Herbaceous Community Composition Research Articles

Multidecadal vegetation transformations of a New Mexico ponderosa pine landscape after severe fires and aerial seeding.

Wildfires and climate change increasingly are transforming vegetation composition and structure, and postfire management may have long-lasting effects on ecosystem reorganization. Postfire aerial seeding treatments are commonly used to reduce runoff and soil erosion, but little is known about how seeding treatments affect native vegetation recovery over long periods of time, particularly in type-converted forests that have been dramatically transformed by the effects of repeated, high-severity fire. In this study, we analyze and report on a rare long-term (23-year) dataset that documents vegetation dynamics following a 1996 post-fire aerial seeding treatment and a subsequent 2011 high-severity reburn in a dry conifer landscape of northern New Mexico, USA. Repeated surveys between 1997 and 2019 of 49 permanent transects were analyzed for differences in vegetation cover, richness, and diversity between seeded and unseeded areas, and to characterize the development of seeded and unseeded vegetation communities through time and across gradients of burn severity, elevation, and soil-available water capacity. Seeded plots showed no significant difference in bare ground cover during the initial years postfire relative to unseeded plots. Postfire seeding led to a clear and sustained divergence in herbaceous community composition. Seeded plots had a much higher cover of non-native graminoids, primarily Bromus inermis, a likely contaminant in the seed mix. High-severity reburning of all plots in 2011 reduced native graminoid cover by half at seeded plots compared with both prefire levels and with plots that were unseeded following the initial 1996 fire. In addition, higher fire severity was associated with increased non-native graminoid cover and reduced native graminoid cover. This study documents fire-driven ecosystem transformation from conifer forest into a shrub-and-grass-dominated system, reinforced by aerial seeding of grasses and high-severity reburning. This unique long-term dataset illustrates that post-fire seeding carries significant risks of unwanted non-native species invasions that persist through subsequent fires-thus alternative postfire management actions merit consideration to better support native ecosystem resilience given emergent climate change and increasing disturbance. This study also highlights the importance of long-term monitoring of postfire vegetation dynamics, as short-term assessments miss key elements of complex ecosystem responses to fire and postfire management actions.

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

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

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

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

Shrubs exhibit competitive interactions with herbaceous plants and shape community assemblage and functional composition in the alpine western Himalaya

AbstractQuestionsIn the alpine region of the Himalaya, shrubs play a vital role in maintaining the diversity and functional composition of associated herbaceous communities through competition–facilitation interactions under varying environmental stress conditions. In this study, we specified the following questions: (1) what is the mode of interaction between dominant shrubs and the associated herbaceous communities; (2) how do differences in resource availability between contrasting microhabitats mediate interactions between shrub and herbaceous communities; and (3) how do dominant shrubs influence the functional composition of herbaceous communities under the canopy as compared to ones in the open?Study areaAn alpine region of the western Himalaya, in India (32.24–33.15° N, 76.51–78.13° E).MethodsField studies were conducted to evaluate the canopy effects of alpine shrubs such as Caragana versicolor, Juniperus polycarpos and Rhododendron anthopogon on species richness, abundance and functional composition of coexisting herbaceous communities.ResultsThe dominant alpine shrubs of the western Himalaya exert competitive interactions with associated herbaceous communities and have low species richness and abundance under their canopies compared to open habitats. Further, contrary to expectations, competitive interactions were more prominent at higher elevations than at lower ones, especially for J. polycarpos and C. versicolor. Although the shrub undercanopies possessed richer soil nutrient pools, this did not contribute toward the facilitation of herbaceous species under the canopies. Moreover, herbaceous species under the canopies were found to exhibit resource‐acquisitive functional strategies, whereas those in the open were resource‐conservative.ConclusionsThe interaction between dominant alpine shrubs and the herbaceous community of the western Himalaya is competitive in nature, which influences species and functional composition and reorganizes herbaceous community assembly. Moreover, under future climate change scenarios the dominance of these shrubs will favour those herbaceous species that possess more competitive and resource‐acquisitive functional strategies.

Grazing intensity effects on herbaceous community composition in burned sagebrush steppe

AbstractThere is limited knowledge of grazing impacts on longer term plant community dynamics following fire in sagebrush steppe. This study evaluated vegetation response to different intensities of deferred rotation cattle grazing over 16 years (2007–2022) on burned Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis (Beetle & Young) Welsh) steppe in eastern Oregon. Treatments were applied in a randomized complete block, which included no grazing on burned (nonuse, n = 5) and unburned (control, n = 5) steppe; and cattle grazing at low (low, n = 4), moderate (moderate, n = 4), and high (high, n = 4) intensities on burned steppe. Vegetation dynamics were evaluated by repeated measures analysis of canopy cover and density of shrub and herbaceous species and functional groups. Herbaceous functional groups were an early‐season bunchgrass (one species, Sandberg bluegrass [Poa secunda J. Presl]), tall perennial bunchgrasses, perennial forbs, annual grass (one species, cheatgrass [Bromus tectorum L.]), and annual forbs. Tall perennial bunchgrass, Sandberg bluegrass, and perennial forb cover and density did not differ among the treatments but did decrease over time in all treatments. The cover of several tall bunchgrass species was generally less in the high treatment, mainly, Idaho fescue (Festuca idahoensis Elmer) and Thurber's needlegrass (Achnatherum thurberianum (Piper) Barkworth). The cover of cheatgrass and annual forbs varied among years but was greater in the burned‐grazed and nonuse treatments than in the control. Native plant cover in the burned treatments (grazed and nonuse) represented 77%–85% of total herbaceous cover versus the control where native plants comprised 91% of the total. Annual weather variability appears to account for most of the compositional dynamics measured in the various grazed and ungrazed treatments.

Shrub encroachment increases soil carbon and nitrogen stocks in alpine grassland ecosystems of the central Tibetan Plateau

Shrub encroachment can significantly affect the ecosystem structure and functioning of alpine grasslands. However, little is known about how this phenomenon alters soil organic carbon stocks (SOCS) and soil nitrogen stocks (STNS) in different alpine grassland ecosystems within the same region. Here we addressed this research gap by investigating the SOCS and STNS of encroached and non-encroached plots in alpine meadow and alpine steppe ecosystems on the central Tibetan Plateau. We found that, in both ecosystems, shrub encroachment significantly increased herbaceous plant richness, diversity, and aboveground biomass, but more notably in alpine meadow. Conversely, shrub encroachment increased belowground plant biomass in alpine steppe but had the opposite effect in alpine meadow, where belowground biomass was four times lower at encroached than at non-encroached plots. Importantly, encroaching shrubs resulted in significantly higher SOCS and STNS in both ecosystems, with alpine steppe showing the greatest carbon and nitrogen gains. The shrub encroached plots had 34.24 % and 97.25 % higher carbon stocks and 70.87 % and 67.34 % higher nitrogen stocks than non-encroached plots at the alpine meadow and alpine steppe, respectively. Increases in carbon and nitrogen stocks occurred mostly at the 10–30 cm soil layer and were highly correlated with shrub-induced changes in the herbaceous community composition. Overall, our results indicated that (1) shrub encroachment promoted carbon and nitrogen accumulation in the soils of both alpine meadow and alpine steppe and (2) the degree of accumulation and the mechanisms behind these changes varied between the two alpine grasslands. Our findings advance the general understanding of the soil carbon and nitrogen dynamics in shrub‐encroached alpine grasslands and highlight the importance of investigating how this phenomenon affects different ecosystems within the same region.

The Coupling Relationship between Herb Communities and Soil in a Coal Mine Reclamation Area after Different Years of Restoration

In a complete organic ecosystem restoration in mining areas, soil and vegetation complement and influence each other. It is of great significance to evaluate the ecological restoration effect on and ecosystem stability of the mining area, with the coupling and coordinating relationship between herb community and soil physicochemical properties after land reclamation. Therefore, this study takes Juxinlong Coal Mine in Dongsheng District of Ordos City, Inner Mongolia Autonomous Region as the study area. The understory herbaceous plant community and soil factors with restoration time of 3 to 7 years were selected as the research objects. In addition, artificial grassland and natural restoration grassland were used as controls to investigate the species composition of herbaceous communities and soil physicochemical properties in different sites. The grey relational coupling model was adopted not only to study the relationship between herb community and soil factors but also to explore the coupling mechanism between herb community and soils’ physicochemical properties. The results included: (1) 51 herbaceous plants were investigated in the study area, among which Gramineae, Compositae, Leguminosae, and Chenopodiaceae were the primary ones, accounting for 70% of the total. (2) With the increase of restoration years, the diversity indices of understory herbaceous plants generally showed an increasing trend, but the diversity indices of understory herbaceous plants with different restoration years was smaller than that of artificial grassland. (3) The results of grey correlation analysis showed that the diversity of herbaceous plant community in the study area was closely related to soils’ available nitrogen and water content. (4) The degree of coupling and coordinating between the diversity of herbaceous plants and soils in the study area presented a trend of first increasing then decreasing during the accumulating restoration years. Among them, the degree of coupling between the diversity of understory herbaceous plants and soil system in the 5-year restoration’s sample plot (0.73) was found to be the highest, which was classified as medium coordination. The lowest coupling degree of herb diversity and soils was identified in the artificial grassland (0.51), which was light incoordination. Therefore, it is suggested that the control of water and nitrogen resources should be strengthened, the herbaceous vegetation should be reasonably selected, and the artificial tending should be carried out in the later stage of understory herbaceous vegetation construction in the study area to better promote the vegetation construction in the mining area.

At high stocking rates, cattle do not functionally replace wild herbivores in shaping understory community composition.

Over a quarter of the world's land surface is grazed by cattle and other livestock, which are replacing wild herbivores, potentially impairing ecosystem structure, and functions. Previous research suggests that cattle at moderate stocking rates can functionally replace wild herbivores in shaping understory communities. However, it is uncertain whether this is also true under high stocking rates and the effects of wild herbivore on plant communities are moderate, enhanced, or simply additive to the effects of cattle at high stocking rates. To evaluate the influence of cattle stocking rates on the ability of cattle to functionally replace wild herbivores and test for interactive effects between cattle and wild herbivores in shaping understory vegetation, we assessed herbaceous vegetation in a long-term exclosure experiment in a semi-arid savanna in central Kenya that selectively excludes wild mesoherbivores (50-1000 kg) and megaherbivores (elephant and giraffe). We tested the effects of cattle stocking rate (zero/moderate/high) on herbaceous vegetation (diversity, composition, leafiness). We also tested how those effects depend on the presence of wild mesoherbivores and megaherbivores. We found that herbaceous community composition (primary ordination axis) was better explained by the presence/absence of herbivore types than by total herbivory, suggesting that herbivore identity is a more important determinant of community composition than total herbivory at high cattle stocking rates. The combination of wild mesoherbivores and cattle stocked at high rates led to increased bare ground and annual grass cover, reduced perennial grass cover and understory leafiness, and enhanced understory diversity. These shifts were weaker or absent when cattle were stocked at high stocking rates in the absence of wild mesoherbivores. Megaherbivores tempered the effects of cattle stocked at high rates on herbaceous community composition but amplified the effects of high cattle stocking rate on bare ground and understory diversity. Our results show that cattle at high stocking rates do not functionally replace wild herbivores in shaping savanna herbaceous communities contrary to previous findings at moderate stocking rates. In mixed-use rangelands, interactions between cattle stocking rate and wild herbivore presence can lead to non-additive vegetation responses with important implications for both wildlife conservation and livestock production.

Interactive effects of prescribed fire and livestock grazing on Seriphium plumosum in South African sour bushveld

Documented increases in woody plant abundance worldwide can reduce land value and grazing capacity. Throughout many grasslands and savannas in South Africa, Seriphium plumosum, an encroaching woody plant, has been rapidly displacing desirable native species. Cost-effective management techniques are required, in order to prevent encroachment. However, the effects of fire and grazing on S. plumosum are not well understood. We established a full factorial, completely randomised experiment to evaluate interactive effects of fire and livestock grazing on woody plant density and native plant diversity. Sixteen 100 m2 plots were aggregated within an area of rangeland and randomly assigned four treatments, including: i) ungrazed, not burned: ii) ungrazed, burned, iii) grazed, not burned, and iv) grazed, burned. Results indicate fire can reduce S. plumosum height (p < 0.01) and canopy cover (p < 0.01), while simultaneously maintaining the herbaceous community composition (p = 0.33). However, this treatment effect was not significant 24 months after a single treatment. This suggests that although fire holds promise for S. plumosum management, follow-up treatments are necessary to control the shrub. Additional research into the effects of altering components of the fire regime in combination with grazing could provide insight into more effective management.

Grazing effects on woody and herbaceous plant biodiversity on a limestone mountain in northern Tunisia.

Mediterranean maquis vegetation is highly biodiverse, but widespread grazing poses a challenge for management and conservation. We sampled woody and herbaceous plants separately on a limestone mountain with strong mesic-xeric gradients in Tunisia’s Parc National de L’Ichkeul, assessed grazing pressure (on a scale of 1–3), and asked whether grazing had a significant effect on plant compositional abundance before and after controlling for environmental covariates. Sites on the more mesic lakeside face of the mountain were most compositionally unique, and forbs contributed most to the herbaceous beta-diversity on the mountain. We used variance partitioning to separate the collective and individual effects of the abiotic environment, grazing, human activity, and space on herbaceous and woody beta-diversity. However, the individual effect of grazing on overall plant community composition was confounded with space, due to the spatially autocorrelated grazing pressure on the mountain. Importantly, we found that herbaceous and woody communities responded differently to increasing levels of grazing intensity: herbaceous beta-diversity was highest between sites with no grazing pressure, while woody beta-diversity peaked under light grazing. Herbaceous community composition was sensitive to any intensity of grazing pressure, and biotic homogenization occured under moderate-to-high grazing pressure. On the other hand, woody community composition remained relatively similar under no to light grazing pressure, but differed under moderate-to-heavy grazing. Using a one-way permutational analysis of variance analysis, we showed that grazing had a significant effect when controlling for abiotic and spatial covariates. Our findings offer insight into the effects of grazing on maquis vegetation at Jebel Ichkeul, acting as a microcosm of similar conservation and management issues elsewhere in the Mediterranean. We suggest that a combination of monitoring and carefully controlled grazing may enhance plant diversity and maintain the region’s biodiverse maquis vegetation, potentially maintaining a key climate refugium for vulnerable endemic species. Importantly, our study provides a useful baseline of the plant assemblages at Jebel Ichkeul with which to compare future vegetation changes.

Precipitation reduction alters herbaceous community structure and composition in a savanna

AbstractQuestionsIn the changing climate scenario, the decline in precipitation is expected to alter water availability for plants, which in turn affects plant community structure and composition. The responses of community composition and structure to declines in precipitation are well documented in other biomes but remain understudied in water‐limited savannas.LocationA savanna ecosystem in southwest China.MethodsWe used a four‐year (2014–2017) precipitation manipulation experiment to examine changes in herbaceous community composition and structure across the species, functional group and community levels under precipitation reduction.ResultsPrecipitation reduction significantly decreased the average height and percentage cover of the herbaceous community, while increasing species richness and the Pielou evenness index. Precipitation reduction significantly decreased average height, percentage cover and relative abundance of graminoids and perennials, but increased those of forbs and annuals. Precipitation reduction prompted a shift in the dominant species of the herbaceous community towards Fimbristylis monostachya.ConclusionsThe results show that precipitation reduction changed the composition and structure of the herbaceous community of this savanna. Furthermore, they provide strong evidence that changes in herbaceous community structure and composition in response to the intensity and duration of precipitation reduction in this savanna exhibited relatively low thresholds, which suggests that the herbaceous community's response to a decline in precipitation was essentially nonlinear. These findings imply that even relatively small declines in precipitation may stimulate shifts in plant community structure and composition and affect the function and stability of savanna ecosystems.

Short-duration rotational grazing leads to improvements in landscape functionality and increased perennial herbaceous plant cover

Livestock grazing can lead to reduced ground cover and altered composition of pastures through the loss of palatable forage species and reduced litter cover. This negatively impacts landscape function and ultimately livestock production. Grazing livestock for short periods with high animal density, followed by long rests to allow pasture recovery (short-duration grazing), could be a way to address these issues. In naturalised pastures, we assessed landscape functioning and compared the abundance of six major plant functional groups at 36 sites on 12 commercial grazing properties. Six of the properties had been managed with short-duration grazing for more than 7 years (in most cases over 10 years), while the six control properties were managed with grazing that was more typical of the region (relatively continuous throughout the year with unplanned rests). Under short-duration grazing, there was approximately 19% greater foliar cover of perennial herbaceous species with a corresponding 14% reduction in foliar cover of introduced annual plants. Attributes relating to biophysical functioning of the landscape were enhanced by short-duration grazing, with environmental factors less important in influencing these landscape function attributes. Higher-value forage species were also more abundant on short-duration grazing properties, especially at higher rainfall sites. Conversely, species that tend to increase under heavy grazing pressures, and are of lower forage value, were less abundant under short-duration grazing. Despite the changes in pasture composition in response to grazing management there was a large amount of unexplained variation in herbaceous community composition. This study demonstrates benefits for landscape function and naturalised pasture composition under short-duration grazing that has been in place for several years compared with more usual grazing practices.

Herbivory and drought generate short-term stochasticity and long-term stability in a savanna understory community.

Rainfall and herbivory are fundamental drivers of grassland plant dynamics, yet few studies have examined long-term interactions between these factors in an experimental setting. Understanding such interactions is important, as rainfall is becoming increasingly erratic and native wild herbivores are being replaced by livestock. Livestock grazing and episodic low rainfall are thought to interact, leading to greater community change than either factor alone. We examined patterns of change and stability in herbaceous community composition through four dry periods, or droughts, over 15years of the Kenya Long-term Exclosure Experiment (KLEE), which consists of six different combinations of cattle, native wild herbivores (e.g., zebras, gazelles), and mega-herbivores (giraffes, elephants). We used principal response curves to analyze the trajectory of change in each herbivore treatment relative to a common initial community and asked how droughts contributed to community change in these treatments. We examined three measures of stability (resistance, variability, and turnover) that correspond to different temporal scales and found that each had a different response to grazing. Treatments that included both cattle and wild herbivores had higher resistance (less net change over 15years) but were more variable on shorter time scales; in contrast, the more lightly grazed treatments (no herbivores or wild herbivores only) showed lower resistance due to the accumulation of consistent, linear, short-term change. Community change was greatest during and immediately after droughts in all herbivore treatments. But, while drought contributed to directional change in the less grazed treatments, it contributed to both higher variability and resistance in the more heavily grazed treatments. Much of the community change in lightly grazed treatments (especially after droughts) was due to substantial increases in cover of the palatable grass Brachiaria lachnantha. These results illustrate how herbivory and drought can act together to cause change in grassland communities at the moderate to low end of a grazing intensity continuum. Livestock grazing at a moderate intensity in a system with a long evolutionary history of grazing contributed to long-term stability. This runs counter to often-held assumptions that livestock grazing leads to directional, destabilizing shifts in grassland systems.

Community-Level Impacts of Management and Disturbance in Western Michigan Oak Savannas

Abstract Midwestern oak savanna systems are typically defined by their open canopy and the co-existence of scattered mature oak trees and a ground layer dominated by herbaceous vegetation. The structure of these systems is thought to be primarily maintained by disturbance such as fire. In this study we examined the plant community of 21 different oak savanna sites in western Lower Michigan, U.S.A., across a coarse disturbance gradient created by different management practices. Herbaceous community composition differed significantly across a variety of management approaches, while overall diversity remained similar. Indicator species analysis (ISA) identified several species commonly associated with mixed oak forest understories (e.g., Maianthemum canadense) as indicators for recently abandoned oak savanna sites, whereas the indicators identified for managed or heavily disturbed sites included common savanna-associates (e.g., Lupinus perennis). Variation in soil characteristics (C:N ratio and pH) and canop...

Top-down vs. bottom-up regulation of herbaceous primary production and composition in an arid, urbanizing ecosystem

Plant growth and composition are regulated by top-down (e.g., herbivory) and bottom-up factors (e.g., resource availability). The relative importance of consumers and multiple resources for net primary production (NPP) and community structure have rarely been studied in drylands, which cover about one third of Earth's land surface, or with respect to increasingly common environmental changes such as urbanization. Urban expansion in drylands is likely to alter both nutrient availability and consumer populations. We explored the relative roles of herbivory, precipitation, and soil nitrogen (N) availability as drivers of aboveground NPP and composition of herbaceous communities in protected native ecosystems in the Sonoran Desert within and surrounding Phoenix, Arizona. Precipitation was the primary driver of production, while soil N availability had little effect on growth. Herbivory was secondarily important relative to precipitation, reducing aboveground biomass by ∼33% regardless of proximity to the city. Protected desert open space supported distinct plant communities within and surrounding the city, but these patterns were more strongly related to bottom-up resources than consumers. Together, our results suggest that urbanization does not significantly affect the relative drivers of plant growth and structure in this arid ecosystem.

Impacts of nitrogen deposition on herbaceous ground flora and epiphytic foliose lichen species in southern Ontario hardwood forests

In this study 70 sugar maple (Acer saccharum Marsh.) dominated plots in Ontario, Canada were sampled in the spring of 2009 and 2010 and herbaceous plant and epiphytic foliose lichen species data were compared against modeled N and S deposition data, climate parameters and measured soil and plant/lichen S and N concentration. Herbaceous plant species richness was positively correlated with temperature and indices of diversity (Shannon Weiner and Simpson's Index) were positively correlated with soil pH but not N or S deposition or standardized foliar N scores. Herbaceous community composition was strongly controlled by traditional factors, but there was a small and significant influence of atmospheric S and N deposition. Epiphytic lichen species richness exhibited a strong negative relationship with standardized foliar N score and only one lichen species (Phaeophyscia rubropulchra) was observed at sites with a standardized foliar N score of 0.76.

Plant-community responses to shrub cover in apáramograssland released from grazing and burning

Shrub encroachment can follow grazing or burning release in páramo grasslands. While encroachment decreases herbaceous species richness in some grassland systems, the effects of this process on the herbaceous community in páramo grasslands are currently unknown. We collected data on shrub cover, herbaceous-species cover and species composition in a páramo grassland 12 years after release from burning and cattle grazing near Zuleta, Ecuador. Topographic and soil measures were also included as predictor variables of differences in community composition. Contrary to studies in other systems, shrub cover did not have a significant effect on herbaceous-species richness, whereas shrub-species richness significantly increased with shrub cover. However, shrub cover was associated with significant shifts in herbaceous–community composition. Most notably, there was an increase in some shade-tolerant forbs and tall-statured wetland grasses with increasing shrub cover, and a corresponding decrease in some short-statured grasses and early successional forbs. These results could indicate that the ameliorative effects of shrubs (e.g. frost and wind protection) in harsh alpine environments may partially compensate for the expected competitive effect of shrubs due to shading.

围封禁牧对小叶锦鸡儿灌丛化草原群落组成和结构的影响

典型草原向灌丛化草原的转变是过度放牧引起的重要结果之一。为研究内蒙古地区小叶锦鸡儿灌丛化草原对围封禁牧的响应，在内蒙古锡林郭勒典型草原退化区选取小叶锦鸡儿成片分布的典型地段，自2003年，分别设置禁牧样区和放牧样区。于2008-2011年连续四年调查禁牧样区小叶锦鸡儿种群生长和生理生化指标及灌丛间群落基本特征，并将2011年禁牧样区与放牧样区小叶锦鸡儿种群与灌丛间群落进行比较。结果显示：（1）围封后小叶锦鸡儿种群开始衰退，主要体现在种群盖度的降低和叶片氮磷含量（特别是磷含量）有所降低，而个体构件生长状况无显著差异；（2）灌丛间群落的物种组成随围封年限增加发生显著变化：禁牧后1年生植物逐渐退出群落，多年生丛生禾草重要值随围封年限增加而显著增加，即围封禁牧对小叶锦鸡儿灌丛化草原的群落恢复有积极作用；（3）小叶锦鸡儿种群盖度与多年生丛生禾草的重要值显著负相关，说明两者之间存在竞争关系，推测多年生丛生禾草的竞争是禁牧后小叶锦鸡儿衰退的重要原因。;Grassland ecosystem is one of the most important terrestrial ecosystems in the world which may impose an impact upon global environmental change. The dynamics of a grassland ecosystem can be affected by disturbance in terms of promoting community succession and changing community structure. Among the disturbing factors, grazing is considered one of the key factors which affect both above-and below-ground processes in the ecosystem. One of the most prominent consequences of overgrazing is the transformation of grassland to shrub-land. In order to protect grassland ecosystem and restore vegetation productivity, a number of measures have been put into practice, among which fencing an area to exclude livestock is often considered to be a simple but effective way. The aim of the present study was to explore the response of the <em>Caragana microphylla</em>-dominated community to fencing and to predict when the degraded community would restore to its normal status after grazing exclusion. The study site was located in Xilingol degraded grassland of Inner Mongolia, China. Long-time grazing resulted in the expansion of <em>C. microphylla</em> populations in this site, and we set different treatments since 2003. Animals were excluded by fencing in the plot of grazing exclusion (EG), while the grazing intensity was controlled at 3 sheep/hm² in the plot of grazing (G). The two plots were adjacent and had similar environment until 2003. We investigated the growth and physiological traits of <em>C. microphylla</em> and the basic characteristics of the herbaceous communities, and compared the temporal changes in plot EG after 5-8 years of fencing and the spatial differences between these two plots after eight years of different utilization regimes. We analyzed the response of the traits of <em>C. microphylla</em> and the composition of herbaceous community to grazing and fencing, and also the relationship between the cover of <em>C. microphylla </em>and the ‘Importance Value’ of different functional groups. (1) The coverage of <em>C. microphylla</em> decreased in the plot EG during the four years while the morphological traits showed no significant variation, indicating that <em>C. microphylla</em> population declined after grazing exclusion. The decline of <em>C. microphylla </em>reflected a decrease of population coverage instead of the growth status of individual modules. Moreover, leaf nitrogen and phosphorus content, especially the latter, decreased after grazing exclusion, which might be associated with the decline of <em>C. microphylla</em> population. (2) Herbaceous community composition showed significant change after eight years of grazing exclusion. Annuals disappeared from the community, while the Important Value of perennial bunch grasses increased significantly in the plot EG both in temporal and spatial comparison. The change of community structure indicated that grazing exclusion had a positive effect on the restoration of <em>C. microphylla</em>-dominated grassland. (3) The coverage of <em>C. microphylla</em> decreased with the increasing Importance Value of perennial bunch grasses, indicating that there existed a competition between <em>C. microphylla</em> and the perennial bunch grasses. The decline of <em>C. microphylla</em> population might be caused by the expansion of well-grown perennial bunch grasses. It is suggested that grazing exclusion impose an effect on the restoration of <em>C. microphylla</em>-dominated community, but re-establishment of the typical steppe with climax community needs at least eight years; in other words, the restoration of <em>C. microphylla</em>-dominated grassland to its original status is a long-term process.

Effects of soil physical characteristics and biotic interferences on the herbaceous community composition and species diversity on the campus of Banaras Hindu University, India

Soil, water and species diversity relationships are central components of the vegetation ecology. In this connection, the present study was performed on the three sites within the campus of Banaras Hindu University of India, to relate herbaceous species diversity to soil physical characteristic and the intensity of biotic interferences. At each site, three, 10 m × 10 m plots were randomly established and within each plot, four quadrats each 50 cm × 50 cm were randomly placed for sampling. For each quadrat, number of individuals and their herbage cover were recorded by species. Soil physical characteristics (soil moisture, water-holding capacity, soil porosity and bulk density), elements of biotic interferences and α-diversity and its components were determined for each plot. The plots were ordinated by Non-metric Multidimensional Scaling (NMS) using Importance Value Indices of the component species. Results showed that the selected locations differed in terms of soil moisture and species diversity parameters due to differences in biotic interferences. NMS ordination yielded three groups corresponding to the three communities experiencing different intensity of land use. NMS axes were substantially related to the soil and herbaceous diversity parameters and suggested that the elements of soil physical characteristics, intensity of biotic interferences and regional herbaceous species pool had profound effect on the organization and determination of herbaceous floristic composition. Further, the sample locations exhibiting greater soil moisture, water-holding capacity, soil porosity and lesser soil bulk density harboured greater herbaceous diversity. A negative relationship between indices of species diversity and soil bulk density revealed that the dry and compact soils due to greater biotic pressure contributed to the loss of species diversity. Reduction in livestock numbers, grazing pressure and soil bulk density could be helpful in the promotion of soil quality and species diversity.

Early environments drive diversity and floristic composition in Mediterranean old fields: Insights from a long-term experiment

While many studies have explored how previous and current environmental conditions affect the performance of individual organisms, their relative importance as drivers of current diversity and composition of communities is virtually unknown. We evaluated the response of herbaceous communities to previous (experienced during early establishment) and current environmental conditions by comparing their composition and diversity in an abandoned Mediterranean cropland planted with Quercus ilex subsp. ballota L. seedlings. These seedlings received different experimental treatments (summer irrigation and artificial shading) during the first three years after planting, and were interrupted from then on. We tested two complementary hypotheses: (i) the previous environments experienced by the herbaceous communities during their establishment have a long-term carry-over effect on diversity and composition of species assemblages and (ii) these communities are influenced by their current environment, particularly by the woody layer and the soil conditions. Overall, we observed an important contribution of initial environmental conditions in determining the current diversity and composition of herbaceous communities. Amelioration of environmental conditions, particularly water stress, during community establishment resulted in a decrease in alpha and beta diversity, possibly as a consequence of decreasing environmental heterogeneity. Previous environments accounted for 26.3% of the explained variance in current community composition. Annuals, legumes and forbs also responded significantly to previous environments, which explained 27.9%, 36.2% and 30.1%, respectively, of the variance in their composition. Our results suggest that those species present at a particular site early in succession pre-empt the site and influence vegetation dynamics on that site for a long time. This study provides important insights for understanding the mechanisms underlying the ecological effects of issues like cropland reforestation and woody vegetation encroachment.