Analysis of Grassland Vegetation of the Southwest Heilongjiang Steppe （China） Using the Power Law

In 1997, we conducted a vegetation survey in three semi-arid natural grasslands （steppes） with different livestock grazing intensities in Southwest Heilongjiang Province, China, The dominant grassland species was the grass Stipa baicalensis Roshev. Grasslands with light, intermediate, and heavy grazing intensities were located 10, 5, and 2 km from a village, respectively. Villagers use the steppe to raise cattle, horses, sheep, and goats. Each of the three grasslands was surveyed by placing 100 quadrats （50 cm×50 cm） along a 50 m line transect. Each quadrat was divided into four equal areas （25 cm×25 cm; S-quadrats） and all plant species occurring in each of these smaller areas were identified and recorded. These data were summarized into frequency distributions and the percentage of S-quadrats containing a given species and the variance of each species were estimated. The power law was applied to these estimates. The power law was used to evaluate the spatial heterogeneity and frequency of occurrence for each species in the grassland community. The lightly grazed grassland exhibited high spatial heterogeneity （caused by large plant size）, the highest species diversity, and a high occurrence of S. baicalensis. In contrast, the heavily grazed grassland exhibited high spatial heterogeneity （caused by patchy populations of small plant size）, low species diversity, and a low occurrence of S. baicalensis. We judged that the heavily grazed grassland was overgrazed and exclusion of livestock from the degraded areas is necessary for recovery.     

Increasing nitrogen deposition enhances post-drought recovery of grassland productivity in the Mongolian steppe

Arid regions are prone to drought because annual rainfall accumulation depends on a few rainfall events. Natural plant communities are damaged by drought, but atmospheric nitrogen (N) deposition may enhance the recovery of plant productivity after drought. Here, we investigated the effect of increasing N deposition on post-drought recovery of grassland productivity in the Mongolian steppe, and we examined the influence of grazing in this recovery. We added different amounts of N to a Mongolian grassland during two sequential drought years (2006 and 2007) and the subsequent 3 years of normal rainfall (2008–2010) under grazed and nongrazed conditions. Aboveground biomass and number of shoots were surveyed annually for each species. Nitrogen addition increased grassland productivity after drought irrespective of the grazing regime. The increase in grassland productivity was associated with an increase in the size of an annual, Salsola collina, under grazed conditions, and with an increase in shoot emergence of a perennial, Artemisia adamsii, under nongrazed conditions. The addition of low N content simulating N deposition around the study area by the year 2050 did not significantly increase grassland productivity. Our results suggest that increasing N deposition can enhance grassland recovery after a drought even in arid environments, such as the Mongolian steppe. This enhancement may be accompanied by a loss of grassland quality caused by an increase in the unpalatable species A. adamsii and largely depends on future human activities and the consequent deposition of N in Mongolia.     

Community dynamics of desert grasslands: influences of climate,landforms, and soils

Abstract. Permanently marked vegetation transects in Big Bend National Park, Texas, USA were monitored to follow temporal dynamics of desert grassland communities on a variety of landforms and soil types over a 26-yr period after the removal of domestic livestock. Historic records indicate that the park area was severely overgrazed prior to its establishment, and our results show that the species present increased in both cover and density after the removal of livestock. However, the timing of recovery corresponded to multiyear periods of above-average precipitation. Little change was observed in between 1955 and 1960, a period dominated by several consecutive years of drought. The cover of two large shrubs common to the Chihuahuan Desert, Larrea tridentata and Flourensia cernua, increased from 1960 to 1967, a period dominated by summer drought and frequent wet winters. The cover and density of forbs, perennial grasses, and most shrubs increased on nearly all landforms between 1967 and 1981, when summers were wetter than average. In contrast, the cover of Larrea tridentata decreased during this period. Comparisons among the plant communities on each landform showed that they diverged through time after domestic livestock were removed. Presumably, differences in topographic position and soil texture influence water availability which was reflected in the species composition on each soil series. Unfortunately, we cannot isolate the effects of recovery from grazing from the effects of climate because the study design did not include control plots located within grazed pastures. Certainly, the directional trajectory of change and the regrowth of grasses into inter-shrub spaces, must, at least in part, be the result of recovery from grazing. However, our data also indicate that the desert grassland communities are sensitive to multi-year periods of above- or below-average precipitation. Clearly, the dynamics between shrubs and grasses cannot be explained by a simple successional paradigm that views increased shrub dominance as retrogression from a climax grassland. Many alternate hypotheses have been forwarded to explain the dynamics that control the vegetation composition in the desert and desert grassland region of North America. Experimental tests of these hypotheses are needed to indentify the interactions between biotic and abiotic factors that control dominance by shrubs or grasses.     

不同放牧制度下呼伦湖流域草原植被冠层截留

冠层截留是降雨过程中的水量分配和流域水平衡的一个重要组成部分,通过水浸泡法和降雨模拟实验研究呼伦湖流域草原3种放牧制度下(休牧、轮牧、自由放牧(超载放牧))植被冠层截留量的变化规律,并利用遥感解译植被归一化指数(NDVI),确定3种放牧制度下草原面积,估算呼伦湖流域草原降雨截留量。研究表明:在休牧、轮牧、自由放牧3种制度下,水浸泡法测定的截留量分别是0.468、0.320、0.271 mm。降雨模拟实验法测得的结果分别是0.957、0.613、0.431 mm。休牧、轮牧、自由放牧草场叶面积指数、盖度、容重、生物量等指标差异显著(P0.05),且单株植被高度、鲜重对截留量影响显著呈线性正相关关系。呼伦湖流域草原一次降雨量为大于等于30 mm全流域降雨,其植被截留量为6.462×106m3。     

Differential Indirect Effects of Excluding Livestock and Rabbits from Chalk Heath on the Associated Leafhopper (Hemiptera: Auchenorrhyncha) Fauna

Preliminary results are presented of sampling the leafhopper assemblages on a field experiment designed to examine the differential effects of rabbits and livestock (mainly sheep) on the vegetation of chalk heath in southern England. Experimental plots that excluded livestock either allowed entry by rabbits or excluded them. Results were compared with those from plots grazed by both livestock and rabbits. After 7 years, exclusion of grazing herbivores had resulted in predictable increases in vegetation height, but no major changes were detected in the species composition of the vegetation. As expected, ungrazed plots had higher species richness and greater abundances of several individual leafhopper species. However, plots grazed only by rabbits had a leafhopper assemblage that was distinct from either ungrazed or mixed grazing plots. It is suggested that rabbit grazing may have subtle effects on grassland invertebrate assemblages that are not necessarily predictable from an examination of the species composition of the vegetation. Chalk heath vegetation contains an unusual mixture of calcicole and calcifuge plant species, but the leafhopper assemblage included a restricted number of calcareous grassland specialist species and only one species strongly associated with acidic grasslands; most leafhoppers recorded were generalist grassland species.     

Refuge effect of an unpalatable forb on community structure and grass morphology in a temperate grassland

The role of unpalatable plant species as biological antiherbivore refuges for palatable species is well-documented at community level particularly in harsh environments. In productive sub-humid temperate grassland subjected to domestic grazing, we examined the protective effect of Eryngium horridum on plant community structure and floristic composition, and evaluated whether these changes impacted on a number of morphological traits of grasses, related to grazing resistance. We also investigated, for a palatable grass species (Stipa neesiana) the existence of morphological differences between protected and unprotected plants and if this eventual variation was either plastic or genetic. The study consisted of a field survey where we compared paired patches, with and without E. horridum, and a greenhouse experiment where we evaluated individuals of S. neesiana coming from both patch types over a 11 months period. Patches dominated by E. horridum had lower richness and cover of forbs than patches without the forb, and similar richness but greater cover of cool-season tussock palatable grasses, which suggests a protective role on the latter. Grasses in these patches also had longer blades and sheaths and lower specific leaf area. The morphological differences of S. neesiana individuals collected from both patch types disappeared after 11 months growth in a common environment which revealed significant phenotypic plasticity in this species. These results suggest the existence of plant-to-plant facilitation in a productive ecosystem not only at community level, through changes in species richness and the promotion of palatable grasses, but also at population level, through plastic changes in aboveground morphological traits. Both facilitation and plasticity, would contribute to the persistence of threatened palatable grasses in the heavy grazed productive ecosystems.     

Variation in small-scale spatial heterogeneity of soil properties and vegetation with different land use in semiarid grassland ecosystem

Soil properties (i.e. soil organic carbon, SOC; soil organic nitrogen, SON; and soil C/N ratio) and vegetation in a semiarid grassland of Inner Mongolia, northern China, were studied with the method of geostatistical analysis. We examined the spatial heterogeneity of soil and plants, and possible impacts of land use on their heterogeneity and on the relationship between soil resources and plant richness. Land use affected small scale spatial heterogeneity in plants and soil. SOC, SON and C/N ratio displayed autocorrelation over a range of ～2 m under most circumstances on sites where livestock grazing had been excluded. The uncontrolled grazing site (UG, i.e. unregulated grazing by excessive livestock) displayed an increased range of spatial autocorrelation and the total amount of variability in soil nitrogen over the other land use types. Plant life forms and plant species exhibited spatial autocorrelation over a range of about 2 m on the grazing exclusion (GE) and mowed (MW) sites, while pattern of spatial autocorrelation for several less common species on the UG site were difficult to predict. Plant species richness was positively related with spatial heterogeneity of SOC, SON and C/N on both GE and MW sites, and with only SOC heterogeneity on the UG site. These suggest that spatial soil heterogeneity plays an active role in maintaining plant species richness. However, we call for caution in generalization of the control of spatial soil heterogeneity over plant richness when multiple modes of disturbances are present, as we found in this study that higher total amount of variation in soil nitrogen and C/N ratio on the over-grazed UG site did not lead to increased plant species richness, and that land use had apparent effects on the patterns of spatial heterogeneity in both vegetation and soil.     

Effect of cattle grazing on soil salinity and vegetation composition along an elevation gradient in a temperate coastal salt marsh of Samborombón Bay (Argentina)

Salt marshes of Samborombón Bay (Argentina) have been grazed sporadically at very low stocking rates, but in the last decade, grazing intensity increased due to agriculture expansion. We investigated the effect of cattle grazing on vegetation and soil salinity on the most extended Spartina densiflora community. This community develops along an elevation gradient where the frequency and duration of tidal flooding and soil salinity increased as elevation decreased. Vegetation and soil data were collected from a national park excluded to cattle grazing for 30 years and from an adjacent commercial livestock farm continuously grazed by cattle. As elevation level decreased, plant cover, richness and diversity of functional groups and species decreased. As we expected, grazing altered soil salinity and vegetation composition in different extent along the elevation gradient. Grazing changed vegetation structure more intensively in the high elevation level because it reduced the competitive exclusion exerted by S. densiflora, allowing the increase in floristic richness. Grazing increased soil salinity and the contribution of salt-tolerant species only in the medium but not in the low elevation level probably because the higher frequency and duration of tidal flooding counterbalanced the increase in evaporation promoted by biomass removal in the low respect to the medium elevation level. While grazing may cause positive impacts for plant conservation in the high elevation level, it may cause negative consequence for livestock production because of the reduction in forage quality along the entire elevation gradient.     

Can switches in disturbance type improve habitat for grassland birds in semi-arid grasslands of South-Eastern Australia?

Grasslands are one of the most endangered and degraded ecosystems globally. Switches in disturbance type can restore grassland function and improve conservation outcomes for fauna, but land-use legacies can limit the capacity of biota to respond positively to shifts in disturbance type, making it difficult to predict ecological outcomes. In semi-arid grasslands of South-Eastern Australia, habitat for grassland birds is managed using livestock grazing, a practice that has continued for >150 years. It is unknown if outcomes for birds can be improved by a switch in disturbance type as alternatives to livestock grazing have not been explored. We compare the effects of status-quo livestock grazing with alternative biomass management tools (no management, ‘crash’ grazing, planned fire) on birds, vegetation structure, and food resources using a BACI design across 3 years (2017–2019). We found crash grazing and fire produced more open, shorter grassy swards, with less exotic grass cover, compared to status-quo grazing or disturbance exclusion. Preferred habitat structure for the critically endangered plains-wanderer (Pedionomus torquatus) was maximized by status-quo grazing. Grassland birds responded in opposing ways to a disturbance switch. Brown songlarks (Megalurus cruralis) and Horsfield's bushlarks (Mirafra javanica) responded positively to disturbance exclusion, while stubble quail (Coturnix pectoralis) responded negatively to crash grazing. Australasian pipits (Anthus novaeseelandiae) were more frequent in response to status-quo grazing. Our findings suggest that multiple disturbance types should be used if the aim is to promote the spectrum of vegetation structures and food sources required to support a diverse grassland bird community in semi-arid grasslands of Australia.     

Long-Term Increase in Aboveground Carbon Stocks Following Exclusion of Grazers and Forest Establishment in an Alpine Ecosystem

Ecosystem stores of carbon are a key component in the global carbon cycle. Many studies have examined the impact of climate change on ecosystem carbon storage, but few have investigated the impact of land-use change and herbivory. However, land-use change is a major aspect of environmental change, and livestock grazing is the most extensive land use globally. In this study, we combine a grazing exclosure experiment and a natural experiment to test the impact of grazer exclusion on vegetation dynamics and ecosystem carbon stores in the short term (12-year exclosures), and the long term (islands inaccessible to livestock), in a heavily grazed mountain region in Norway. Following long-term absence of sheep, birch forest was present. The grazing-resistant grass Nardus stricta, dominated under long-term grazing, whilst the selected grass Deschampsia flexuosa and herb species dominated the vegetation layer in the long-term absence of sheep. The established birch forest led to vegetation carbon stocks being higher on the islands (0.56 kg C m^?2 on the islands compared to 0.18 kg C m^?2 where grazed) and no difference in soil carbon stocks. In the short-term exclusion of sheep, there were minor differences in carbon stocks reflecting the longer term changes. These results show that aboveground carbon stocks are higher in the long-term absence of sheep than in the continual presence of high sheep densities, associated with a vegetation state change between tundra and forest. The reduction of herbivore populations can facilitate forest establishment and increase aboveground carbon stocks, however, the sequestration rate is low.     

Asymmetric effects of grazing intensity on macroelements and microelements in grassland soil and plants in Inner Mongolia Grazing alters nutrient dynamics of grasslands

Grazing is a traditional grassland management technique and greatly alters ecosystem nutrient cycling. The effects of grazing intensity on the nutrient dynamics of soil and plants in grassland ecosystems remain uncertain, especially among microelements. A 2‐year field grazing experiment was conducted in a typical grassland with four grazing intensities (ungrazed control, light, moderate, and heavy grazing) in Inner Mongolia, China. Nutrient concentration was assessed in soil and three dominant plant species (Stipa krylovii, Leymus chinensis, and Cleistogenes squarrosa). Assessed quantities included four macroelements (carbon (C), nitrogen (N), phosphorus (P), and magnesium (Mg)) and four microelements (copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn)). Soil total C, total N, total P, available N, and available P concentrations significantly increased with grazing intensity but soil Mg, Cu, Fe, Mn, Zn concentrations had no significant response. Plant C concentration decreased but plant N, P, Mg, Cu, Fe, Mn, and Zn concentrations significantly increased with grazing intensity. In soil, macroelement dynamics (i.e., C, N, and P) exhibited higher sensitivity with grazing intensity, conversely in plants, microelements were more sensitive. This result indicates macroelements and microelements in soil and plants had asymmetric responses with grazing intensity. The slopes of nutrient linear regression in C. squarrosa were higher than that of S. krylovii and L. chinensis, indicating that C. squarrosa had higher nutrient acquisition capacity and responded more rapidly to heavy grazing. These findings indicate that short‐term heavy grazing accelerates nutrient cycling of the soil–plant system in grassland ecosystems, elucidate the multiple nutrient dynamics of soil and plants with grazing intensity, and emphasize the important function of microelements in plant adaptation in grazing management.     

Effects of grazing on plant communities and successional processes vary along an aridity gradient at a northern temperate grassland

The intermountain grasslands of North America reach their most northern geographic extent in interior British Columbia’s Cariboo-Chilcotin region. Here, this study examined the long-term effects of livestock grazing exclusion and reductions in grazing severity on plant community characteristics including plant and litter cover, species richness and abundance of leading species of 33 grassland sites across a broad aridity and soil property gradient. Across the aridity gradient, grazing reduced species richness, plant cover, and litter cover. However, the effects of grazing on dominant species varied across the gradient. In more arid grasslands, historical grazing substantially reduced cover of late-seral native bunchgrass Psuedoroegnaria spicata, and repeated measurements indicate that very long time periods are necessary for successional processes associated with recovery of native bunchgrasses. At the cool-wet end of the aridity gradient, successional processes are more rapid but dominated by exotic species Poa pratensis and Tragopogon pratensis. Recent (past 20 years) light grazing and rest-rotation have favored Poa pratensis at the expense of native needlegrasses (Achnatherum spp. and Hesperostipa spp.). We suggest that absence of a dominant large-stature native bunchgrass for mesic grasslands was a key factor in the invasion and dominance of Poa pratensis.

     

禁牧对伊犁绢蒿种群数量特征及其构件生物量分配的影响

该研究于2020年5月、7月和9月在天山北坡中段试验区的禁牧区(5a)和放牧区分别进行调查采样,测定分析不同月份禁牧区和放牧区伊犁绢蒿种群数量特征(高度、盖度、密度和生物量)及构件(茎、叶、根)生物量,以揭示植物种群特征和构件生物量对禁牧的响应规律,为退化草地的修复治理以及合理利用提供依据。结果表明：(1)与放牧区相比,5月、7月、9月禁牧区的伊犁绢蒿种群的高度、盖度和生物量均显著增加(P<0.05),其中高度增幅为69.90%～95.53%,盖度增幅为186.53%～297.82%,生物量增幅为86.24%～631.83%。(2)随着月份的推移,禁牧区与放牧区伊犁绢蒿单株生物量、茎生物量和叶生物量均呈先降后增的变化趋势,而根生物量呈增加的趋势;禁牧改变了植株构件比例,与放牧区相比,禁牧区茎、叶的生物量所占单株生物量的比例在7月、9月均显著增加,而根生物量所占比例显著降低。研究认为,禁牧有利于地上植被的恢复,使其构件结构改变,是恢复退化荒漠草地植物的有效措施。     

Effects of open grazing and livestock exclusion on floristic composition and diversity in natural ecosystem of Western Saudi Arabia

Livestock grazing is one of the main causes of rangeland degradation in Saudi Arabia. Fencing to exclude grazers is one of the main management practices used to restore vegetation and conserve biodiversity. The main objectives of this study were to investigate the changes in plant diversity and abundance, floristic composition and plant groups of the major life forms in response to thirty-five years of grazing exclosure in western Saudi Arabia. These vegetation attributes and palatability were compared in 30 sampling stands located in the excluded and grazed sites. Our results showed that livestock exclusion significantly increased covers, density and species richness of annuals, grasses, perennial forbs, shrubs and trees. Exclosure enhanced the abundance and richness of palatable species and depressed the development of weedy species. About 66.7% of the recorded species at the excluded site were highly palatable compared to 34.5% at the grazed site. In contrary, about 55.2% unpalatable species were found in the grazed site compared to 25.8% in the protected site. Jaccard’s similarity index between the excluded and grazed sites showed lower values of 0.39%, 0.40% and 0.31% at levels of families, genus and species, respectively. The results suggest that establishing livestock exclusion may be a useful sustainable management tool for vegetation restoration and conservation of plant diversity in degraded rangelands of arid regions.     

The effect of grazing on winter survival of midday gerbil (Meriones meridianus) of different genders

The objective of this study was to investigate the effects of grazing on midday gerbil (Meriones meridianus) population characteristics and survival of animals of different genders. The experiment used a randomized complete block design and was conducted in Alxa Left Banner, Inner Mongolia, China, in 2002 (The agricultural reclamation plots set up in 1994). From April 2006 to October 2010, midday gerbils were live‐trapped in 3 light grazing plots, 3 overgrazed plots, and 3 grazing exclusion plots. The quantity of vegetation was investigated in the two different grazing intensity areas and grazing exclusion area to determine the relationship between gerbils and plant food availability. The results suggested that there was higher gerbil density, individual body mass, and daily body mass growth rate in the grazing exclusion sites than the other sites across the whole year. Females had higher survival in grazing exclusion areas than in other treatments, but the males’ survival showed the opposite pattern. Our results indicated that grazing negatively influenced the midday gerbil population by reducing food availability. Grazing influenced the survival rates of male midday gerbils positively, but had negative effects on females. The reason for gendered differences in survival rates of midday gerbils requires further investigation.     

Complex effects of grazing treatment on an annual in a species-poor grassland community

Abstract. The effects of grazing upon the establishment, survival, growth and reproduction of a grassland annual Geranium dissectum growing in a sward dominated by grasses were examined in a replicated grazing experiment with sheep. Seeds were sown in both summer and autumn, and grazing was controlled to produce two levels of grazing in winter, two levels in spring, and two in summer, combined in a 2 x 2 x 2 factorial experimental design. Higher intensities of grazing in the period immediately before emergence benefitted plant establishment, but subsequent survival showed many interactions between factors, demonstrating that under certain conditions and at certain times grazing was detrimental. It is suggested that the frequency of G. dissectum in the grassland was low because the heavy grazing conditions that foster seedling emergence also jeopardize subsequent survival. This may also be why productive grassland communities in general contain few palatable dicots.     

Grassland responses to multiple disturbances on the New England Tablelands in NSW,Australia

Disturbances have played a significant role in shaping vegetation patterns following European settlement and agricultural development in Australia, particularly over the last 100 years. However, little is known about the temporal dynamics of plant communities in relation to disturbances and their interactions. In this study we examined the response and recovery of temperate grassland communities to disturbance, using a multi-factorial experiment involving grazing exclusion (absent and present), fire (absent and present), soil cultivation (none, 5 and 20 cm) and soil amendment (none, fertiliser and fertiliser plus clover seeds) on the New England Tablelands in NSW, Australia. Temporal patterns of plant communities were analysed using detrended correspondence analysis for eight surveys over 24 months. Vegetation patterns at each survey were analysed using principal components analysis. The effects of treatments on malleability (Bray–Curtis dissimilarity) of plant communities were analysed using a linear mixed model, and the relationships between malleability and plant species groups were depicted using a generalised additive model and further analysed using a linear model. Perennial native grasses and a non-native forb (Hypochaeris radicata) initially dominated the vegetation, but after the disturbances H. radicata and other non-native species dominated. Compositional changes from the initial states were greatest in the first spring (7 months after treatment application), and then the vegetation tended to recover towards its original state. Soil cultivation resulted in the greatest deviation in community composition, followed by soil amendment, fire, with grazing exclusion the least. The recovery process and rate of recovery varied with treatment reflecting the dominance of soil cultivation and its interactions with other forms of disturbances. Soil amendment and grazing exclusion tended to reduce the effects of soil cultivation. Malleability was negatively related to perennial native grasses; positively to other non-native species, annual native grasses and perennial native sedges/rushes; and negatively to H. radicata when its cover was below 18%, but positively when above 18%. The degree of malleability reflected the high resilience of the vegetation to disturbance, and was mainly due to the recovery of perennial native grasses and H. radicata. This resilience demonstrated that the small-scale disturbances did not cause vegetation to cross an ecological threshold and that the present vegetation is resilient to common disturbances occurring at small scales. The results also suggest (1) that the present vegetation has developed mechanisms to adapt to these disturbances, (2) the importance of disturbance scale or (3) that the ecological threshold had already been crossed and the present vegetation is in a degraded state compared with its original state before the end of the 19th century.     

Spontaneous steppe restoration on abandoned cropland in Kazakhstan: grazing affects successional pathways

After the collapse of the Soviet Union in 1991, 12 million ha of cropland were abandoned in the steppe zone of Kazakhstan. At the same time livestock numbers crashed, leaving large areas of steppe without any grazing. We aimed to investigate, to which degree spontaneous succession on former cropland leads to the restoration of native steppe vegetation, and how this process is influenced by changing grazing patterns. We recorded biomass and vegetation characteristics as well as local soil and landscape variables in 151 quadrats of 100 m², covering 89 plots on former cropland abandoned in the early 1990s and 62 reference plots of near-natural steppe grassland that was never ploughed. About half of the plots of each category were located in a remote region where grazing has been absent for ca. 20 years, whereas the other half was located in a region with moderate livestock grazing. While there were no differences in the diversity, structure and plant life form composition of currently grazed and un-grazed near-natural steppe grasslands, corresponding successional plots on abandoned arable land exhibited significant differences. Grazed plots on former fields showed higher species richness and a higher cover of dwarf shrubs (mostly Artemisia spec.), ruderals and perennial herbs. At the same time, immigration of typical steppe species was much more successful. Contrary, in the absence of any grazing we found species-poor swards dominated by Stipa lessingiana and Leymus ramosus exhibiting an increasing frequency of wildfires due to litter accumulation. After 15–20 years, secondary steppe grasslands still differed substantially from their near-natural references. Our results suggest that grazing is mandatory to fully restore the original near-natural steppe vegetation and the underlying processes of pyric herbivory.     

黄土高原半干旱区退化草地恢复与利用过程研究

草地退化表现为土壤和植被遭到彻底破坏,草地演替过程受到强烈抑制.实验采用长期(30年)封禁措施,定位监测退化草地从次生半裸地演变为近似原生植被(进展演替)的变化过程.结果表明,随着封禁时间的变化,退化草地恢复演替经历了4个阶段,群落盖度、植株密度、物种丰富度和多样性指数、地上生物量和地下生物量在草地群落恢复过程中逐渐增加,其特征变化出现的峰值均在封禁第20年(地下生物量峰值在第15年),其中地上生物量最高达520.5 g/m2;直到封禁的第20~25年,以本氏针茅为建群种的草原群落衰败退化现象明显,而大针茅种群密度剧增;在封禁的第26年以上以大针茅为优势的群落生长较为稳定,从目前群落演替进程看,大针茅有替代本氏针茅的趋势.另外,在草原沟道两侧以斑块状聚集分布有中旱生灌木,群落的演替进入了一个新的阶段.随着封禁时间的延续,退化草地从自然封禁恢复的0~26年,通过侵入-竞争-扩散-定居的几个演替阶段,目前形成以大针茅为建群种相对稳定的"亚顶级".虽然草地生物量有一定下降,但草地质量提高,物种多样性丰富,促进草地的进展演替.草地植物群落主要由禾本科、豆科和菊科组成;多年生植物、C3和旱生物种可以作为草地演替过程和植被恢复的指示物种.长期封育对草地物种更新和生态系统稳定性有负面影响,因此,合理的封育时间是草地生态恢复中非常重要的一个因素.本研究提出,在黄土区退化草地封育10~15年后可以开始进行合理的利用,例如通过两年一次刈割和轻度放牧(2只羊/hm2).本研究可为干旱区、半干旱区相似的退化草地恢复提供理论依据.     

青藏高原高寒灌丛植被对长期放牧强度试验的响应特征

在青藏高原中国科学院海北高寒草甸生态系统定位研究站对金露梅高寒灌丛草场植被开展了长期不同放牧强度试验,分别在短期(4年)、中期(11年)和长期(18年)放牧阶段研究不同放牧干扰强度对草地植物物种多样性、群落结构、地上生物量和草场质量的影响.研究表明,在不同放牧阶段,随着放牧强度增加植物群落的高度和盖度都降低.在中期放牧干扰阶段,物种多样性数和均匀度指数随着放牧强度增加呈现典型的单峰曲线模式;在长期放牧干扰阶段,随着放牧强度增加,占优势地位的灌木和禾草被典型杂类草替代,其中的重度放牧干扰简化了高寒灌丛植被群落结构,减少了地上现存生物量,特别是可食优良牧草生物量.植被对放牧的响应除了与放牧强度和放牧时间阶段密切相关外,还与该地区水热条件的变化有一定的相关性.针对长期放牧干扰的反应特性可将金露梅灌丛草场中植物划分为增加型、敏感型、忍耐型和无反应型4种类型.除了丰富度指数、多样性指数和均匀度指数外,其它一些特征参数并不支持著名的中度干扰假说.本研究发现,长期重度放牧促进了青藏高原高寒草地退化,适度放牧有利于高寒灌丛草场的生物多样性保护和牧草利用;"取半留半"的放牧原则在青藏高原草场放牧管理实践中值得推荐,它将有利于防止草场退化,提高牧草利用率和维持较高的生物多样性.