-based landscape-scale studies of encroachment, what is missing is a broad scale, ground-based study to understand common regional drivers, especially at high elevations, where land management has often played a less direct role. With this study we ask: what are the climate and landscape conditions conducive to woody plant encroachment at the landscape scale, and how has historical climate variation affected tree recruitment in subalpine meadows over time? We measured density of encroaching trees across 340 subalpine meadows in the central Sierra Nevada, California, USA, and used generalized additive models (GAMs) to determine the relationship between landscape-scale patterns of encroachment and meadow environmental properties. We determined ages of trees in 30 survey meadows, used observed climate and GAMs to model the relationship between timing of recruitment and climate since the early 1900s, and extrapolated recruitment patterns into the future using downscaled climate scenarios. Encroachment was high among meadows with lodgepole pine (Pinus contorta Douglas ex Loudon var. murrayana (Balf.) Engelm.) in the immediate vicinity, at lower elevations, with physical conditions favoring strong soil drying, and with maximum temperatures above or below average. Climatic conditions during the year of germination were unimportant, with tree recruitment instead depending on a 3-year seed production period prior to germination and a 6-year seedling establishment period following germination. Recruitment was high when the seed production period had high snowpack, and when the seedling establishment period had warm summer maximum temperatures, high summer precipitation, and high snowpack. Applying our temporal model to downscaled output from four global climate models indicated that the average meadow will shift to forest by the end of the 21st century. Sierra Nevada meadow encroachment by conifers is ubiquitous and associated with climate conditions increasingly favorable for tree recruitment, which will lead to substantial changes in subalpine meadows and the ecosystem services they provide.
Data from: Contrasting habitat and landscape effects on the fitness of a long-lived grassland plant under forest encroachment: do they provide evidence for extinction debt?
1. Habitat loss, fragmentation and transformation threaten the persistence of many species worldwide. Population and individual fitness are often compromised in small, degraded and isolated habitats, but extinction can be a slow process and extinction debts are common. 2. Long-lived species are prone to persist as remnant populations in low quality habitats for a long time, but the population and individual-level mechanisms of extinction debt remain poorly explored so far. 3. We here investigate the mechanisms involved in the long-term persistence of the common grassland specialist, long-lived, clonal plant Aphyllanthes monspeliensis L. (Asparagaceae). after forest encroachment into semi-natural Mediterranean calcareous grasslands in Catalonia (NE Iberian Peninsula). For this purpose we assess vegetative (aboveground and belowground) and reproductive plant performance indicators and their habitat and landscape (current and historical) drivers. 4. We confirm the existence of an extinction debt for this species, since current plant frequency is related to historical but not current connectivity, and we also find a positive effect of historical connectivity on seed set. In addition, current tree cover negatively affects individual size and aboveground/belowground biomass ratio, and biotic soil acidification leads to a reduction in the flowering probability of individuals and stems. 5. However, we also find that current connectivity negatively affects flowering and that tree cover enhances seed set. The forestation process, thus, also exerts a positive effect on some fitness traits, probably by providing a moister environment. 6. Synthesis. Habitat loss and deterioration result in a decreased vegetative performance of Aphyllanthes monspeliensis, a grassland specialist, but show contrasting effects on its reproductive performance. However, further forest encroachment would increase light competition and soil acidification, threatening its persistence and promoting the payment of the extinction debt if no conservation measures are taken.
Data from: How shrub encroachment under climate change could threaten pollination services for alpine wildflowers: a case study using the alpine skypilot, Polemonium viscosum
Under climate change, shrubs encroaching into high altitude plant communities disrupt ecosystem processes. Yet effects of encroachment on pollination mutualisms are poorly understood. Here, we probe potential fitness impacts of interference from encroaching Salix (willows) on pollination quality of the alpine skypilot, Polemonium viscosum. Overlap in flowering time of Salix and Polemonium is a precondition for interference and was surveyed in four extant and 25 historic contact zones. Pollinator sharing was ascertained from observations of willow pollen on bumble bees visiting Polemonium flowers and on Polemonium pistils. We probed fitness effects of pollinator sharing by measuring the correlation between Salix pollen contamination and seed set in naturally pollinated Polemonium. To ascertain whether Salix interference occurred during or after pollination, we compared seed set under natural pollination, conspecific pollen addition, and Salix pollen addition. In current and past contact zones Polemonium and Salix overlapped in flowering time. After accounting for variance in flowering date due to latitude, Salix and Polemonium showed similar advances in flowering under warmer summers. This trend supports the idea that sensitivity to temperature promotes reproductive synchrony in both species. Salix pollen is carried by bumble bees when visiting Polemonium flowers and accounts for up to 25% of the grains on Polemonium pistils. Salix contamination correlates with reduced seed set in nature and when applied experimentally. Postpollination processes likely mediate these deleterious effects as seed set in nature was not limited by pollen delivery. Synthesis: As willows move higher with climate change, we predict that they will drive postpollination interference, reducing the fitness benefits of pollinator visitation for Polemonium and selecting for traits that reduce pollinator sharing.
Data from: Dynamics of marsh-mangrove ecotone since the mid-Holocene: a palynological study of mangrove encroachment and sea level rise in the Shark River Estuary, Florida
Sea level rise and the associated inland shift of the marsh-mangrove ecotone in south Florida have raised many scientific and management concerns in recent years. Holocene paleoecological records can provide an important baseline to shed light on the long-term dynamics of vegetation changes across this ecotone in the past, which is needed to predict the future. In this study, we present palynological, X-ray fluorescence, and loss-on ignition data from four sedimentary cores recovered from a 20-km marine-to-freshwater transect along the Shark River Estuary, southwest Everglades, to document the patterns and processes of coastal vegetation changes in response to sea level rise since the mid-Holocene. Our record indicates that freshwater marsh progressively replaced marl prairies at the Shark River Estuary between 5700 and 4400 cal yr BP. As marine transgression continued, marine influence reached the threshold necessary for mangroves to establish at the current mouth of the Shark River Slough at 3800 cal yr BP. During the next 3000 years, although sea level rise in the Western North Atlantic slowed down to 0.4 mm/yr, a spatial and temporal gradient was evident as the marsh-mangrove ecotone shifted inland by 20 km from 3800 to 800 cal yr BP, accompanied by a gradual landward replacement of freshwater marsh by mangrove forest. If sea level continues to rise at 2.33 mm/yr in the 21st century in south Florida, it is possible that marine influence will reach the threshold for mangroves to establish in the central Everglades, and we could expect a much more aggressive mangrove encroachment toward the northern and interior parts of south Florida in the next few centuries.
Data from: Fire-sensitive species dominate seed rain in a long unburned Cerrado: implications for plant community diversity and woody encroachment in savannas
Woody encroachment is becoming common in tropical savannas. Seed rain data and seed addition experiments in a long unburned Brazilian savanna indicate that abundant seed rain of fire-sensitive species can surpass limitations to recruitment and lead to woody encroachment. Thus, active fire management may be required to maintain savanna diversity.
Data from: Social-ecological landscape patterns predict woody encroachment from native tree plantings in a temperate grassland
Afforestation is often viewed as the purposeful planting of trees in historically non-forested grasslands, but an unintended consequence is woody encroachment, which should be considered part of the afforestation process. In North America's temperate grassland biome, Eastern redcedar (Juniperus virginiana L.) is a native species used in tree plantings that aggressively invades in the absence of controlling processes. Cedar is a well-studied woody encroacher, but little is known about the degree to which cedar windbreaks, which are advocated for in agroforestry programs, are contributing to woody encroachment, what factors are associated with cedar spread from windbreaks, nor where encroachment from windbreaks is occurring in contemporary social–ecological landscapes. We used remotely sensed imagery to identify the presence and pattern of woody encroachment from windbreaks in the Nebraska Sandhills. We used multimodel inference to compare three classes of models representing three hypotheses about factors that could influence cedar spread: (a) windbreak models based on windbreak structure and design elements; (b) abiotic models focused on local environmental conditions; and (c) landscape models characterizing coupled human-natural features within the broader matrix. Woody encroachment was evident for 22% of sampled windbreaks in the Nebraska Sandhills. Of our candidate models, our inclusive landscape model carried 92% of the model weight. This model indicated that encroachment from windbreaks was more likely near roadways and less likely near farmsteads, other cedar plantings, and waterbodies, highlighting strong social ties to the distribution of woody encroachment from tree plantings across contemporary landscapes. Cedar control efforts are insufficient for nearly one-quarter of windbreaks in the Nebraska Sandhills. Our model findings indicate where additional investments into cedar control can be prioritized to prevent cedar spread from windbreaks. This approach can serve as a model in other temperate regions to identify where woody encroachment resulting from temperate agroforestry programs is emerging.
Growth and photosynthetic responses of encroaching tree seedlings to CO2 and stress interactions
<p>1. Woody encroachment in southern African savanna has been partly attributed to rising atmospheric CO2 fertilising the growth of C3 trees but less so that of competing C4 grasses. However, growth conditions (resource availability, competition, rooting space, and herbivory) must be suitable for the effects of elevated CO2 (eCO2) to be realised.</p> <p>2. This research investigated the interactions between the positive effect of eCO2 on tree seedling growth and limitations imposed by drought, disturbance, and competition with C4 grasses. Seedlings of the prolific encroacher C3 tree Vachellia karroo were grown at ambient (400 ppm) or eCO2 (800 ppm) in Open-Top Chambers and exposed to a variety of stresses and disturbances typical of savanna systems. Photosynthetic, growth and allocation responses to eCO2 and other treatments were determined.</p> <p>3. Unsurprisingly, we show strong growth and water-saving responses of V. karroo seedlings to eCO2 when in the absence of competition and herbivory. However, the addition of either competition or simulated herbivory in the first season of growth moderated this, while neither drought nor shading diminished the eCO2 effect relative to similarly treated plants grown at ambient [CO2].</p> <p>4. Synthesis. We demonstrate that eCO2-induced C3 stimulation in encroaching savanna species such as V. karroo will be inconsistent across time and space. This research does not detract from the suggestion that increasing atmospheric CO2 is implicated in woody encroachment, but rather that eCO2 benefits to C3 tree seedlings are only realised when growth conditions are suitable. Inconsistencies in eCO2 response will translate into spatial and temporal variation in seedling responses to eCO2 and CO2-driven woody encroachment, explaining some of the variability observed in woody encroachment across geographic regions and disturbance gradients.</p>
Raw data for article "Use of molasses-based blocks to modify grazing patterns and increase Highland cattle impacts on Alnus viridis-encroached pastures"
<p>Data supporting the conclusions of the article <strong>"Use of molasses-based blocks to modify grazing patterns and increase Highland cattle impacts on Alnus viridis-encroached pastures" </strong>published in the journal <strong>F<em>rontiers in Ecology and Evolution</em></strong>.</p> <p>Authors: Mia Svensk, Ginevra Nota, Pierre Mariotte, Marco Pittarello, Davide Barberis, Michele Lonati, Eric Allan, Elisa Perotti and Massimiliano Probo.</p> <p> </p> <p> </p>
Watershed and fire severity are stronger determinants of soil chemistry and microbiomes than within-watershed woody encroachment in a tallgrass prairie system
<p>Fire can impact terrestrial ecosystems by changing abiotic and biotic conditions. Short fire intervals maintain grasslands and communities adapted to frequent, low-severity fires. Shrub encroachment that follows longer fire intervals accumulates fuel and can increase fire severity. This patchily distributed biomass creates mosaics of burn severities in the landscape—pyrodiversity. Afforded by a scheduled burn of a watershed protected from fires for 27 years, we investigated effects of woody encroachment and burn severity on soil chemistry and soil-inhabiting bacteria and fungi. We compared soils before and after fire within the fire-protected, shrub-encroached watershed and soils in an adjacent, annually burned and non-encroached watershed. Organic matter and nutrients accumulated in the fire-protected watershed but responded less to woody encroachment within the encroached watershed. Bioavailable nitrogen and phosphorus and fungal and bacterial communities responded to high-severity burn regardless of encroachment. Low-severity fire effects on soil nutrients differed, increased bacterial but decreased fungal diversity and effects of woody encroachment within the encroached watershed were minimal. High-severity burns in the fire-protected watershed led to a novel soil system state distinct from non-encroached and encroached soil systems. We conclude that severe fires may open grassland restoration opportunities to manipulate soil chemistry and microbial communities in shrub-encroached habitats.</p>
Woody encroachment of grasslands: near-surface thermal implications through the lens of an astronomical event
<p><span>Temperature </span><span>has long been understood</span><span> as a fundamental condition that </span><span>influences</span><span> ecological </span><span>patterns and </span><span>processes</span><span>.</span><span> </span><span>H</span><span>eterogeneity</span><span> in landscapes that is</span><span> </span><span>structured </span><span>by </span><span>ultimate (</span><span>climate</span><span>) and proximate</span><span> </span><span>(</span><span>vegetation, topography, disturbance events, and land use</span><span>) forces</span><span> serve to shape </span><span>thermal patterns across multiple </span><span>spatio</span><span>-temporal scales</span><span>. </span><span>Thermal landscapes of grasslands are likely shifting as woody encroachment fragments these </span><span>eco</span><span>systems and studies quantifying thermal fragmentation in grassland systems </span><span>resulting from</span><span> woody encroachment are lacking. </span><span>We utilized the</span><span> August </span><span>21</span><span>st</span><span>, 2017 solar eclipse </span><span>to mimic a rapid sunrise/sunset event across a landscape characterized as a grassland to </span><span>experimentally manipulate</span><span> </span><span>levels of </span><span>solar radiation </span><span>in </span><span>the system. We then quantified changes in near surface temperatures </span><span>resulting from changes in</span><span> solar radiation</span><span> levels during the eclipse. </span><span>Temperatures were monitored across </span><span>three </span><span>grassland </span><span>pastures</span><span> in central Oklahoma</span><span> </span><span>that were characterized by d</span><span>ifferent </span><span>densities (low, </span><span>medium, and high)</span><span> of </span><span>Juniperus virginiana</span><span> to understand the impact of woody encroachment on</span><span> diurnal temperature patterns and thermal heterogeneit</span><span>y in </span><span>a grassland's </span><span>thermal landscape</span><span>. The largest temperature range</span><span> across sites</span><span> that occurred during the eclipse was in the mixed grass </span><span>vegetation</span><span>. Similarly, the largest change in thermal heterogeneity occurred in the grassland with the lowest amount of woody encroachment. </span><span>T</span><span>hermal heterogeneity was lowest in the highly encroached </span><span>grassland, which</span><span> also experienced the lowest overall change in thermal heterogeneity during the eclipse. </span><span>Time-series models suggested that solar radiation was the most influential factor in predicting changes in thermal heterogeneity as opposed to ambient temperature alone. </span><span>These results suggest that highly encroached grasslands may experience lower diurnal variability of temperatures at the cost of a decrease in the overall thermal heterogeneity of that landscape. </span><span>It appears that fine-scale </span><span>spatio</span><span>-temporal thermal variation </span><span>is largely driven</span><span> by solar radiation, which can be influenced by vegetation heterogeneity inherent within a landscape.</span><span> </span></p>
Soil carbon is mostly grass-derived in tropical savannas, even under woody encroachment
<p>Tropical savannas have been increasingly targeted for carbon (C) sequestration from afforestation, assuming large gains in soil organic C (SOC) with increasing tree cover. Because savanna SOC is also derived from grasses, this assumption may not reflect real changes in SOC under afforestation, but grass contributions to SOC and changes in SOC with increasing tree cover remain poorly synthesized. Here, we combine a case study from Kruger National Park, South Africa, with data synthesized from tropical savannas globally to show that grass-derived C constitutes more than half of total SOC to a soil depth of 1-meter, even in soils directly under trees. The largest SOC concentrations were associated with the largest grass contributions (> 70% of total SOC). Regionally and across the tropics, SOC concentration was not explained by tree cover. Both SOC gain and loss were observed following increasing tree cover, and on average SOC storage within 1-meter profile only increased by a negligible and non-significant 6% (SE = 4%, n = 44). These results underscore the substantial contribution of grasses to SOC and the considerable uncertainty in SOC responses to increasing tree cover, challenging the widespread assumption that afforestation universally and substantially enhances SOC storage across tropical savannas.</p>
Top-down and bottom-up controls limit woody encroachment into persistent temperate rainforest meadows
<p><span>These data describe soils, woody plant seedlings, and ungulate herbivory in and around temperate montane meadows in the Oregon Coast Range, USA. Meadows such as these are a global study system for the accelerating phenomenon of woody encroachment, but study this phenomenon into meadows in western Oregon has been conducted almost entirely in the western and High Cascades, with only two extant observational studies of grassy balds in the Coast Range. These data describe factors limiting woody encroachment into meadows in the Oregon Coast Range, including bottom-up control by soil properties, plant-plant interactions, and top-down control by large herbivores.<b> </b>I measured chemical and physical properties of soils (depth of organic layer; bulk density of top 3 cm of mineral soil; and mineral soil profiles: particle size distribution, pH, % total C, % total N) to a depth of 50 cm in meadow and forest. I recorded community, density, and proportion browsed for shrubs, conifers, and deciduous trees ≤2 m tall along transects from meadow into forest. I experimentally planted 20 <i>Pseudotsuga menziesii</i> (Douglas-fir) seedlings in each of five meadows (<em>n </em>= 100) and factorially manipulated aboveground neighboring plant presence and ungulate herbivore access. I found that m</span><span>eadow soils were lower in C and C:N; slightly lower in N, and similar in plant-available water (derived from particle size distribution) and pH relative to forest soils. Shrubs were most dense, but experienced the lowest browse pressure, near the meadow edge; while trees were sparse and varied by site—although at one site, browse pressure was heavier in meadow than forest. Seedling survival and growth varied by site, herbivory reduced growth, and total soil N best explained residual variation in seedling growth among sites.</span><span><b> </b>My findings indicate that ungulate herbivores exert top-down control on woody encroachment into temperate montane meadows, perhaps in concert with local N-limitation.</span></p>
Data from: Native lagomorphs suppress grass establishment in a shrub‐encroached, semiarid grassland
Shrub encroachment into arid grasslands has been associated with reduced grass abundance, increased soil erosion, and local declines in biodiversity. Livestock overgrazing and the associated reduction of fine fuels has been a primary driver of shrub encroachment in the southwestern United States, but shrublands continue to persist despite livestock removal and grassland restoration efforts. We hypothesized that herbivory feedbacks from native mammals may contribute to continued suppression of grasses after the removal of livestock. Our herbivore exclusion experiment in southeastern Arizona included five treatment levels and allowed access to native mammals based on their relative body size, separating the effects of rodents, lagomorphs, and mule deer. We included two control treatments and replicated each treatment 10 times (n = 50). We introduced uniform divisions of lawn sod (Cynodon dactylon) into each exclosure for 24-hour periods prior to (n = 2) and following (n = 2) the monsoon rains and used motion-activated cameras to document herbivore visitations. In the pre-monsoon trials, treatments that allowed lagomorph access had less sod biomass relative to other treatments (p < 0.001), averaging 44% ( 36%) and 29% ( 45%) remaining biomass after the 24-hour trial periods. Following the onset of monsoons, differences in remaining biomass among treatments disappeared. Desert cottontails (Sylvilagus audubonii) were detected more frequently than any of the other 11 herbivore species present at the site, accounting for 83% of detections during the pre-monsoon trials. Significantly more (p < 0.001) desert cottontails were detected during the pre-monsoon trials (2,077) compared to the post-monsoon trials (174), which coincided with biomass removal from lagomorph accessible treatments. We conclude that desert cottontails are significant consumers of herbaceous vegetation in shrub-encroached arid grasslands and they, along with other native herbivores, may act as a biotic feedback contributing to the competitive advantage and persistence of shrubs.
Data from: Shrub encroachment does not reduce the activity of some soil enzymes in Mediterranean semiarid grasslands
Shrub encroachment is a worldwide phenomenon with implications for desertification and global change. We evaluated its effects on the activities of urease, phosphatase and b-glucosidase in Mediterranean semiarid grasslands dominated by Stipa tenacissima by sampling 12 sites with and without resprouting shrubs along a climatic gradient. The presence of shrubs affected the evaluated enzymes at different spatial scales. Soils under S. tenacissima tussocks and in bare ground areas devoid of vascular plants had higher values of phosphatase and urease when the shrubs were present. For the b-glucosidase, this effect was site-specific. At the scale of whole plots (30 m 30 m), shrubs increased soil enzyme activities between 2% (b-glucosidase) and 22% (urease), albeit these differences were significant only in the later case. Our results indicate that shrub encroachment does not reduce the activity of extracellular soil enzymes in S. tenacissima grasslands.
Data from: Shrub encroachment can reverse desertification in semi-arid Mediterranean grasslands
The worldwide phenomenon of shrub encroachment in grass-dominated dryland ecosystems is commonly associated with desertification. Studies of the purported desertification effects associated with shrub encroachment are often restricted to relatively few study areas, and document a narrow range of possible impacts upon biota and ecosystem processes. We conducted a study in degraded Mediterranean grasslands dominated by Stipa tenacissima to simultaneously evaluate the effects of shrub encroachment on the structure and composition of multiple biotic community components, and on various indicators of ecosystem function. Shrub encroachment enhanced vascular plant richness, biomass of fungi, actinomycetes and other bacteria, and was linked with greater soil fertility and N mineralization rates. While shrub encroachment may be a widespread phenomenon in drylands, an interpretation that this is an expression of desertification is not universal. Our results suggest that shrub establishment may be an important step in the reversal of desertification processes in the Mediterranean region.
Ecosystem sulfur accumulation following woody encroachment drives a more open S-cycle in a subtropical savanna
<p>Globally widespread woody encroachment into grass-dominated ecosystems has substantial consequences for carbon (C), nitrogen (N), and phosphorus (P) cycles. Despite its significance as an essential macronutrient, however, little is known regarding potential changes in the sulfur (S) cycle. We quantified S concentrations, stoichiometric relationships, and δ<sup>34</sup>S values in the plant-soil environment to investigate landscape-scale changes in the S cycle following grassland-to-woodland transitions in a subtropical savanna. Plant tissues of woody species had significantly higher S concentrations and δ<sup>34</sup>S values than those of herbaceous species, resulting in a landscape-scale correspondence between spatial patterns of S and δ<sup>34</sup>S in surface soils and vegetation distribution, with higher S and δ<sup>34</sup>S in soils beneath woody patches. These patterns were more subtle at soil depths > 5 cm. Woody plants had higher N:S ratios but comparable P:S ratios relative to herbaceous species, which contributed to contrasting spatial patterns between N:S and P:S ratios in surface soils. Sulfur in surface soils increased proportionally less relative to N, but proportionally more compared to P. Our findings indicate that grassland-to-woodland transitions amplify landscape-scale S dynamics, especially in surface soils, and create a S-enriched environment that enables woody plants to acquire sufficient S relative to demand to support their continued productivity and proliferation.</p>
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