Demographic variation in the Australian desert cassia under grazing pressure

Summary Demographic variation was examined in 3 populations of the Australian desert shrub Cassia nemophila which vary in their grazing histories. Age-specific life tables were constructed from 50 years of observations on mortality and recruitment at the Koonamore Vegetation Reserve in South Australia. Population projection matrix models were used to examine population responses to grazing pressure. The predicted population growth rates, reproductive values and stable age distributions are evaluated and compared with observed results. Grazing by sheep or rabbits, in high populations, prevents shrub recruitment and causes local population extinction. However, where protected from sheep and with low rabbit pressure, Cassia populations have increased. Current sheep grazing practices and rabbit population levels if continued will have a drastic affect on Cassia populations and other shrub species, and on the structure and composition of the Australian arid shrublands in general.     

轮牧时间对荒漠草原土壤种子库特性的影响

探讨季节性放牧下土壤种子库的特性对荒漠草原合理利用具有重要作用.以宁夏荒漠草原为对象,通过封育(FY)、传统时间轮牧(FG)、延迟开始轮牧(YG)、提前结束轮牧(TG)、延迟开始并提前结束轮牧(YT)和自由放牧(ZY)处理,研究了0～5、5～10、10～15 cm土层土壤种子库物种组成、种子库密度、垂直分布、多样性及其...     

放牧对短花针茅荒漠草原土壤团聚体组成及稳定性的影响

放牧作为草地的主要利用方式,对草地生态系统的结构和功能具有显著影响。目前长期放牧对草地土壤团聚体组成及其稳定性的影响还知之甚少。本研究依托2004年建立的内蒙古短花针茅荒漠草原长期放牧试验平台,设置围封禁牧(对照)、轻度、中度和重度放牧强度,研究不同放牧强度下土壤团聚体组成和稳定性的变化,并结合相关土壤理化指标探究影响土壤团聚体稳定性的主要因素。结果表明: 放牧显著改变了不同粒径土壤团聚体的组成。与对照相比,大团聚体(0.25～2 mm)含量在轻度放牧下保持不变,而在中度和重度放牧下显著下降;重度放牧显著降低了小团聚体(0.053～0.25 mm)含量,而中度和重度放牧显著增加了微团聚体(<0.053 mm)含量。土壤团聚体稳定性在轻度放牧下维持较高水平,但在中度和重度放牧下显著降低。土壤团聚体稳定性与大团聚体含量呈显著正相关,与微团聚体含量呈显著负相关。土壤pH值、容重、有机碳及其他理化性质共同作用于土壤团聚体组成进而对其稳定性产生影响。综上,对于荒漠草原而言,适度放牧可以维持较高的土壤团聚体稳定性。     

科尔沁草甸草地放牧和割草条件下土壤种子库研究

研究了科尔沁沙地草甸草地在放牧、割草两种利用方式下的土壤种子库的大小、组成及其与地上植被的关系．结果表明,在自由放牧下,土壤种子库密度为6158±1647粒·m^-2,在割草利用下,土壤种子库密度为8312±2540粒·m^-2．放牧干扰下,种子库组成以矮小、短命的一年生植物为主,其中一二年生植物占81．66％,占比例最大的前4种植物分别为虎尾草(38．55％)、灰绿藜(15．42％)、毛马唐(14．95％)和狗尾草(9．83％),多年生植物种子密度仅为1129±302粒·m^-2;割草干扰下,一二年生植物占68．08％,其中狗尾草占52．7％;而割草地多年生植物种子密度为2653±811粒·m^-2．放牧地土壤种子库密度与地上植物相关不显著;割草地土壤种子库密度与地上植物多度显著相关(r=0．76,P<0．01)．放牧地种子库的Shannon—Wiener指数、丰富度指数分别为2．96和2．98,明显小于割草地的3．10和5．09,表明自由放牧更易使物种多样性下降．     

Evidence for the retention of genetic variation in Erodium seed dormancy by variable rainfall

Summary The periodic occurrence of summer/early autumn precipitation in the California annual grassland can result in the formation of early and late emerging cohorts of Erodium botrys and E. brachycarpum. The occurrence of early rainfall and the timing of such rainfall are highly variable from year to year. A series of field watering experiments in 1980–81 were used to simulate early emergence conditions that would result from significant rainfall (1 cm) occurring in mid-July, late August, and mid-September. Net reproduction was used to estimate fitness differentials between Erodium cohorts emerging in response to a watering treatment (early emerging cohorts) and Erodium cohorts emerging with the onset of winter rains in mid-October (late emerging cohorts). Survival was lower and gross reproduction was higher among early emerging cohorts than late emerging cohorts. For both species, net reproduction of the early cohort was lower than that of the late cohort under the July watering treatment and higher than that of the late cohort under the August watering treatment.Early cohorts, formed in response to rainfall in mid-September, 1982, were also compared demographically to later cohorts emerging in October. Compared to late cohorts, net reproduction, gross reproduction and survival were higher for the early cohorts.Common garden experiments indicate that differences in the duration of seed dormancy between the progenies of early and late emerging plants reflect a significant genetic component. Progency produced by early cohorts of E. brachycarpum from all three watering treatments possessed more extended seed dormancy than progeny of late cohorts. In E. botrys, progeny from early cohorts emerging in response to the July watering treatment were also more dormant than late progeny. In contrast, early cohorts of E. botrys emerging in response to the September watering treatment produced seed less dormant than seed produced by late cohorts. When combined with demographic data, indicating that fitness differentials between early and late cohorts varied with changes in the date of early emergence, genetic results suggest that year to year variation in early rainfall may act to retain genetic variation in the duration of seed dormancy.     

Root and shoot growth, seed composition, and yield components of no-till rainfed soybean under variable potassium

Spatial soil-K availability for no-till soybean [Glycine max (L.) Merr.] has not been studied extensively. We characterize soybean growth- and yield-component and quantify root parameters as a function of soil depth in K-stratified soils with 1 M ammonium acetate extractable-K ranges 60–290 at 0–10 cm increment and 50–90 mg kg^?1 at the 10–20 cm increment. Shoots and roots (five depth increments to 50 cm) were collected during development and grain at harvest during 2 years. Soil K at or above the critical level (104 mg K kg^?1) increased early-season leaf area and root K-uptake rates early and late in reproductive development. Greater number of seeds plant^?1 increased yield for soils with K near the critical level. Soil-K above the critical level increased luxury K-uptake without improving yield, seed-K concentration and accumulation, or seed oil and protein concentration. Greater root length density (>41% of the total) in surface soil coupled with previous results showing greater water content throughout the season in surface soil compared to deeper layers illustrates K stratification caused by no-till may enhance soybean K-uptake.     

Flooding, soil seed bank dynamics and vegetation resilience of a hydrologically variable desert floodplain

1. This paper explores soil seed bank composition and its contribution to the vegetation dynamics of a hydrologically variable desert floodplain in central Australia: the Cooper Creek floodplain. We investigated patterns in soil seed bank composition both temporally, in response to flooding (and drying), and spatially, with relation to flood frequency. Correlations between extant vegetation and soil seed bank composition are explored with respect to flooding. 2. A large and diverse germinable soil seed bank was detected comprising predominantly annual monocot and annual forb species. Soil seed bank composition did not change significantly in response to a major flood event but some spatial patterns were detected along a broad flood frequency gradient. Soil seed bank samples from frequently flooded sites had higher total germinable seed abundance and a greater abundance of annual monocots than less frequently flooded sites. In contrast, germinable seeds of perennial species belonging to the Poaceae family were most abundant in soil seed bank samples from rarely flooded sites. 3. Similarity between the composition of the soil seed bank and extant vegetation increased following flooding and was greatest in more frequently flooded areas of the floodplain, reflecting the establishment of annual species. The results indicate that persistent soil seed banks enable vegetation in this arid floodplain to respond to unpredictable patterns of flooding and drying.     

不同放牧强度下短花针茅荒漠草原植被的空间异质性

分析不同放牧强度下植物群落中物种的空间分布特征, 有助于阐明群落在放牧胁迫下的演替规律。该研究基于幂函数法则, 探讨了不同放牧强度下短花针茅(Stipa breviflora)荒漠草原群落植物的频率和空间异质性。结果表明: 不同放牧强度下物种空间分布与幂函数法则能很好地吻合; 不同物种空间异质性具有特异性, 随着放牧强度的增加, 提高群落空间异质性的物种分别由无芒隐子草(Cleistogenes songorica)、冷蒿(Artemisia frigida)、短花针茅、银灰旋花(Convolvulus ammannii)等多个物种逐渐转变为以无芒隐子草、短花针茅为主的少数物种, 同时, 物种空间异质性大于群落空间异质性的物种数逐渐减少。     

Increased autumn rainfall disrupts predator–prey interactions in fragmented boreal forests

There is a pressing need to understand how changing climate interacts with land‐use change to affect predator–prey interactions in fragmented landscapes. This is particularly true in boreal ecosystems facing fast climate change and intensification in forestry practices. Here, we investigated the relative influence of autumn climate and habitat quality on the food‐storing behaviour of a generalist predator, the pygmy owl, using a unique data set of 15 850 prey items recorded in western Finland over 12 years. Our results highlighted strong effects of autumn climate (number of days with rainfall and with temperature <0 °C) on food‐store composition. Increasing frequency of days with precipitation in autumn triggered a decrease in (i) total prey biomass stored, (ii) the number of bank voles (main prey) stored, and (iii) the scaled mass index of pygmy owls. Increasing proportions of old spruce forests strengthened the functional response of owls to variations in vole abundance and were more prone to switch from main prey to alternative prey (passerine birds) depending on local climate conditions. High‐quality habitat may allow pygmy owls to buffer negative effects of inclement weather and cyclic variation in vole abundance. Additionally, our results evidenced sex‐specific trends in body condition, as the scaled mass index of smaller males increased while the scaled mass index of larger females decreased over the study period, probably due to sex‐specific foraging strategies and energy requirements. Long‐term temporal stability in local vole abundance refutes the hypothesis of climate‐driven change in vole abundance and suggests that rainier autumns could reduce the vulnerability of small mammals to predation by pygmy owls. As small rodents are key prey species for many predators in northern ecosystems, our findings raise concern about the impact of global change on boreal food webs through changes in main prey vulnerability.     

长期不同放牧强度下荒漠草原建群种短花针茅(Stipa breviflora)叶片解剖结构变化

植物叶片解剖结构随放牧强度的变化而发生变化。以短花针茅荒漠草原长期(19年)固定监测放牧样地为研究区,以建群种短花针茅叶片作为实验材料,测量不同放牧强度(对照(CK)、轻度放牧(LG)、中度放牧(MG)、重度放牧(HG))下短花针茅叶片保护组织、输导组织、机械组织、同化组织有关的14个解剖结构指标,分析短花针茅叶片解剖结构随长期不同放牧强度的变化,从生理角度探讨荒漠草原植物对长期放牧的适应及响应。研究结果表明：(1)叶片层次上,短花针茅通过增加其叶片横切面面积和叶厚度去适应放牧干扰;(2)保护组织方面,与对照区相比,重度放牧区角质层厚度增加,但未达到差异显著性(P>0.05),角质层厚度占叶厚度比例随放牧强度的增加整体呈增加趋势;(3)输导组织方面,木质部面积随放牧强度的增加呈下降趋势,而韧皮部面积、主导管面积、维管束面积等指标均随放牧强度的增加呈增加趋势,但没有达到显著性(P>0.05);(4)机械组织方面,厚壁组织面积和厚壁组织占叶面积比例均随放牧强度的增加呈增加趋势;(5)同化组织方面,随着放牧强度的增加,短花针茅叶片叶肉组织面积显著增加(P<0.05)。可见,...     

荒漠草原不同放牧强度背景下添加氮水对凋落物分解的影响

凋落物分解是连接生态系统地上、地下过程的重要环节,决定了生态系统养分循环速率,但到目前为止对凋落物分解在荒漠草地生态系统受放牧以及外源资源补给影响的研究较少。本研究通过对不同放牧强度(对照、轻牧、中牧和重牧)短花针茅草原群落进行添加氮素(10.0 g N m~(-2) a~(-1))和增水(108 mm/a)处理,探讨群落水平凋落物分解速率的变化。研究结果显示,过去不同强度放牧历史对群落凋落物分解影响极显著(P0.0001)。凋落物前期分解(135 d)过程中,凋落物初始C∶N比与凋落物分解速率常数呈显著负相关关系,表明凋落物可降解性在凋落物前期分解中起主要作用。轻度放牧影响下凋落物分解速度最快,这与该条件下凋落物C∶N比显著低于其他放牧强度下的有关,说明适度放牧不仅有利于群落维持,也在一定程度上有利于生态系统养分循环。当凋落物分解更长时间(870 d)后,对照区凋落物分解速率显著低于放牧处理样地,但凋落物初始C∶N比对凋落物分解速率没有显著影响。进一步分析显示,不同放牧强度背景下长期凋落物分解速率与分解环境的土壤微生物多样性成正相关关系,与群落盖度呈极显著(P0.001)负相关关系。添加氮素显著(P0.05)降低凋落物分解速度,但对凋落物氮含量无显著影响。生长季加水未影响凋落物质量及凋落物分解速度。研究结果表明,凋落物前期分解受凋落物质量影响,但较长时间凋落物分解则与分解过程中接受到的太阳辐射量有关。     

Summer irrigation,grazing and seed addition differentially influence community composition in an invaded serpentine grassland

Community assembly theory predicts that resource availability, biotic interactions, and dispersal dynamics will determine community composition. Recent work has demonstrated that manipulating these processes or “filters” to exclude exotic species may assist in restoring invaded plant communities. In this study, we began by manipulating an abiotic filter, summer water availability, on the theory that irrigation prior to the growing season could trigger the germination of exotic species during unfavorable environmental conditions. First, we performed a greenhouse experiment to assess the germination traits of 23 native and exotic species at low (16°C, spring) and high (30°C, summer) temperatures. At summer temperatures, we found high emergence of many exotic and native grasses and low emergence of native forbs suggesting that summer irrigation may help deplete the exotic seed bank. In a second experiment, we established field plots to test the efficacy of summer irrigation and simultaneously manipulated a biotic and a dispersal filter, subjecting some plots to grazing and/or native seed addition. Summer irrigation and seed addition had no effect on percent cover or species richness while grazing reduced native cover but increased native species richness and soil nitrogen content. Our data suggest that manipulating grazing (a biotic filter) may be more effective than altering abiotic or dispersal filters when restoring invaded serpentine grassland. However, summer irrigation may also be effective, if applied at lower temperatures or for longer periods.     

Evaluating germinability of eight desert halophytes under long-term seed storage: Implications for conservation

Ex situ conservation in seed banks is a potential complementary conservation strategy for native plant species.It is well established that ex situ seed banking of native wild plants prolongs seed viability and thereby preserves genetic and species diversity for future use.We evaluated ex situ storage potential of eight halophytic species from deserts in the United Arab Emirates(UAE)by studying seed germination.Specifically,we examined the germinability of freshly collected seeds and seeds stored for three years in a seed bank.We also examined the effect of light conditions on fresh and stored seed germination.Fresh seeds of seven of the eight species tested had a higher germination rates under 12/12 h light/dark fluctuations than did those exposed to total darkness.Storage reduced light sensitivity in Halocnemum strobilaceum,Suaeda aegyptiaca,Salsola drummondii and Salsola imbricata,but increased the requirement for light in Arthrocnemum macrostachyum.In Anabasis setifera,storage decreased germination percentage when there was a 12-hour light/dark fluctuation,but increased germination rate when exposed to the dark treatment.Storage significantly reduced germination in both the light/dark and dark treatments in Suaeda vermiculata and S.aegyptiaca.Germination speed also responded differently to storage;whereas Timson's index significantly increased in A.macrostachyum and H.strobilaceum,it significantly decreased for S.drummondii,S.aegyptiaca and S.vermiculata.Germination of these species at a range of temperatures requires further testing;additionally,we strongly suggest that these laboratory findings be complemented by field studies.     

Early changes in species composition of upland sown grassland under extensive grazing management

Abstract. Due to economic pressures and policy changes Lolium perenne‐Trifolium repens sown swards in upland UK sheep systems are likely to become less intensively managed. We present results from the first 5 yr of a long‐term experiment studying vegetation change under more extensive grazing management at three sites. One treatment was representative of current, intensive management and 5 were unfertilized with different intensities of seasonal grazing. The species composition of unfertilized, ungrazed swards changed dramatically within 2 yr and the sown species had virtually disappeared by year 5. Ranunculus repens, Poa trivialis, Agrostis gigantea, Juncus spp. and Carex spp. became dominant at the wettest site. Grasses were dominant at the other sites. In contrast, the sown species were retained in the unfertilized, grazed treatments; there were small shifts in abundance of the species present initially and few additions or losses of species. Some colonizing species were present in the seed bank whereas others with a transient seed bank appeared to have invaded from neighbouring vegetation. Implications of these results for compensation schemes to reduce animal output and increase biodiversity are discussed.     

Habitat protection,cattle grazing and density‐dependent reproduction in a desert tree

Anthropogenic activities usually trigger changes in the population density of plants. Thus, land management practices can influence density‐dependent demographic parameters and species interactions. We investigated plant‐pollinator interactions and reproduction in Prosopis flexuosa, the largest tree species in the Central Monte desert of Argentina, an important economic and cultural resource for humans and a functionally prominent species. We hypothesized that reproductive output of P. flexuosa would be limited at low densities, and that exclusion of catle grazing would enhance population density and consequently interaction frequency with pollinators and reproductive success. The study was conducted in and around Ñacuñán Biosphere Reserve (Mendoza, Argentina), where cattle grazing has been excluded for over 35 years. Working in five pairs of protected and cattle grazed 1‐ha plots, we recorded density of adult trees, pollinator visitation frequency to inflorescences and seeds per inflorescence in focal trees. Adult tree density was higher in protected plots than in cattle grazed plots. Density of reproductive trees was positively correlated with seed production, suggesting positive density dependence for reproduction (Allee effect). Pollinator visitation to inflorescences and seed production was higher in protected plots compared with plots under cattle grazing. Suppression of anthropogenic degradation has resulted in higher adult tree density in protected plots, indirectly higher pollinator visitation to inflorescences and higher reproductive success of trees. Increased frequency of plant‐pollinator interactions and tree reproduction suggest success of management practices aimed at protecting P. flexuosa woodlands.     

埋藏条件下3种干旱荒漠植物的种子休眠释放和土壤种子库

对种子休眠的自然释放及其作用因素的研究, 是了解种子休眠生态学、种群适应机制的重要途径。以内蒙古阿拉善干旱荒漠区的3种主要植物牛枝子(Lespedeza potaninii)、唐古特白刺(Nitraria tangutorum)和骆驼蒿(Peganum nigellastrum)为材料, 研究了种子在野外埋藏18个月期间和4个埋深条件下的休眠释放特性和土壤种子库。3种植物种子在野外埋藏时(采收后5 ℃冷藏6个月)的休眠率分别为98%、95%和3%。结果显示, 埋藏过程中, 3种植物种子的休眠释放表现出不同的变化特性。对牛枝子而言, 置于地表(0 cm)的种子比埋藏于土中的种子的休眠释放快, 埋藏期末, 埋深0、2、5和10 cm的种子的休眠率分别为64%、87%、86%和82%。唐古特白刺种子埋藏6个月后, 各埋深的休眠已完全释放, 释放速率随埋深增加而加快。骆驼蒿种子具有典型的季节性休眠循环特性, 休眠率各年度最高点出现在10月份, 释放速率随埋深增加呈减慢趋势。埋藏期末不同埋深条件下, 牛枝子、唐古特白刺和骆驼蒿种子的平均田间萌发率分别为11%、12%和8%; 平均室内萌发率分别为3%、74%和42%; 而平均死种子率分别为3%、15%和10%。根据Thompson和Grime (1979)的土壤种子库分类体系, 供试的3种植物都属于持久土壤种子库类型。     

Arbuscular mycorrhizal fungi associated with vegetation and soil parameters under rest grazing management in a desert steppe ecosystem

The impact of rest grazing on arbuscular mycorrhizal fungi (AMF) and the interactions of AMF with vegetation and soil parameters under rest grazing condition were investigated between spring and late summer in a desert steppe ecosystem with different grazing managements (rest grazing with different lengths of resting period, banned or continuous grazing) in Inner Mongolia, China. AMF diversity and colonization, vegetation biomass, soil properties and soil phosphatase activity were examined. In rest grazing areas of 60 days, AMF spore number and diversity index at a 0–10 cm soil depth as well as vesicular and hyphal colonization rates were higher compared with other grazing treatments. In addition, soil organic matter and total N contents were highest and soil alkaline phosphatase was most active under 60-day rest grazing. In August and September, these areas also had the highest amount of aboveground vegetation. The results indicated that resting grazing for an appropriate period of time in spring has a positive effect on AMF sporulation, colonization and diversity, and that under rest grazing conditions, AMF parameters are positively correlated with some soil characteristics.     

Effect of grazing removal on aboveground vegetation and soil seed bank composition in sub-alpine grasslands of northern Iran

Background: Knowledge about vegetation and soil seed bank composition and the processes that contribute to vegetation recovery after the removal of heavy grazing is lacking in sub-alpine ecosystems.

Aims: In order to assess the effects of large herbivores on above-ground vegetation (AGV) and soil seed bank (SSB) characteristics, intensively sheep-grazed areas were compared to adjacent areas where grazing had been removed 10 years previously in a sub-alpine grassland of northern Iran.

Methods: A total of 40 4-m² (2 m × 2 m) plots were established in each treatment, and soil samples were collected from each plot within a depth of 10 cm. Plant species composition was determined for each plot during the flowering stage of herbaceous species in June 2011. The seedling emergence method was used to estimate SSB composition.

Results: A total of 45 species (23 annuals and 22 perennials) emerged from the soil samples of the grazed area, while the number of species emerged from the soil samples of the previously grazing area was 76 (37 annuals and 39 perennials). The removal of grazing led to a significant increase in species richness and seed density in the SSB. Species turnover of AGV was higher, and that of the SSB was similar for grazed areas compared with areas that were formerly grazed. Detrended correspondence analysis ordination of AGV composition showed a clearly separate structure between grazed plots and plots where grazing has been removed, while the segregation was less clear for SSB composition.

Conclusions: We concluded that restoration of locally degraded sites cannot rely on the SSB when grazing is stopped simultaneously and unvegetated gaps are colonised by vegetative growth rather than by seed.     

Divergent changes in plant community composition under 3-decade grazing exclusion in continental steppe

An understanding of the factors controlling plant community composition will allow improved prediction of the responses of plant communities to natural and anthropogenic environmental change. Using monitoring data from 1980 to 2009, we quantified the changes in community composition in Leymus chinensis and Stipa grandis dominated grasslands in Inner Mongolia under long-term grazing-exclusion and free-grazing conditions, respectively. We demonstrated that the practice of long-term grazing exclusion has significant effects on the heterogeneity, the dominant species, and the community composition in the two grasslands. The community composition of L. chinensis and S. grandis grasslands exhibited directional changes with time under long-term grazing exclusion. Under free grazing, the L. chinensis community changed directionally with time, but the pattern of change was stochastic in the S. grandis community. We attributed the divergent responses to long-term grazing exclusion in the S. grandis and L. chinensis grasslands to litter accumulation and changes in the microenvironment after grazing exclusion, which collectively altered the growth and regeneration of the dominant species. The changes in the grazed grasslands were primarily determined by the selective feeding of sheep during long-term heavy grazing. Overall, the responses of the community composition of the Inner Mongolian grasslands to long-term grazing exclusion and heavy grazing were divergent, and depended primarily on the grassland type. Our findings provide new insights into the role of grazing in the maintenance of community structure and function and therefore have important implications for grassland management.