利用数字摄影测量探测橡树草原变化

数字摄影测量的一个新的应用，即自然景观的变化监测，它是我们近年围绕遥感生态测量学所做的一些初步尝试。用加州橡树草原区１９７０ 年和１９９５ 年１∶１２ ，０００ 比例尺航空摄影立体像对，构建有别于数字地形模型的数字表面模型。由于数字表面模型上记录着树冠和其它景观的三维表面坐标，我们可以测量树高与树冠郁闭度，从而比较２５ 年间橡树草原的变化。用数字表面模型有助于改善遥感多光谱分析所探测到变化的精度，使融合三维表面数据和多光谱数据以提高遥感信息提取的精度成为可能。

基于MODIS/NDVI的锡林郭勒草原植被变化及其归因

在国家“生态文明”战略背景下,蒙古高原作为中国北方重要生态屏障的作用更加凸显。锡林郭勒作为蒙古高原南端的典型内流区与生态敏感区,其草原植被覆盖变化规律在一定程度上能表征中国北方整体生态系统功能的变化态势。本文采用MODIS13Q1的产品NDVI数据集,运用势分析法、偏相关性及残差分析法,分析锡林郭勒草原2000—2015年生长期植被覆盖度时空演变特征,探讨草原植被变化的归因。研究结果表明:16年来锡林郭勒草原的生长期植植被覆盖每年以0.0021的速率缓慢增长,但其变化趋势存在明显地域差异性;比较发现,降水因子是影响锡林郭勒地区植被覆盖变化的主要因素;人类活动对该地区植被覆盖变化的改善促进和破坏抑制作用并存,总体是正向干扰效应大于负向干扰效应,人类活动对植被覆盖度改善作用的面积占34.911%,京津风沙源地治理工程等在草原生态环境改善中发挥重要作用;而人类活动导致生态环境遭到破坏的面积占19.348%,尤其是草原露天煤矿开采严重破坏草原植被系统。

Research Progress of Vulnerability Assessment on Grassland Ecosystem under Climate Change

[Objective] The aim was to review on vulnerability evaluation of grassland ecosystem under climate change.[Method] In the research,vulnerability evaluation methods of ecosystem under climate changes,at home and abroad,and the related cases and research progress in China were reviewed.In addition,the future of evaluation research was predicted.[Result] Ecosystem vulnerability to climate change is an important part of climate change research.It is necessary and urgent to improve evaluation methods and reduce uncertainty of future vulnerability evaluation to make evaluation more comprehensive and objective.Furthermore,evaluation on effects of human activity on natural ecosystem vulnerability should be highlighted.[Conclusion] The research provided reference for establishment of a new management model for ecosystem adaptability to climate and sustainable development of grasslands in northern China.

Seasonal characteristics of Carex lasiocarpa biomass and nutrient accumulation in the typical wetland of Sanjiang Plain, China

Seasonal dynamics of above- and belowground biomass and nutrient characteristics （nitrogen, carbon, and phosphorus） of Carex lasiocarpa were investigated in the typical wetland of Sanjiang Plain, China from May 2007 to September 2008. The results show that the changes of aboveground biomass during the growing season are best described by the twice function curve model, whereas the changes of belowground biomass follow the exponential increase curve model. Both the organic carbon contents in the above- and belowground plant parts show significant positive linear correlations with the growing time, and the coefficients R2 are 0.983 and 0.746, respectively. The carbon accu-mulations of the above- and belowground plant parts during the growing season show the same dynamics as those of the biomass. However, the nitrogen contents and accumulation in C. lasiocarpa aboveground and belowground parts show exponential increase during the growing season. The dynamics of C. lasiocarpa phosphorus contents follows the twice function curve model, whereas the accumulation of phosphorus shows the linear increase. The ratios of C/N in different parts of C. lasiocarpa fit the negative linear relations with total nitrogen content in the growing season. Moreover, the ratios of C/P in C. lasiocarpa plant also fit the negative linear relations with total phosphorus content. The results show that nitrogen is the primary limiting nutrient for C. lasiocarpa growth as compared with carbon and phosphorus.

Diurnal and Seasonal Dynamics of Soil Respiration at Temperate Leymus Chinensis Meadow Steppes in Western Songnen Plain, China

To evaluate the diurnal and seasonal variations in soil respiration （Rs） and understand the controlling factors, we measured carbon dioxide （CO2） fluxes and their environmental variables using a LI-6400 soil CO2 flux system at a temperate Leymus chinensis meadow steppe in the western Songnen Plain of China in the growing season （May-October） in 2011 and 2012. The diurnal patterns of soil respiration could be expressed as single peak curves, reaching to the maximum at 11：00-15：00 and falling to the minimum at 21：00-23：00 （or before dawn）. The time-window between 7：00 and 9：00 could be used as the optimal measuring time to represent the daily mean soil CO2 efflux. In the growing season, the daily value of soil CO2 efflux was moderate in late spring （1.06-2.51μnol/（m2.s） in May）, increased sharply and presented a peak in summer （2.95-3.94 μmol/（m2.s） in July）, and then decreased in autumn （0.74-0.97 μmol/（m2.s） in October）. Soil temperature （Ts） exerted dominant control on the diurnal and seasonal variations of soil respiration. The temperature sensitivity of soil respiration （Q10） exhibited a large seasonal variation, ranging from 1.35 to 3.32, and decreased with an increasing soil temperature. Rs gradually increased with increasing soil water content （Ws） and tended to decrease when Ws exceeded the optimum water content （27%） of Rs. The Ts and Ws had a confounding effect on Rs, and the two-variable equations could account for 72% of the variation in soil respiration （p 〈 0.01）.

A Time-dependent Stochastic Grassland Fire Ignition Probability Model for Hulun Buir Grassland of China

Grassland fire is one of the most important disturbance factors in the natural ecosystems.This paper focuses on the spatial distribution of long-term grassland fire patterns in the Hulun Buir Grassland located in the northeast of Inner Mongolia Autonomous Region in China.The density or ratio of ignition can reflect the relationship between grassland fire and different ignition factors.Based on the relationship between the density or ratio of ignition in different range of each ignition factor and grassland fire events,an ignition probability model was developed by using binary logistic regression function and its overall accuracy averaged up to 81.7%.Meanwhile it was found that daily relative humidity,daily temperature,elevation,vegetation type,distance to county-level road,distance to town are more important determinants of spatial distribution of fire ignitions.Using Monte Carlo method,we developed a time-dependent stochastic ignition probability model based on the distribution of inter-annual daily relative humidity and daily temperature.Through this model,it is possible to estimate the spatial patterns of ignition probability for grassland fire,which will be helpful to the quantitative evaluation of grassland fire risk and its management in the future.

An Assessment of Climate Change and Available Water Resources in the Lowveld Region, Swaziland

Long-term climatic data （maximum temperature, minimum temperature, rainfall and evaporation） for Big Bend in the Lowveld, a semi-arid region of Swaziland, were analysed for any changes or variations. Evaporation and rainfall data were analysed to assess water resources availability in the region. Analysis of the available data shows that there is no indication of decrease in rainfall with time, but the results show that there has been a steady increase in minimum temperatures over the last 25 years. The average effective water resources index, measured as the difference between mean annual rainfall and mean annual evaporation, for the region in the period from 1965 to 2001 was -1,500 mm. The large negative index implies low available water for the region, a situation that is likely to affect agricultural, hydropower and other water related development activities in the region. The negative effective water index implies deficits in the region＇s water resources which call for better management of the region＇s water resources. In the agriculture sector, this requires promoting technologies and practices that provide for water saving, improved water use performance and high water productivity. These include soil conservation tillage, wastewater reuse, runoff harvesting and soil fertility interventions through application of fertilizers, manures and mulches, and agronomic management. There is need for more analysis for the other regions in order to get a countrywide picture of the climate as well as water resources situations.

Ecological Consequences of Land Use Change: Forest Structure and Regeneration across the Forest-grassland Ecotone in Mountain Pastures in Nepal

The ecotone, the spatial transition zone between two vegetation communities, is claimed to have more species than the adjoining communities. However, empirical studies do not always confirm higher richness at the ecotone. The ecotone position and structure are dynamic over time and space and it is driven by the changes in climate, land use or their interaction. In this context, we assessed the forest- grassland ecotone of temperate mountains in central Nepal by i） comparing species composition and richness across the ecotone, ii） analyzing if the forestgrassland ecotone is shifting towards the grassland center by colonizing them with trees, and iii） discussing the consequence of changed disturbance regime in the dynamics of this ecotone and the surrounding grasslands. We analyzed vegetation data sampled from belt transects laid across the forest- grassland ecotone in semi-natural grassland patches. Vegetation data consisting of species richness and composition, and size structure and regeneration of the two most dominant tree species, namely Rhododendron arboreum and Abies spectabilis, from the transects, were used to analyze the trend of the forest-grassland ecotone. Forest and grasslands were different in terms of floristic composition and diversity. Vascular plant speciesrichness linearly increased while moving from forest interior to grassland center. Spatial pattern of tree size structure and regeneration infers that forest boundary is advancing towards the grasslands at the expense of the grassland area, and tree establishment in the grasslands is part of a suceessional process. Temporally, tree establishment in grasslands started following the gradual decline in disturbance. We argue that local processes in terms of changed land use may best explain the phenomenon of ecotone shift and consequent forest expansion in these grasslands. We underpin the need for further research on the mechanism, rate and spatial extent of ecotone shift by using advaneed tools to understand the process indepth.

Abandoned Altitudes? Decrease and Expansion of Grassland in the Hinterland of Popayán, Southern Colombian Andes

Due to their ecological disadvantages, many mountain regions have experienced land-use abandonment and shrub encroachment on former grassland at higher altitudinal zones--especially during recent decades of urbanization. But does this trend also apply to the hinterland of urban settlements？ By using the Southern Colombian example of Popay^n, a medium-sized city located in the Northern Andes, we can show that the landscape changes observed between 1989 and 2010 can hardly be related to agricultural abandonment. Hypsometric variations of land-cover change indicate that, until 2001, woods or shrubland expanded faster at the lower altitudinal range adjacent to the city than at the more remote higher zones. In contrast, after 2001 grassland areas increased on former woods or shrnbland at all altitudinal belts. Both periods thus present developments that can be interpreted as the result of land-use expansion below 2000 m asl and land-use persistence in the tierrafrla of the mountain city＇s hinterland.

Response of Biomass Spatial Pattern of Alpine Vegetation to Climate Change in Permafrost Region of the Qinghai-Tibet Plateau,China

Alpine ecosystems in permafrost region are extremely sensitive to climate changes.To determine spatial pattern variations in alpine meadow and alpine steppe biomass dynamics in the permafrost region of the Qinghai-Tibet Plateau,China,calibrated with historical datasets of above-ground biomass production within the permafrost region's two main ecosystems,an ecosystem-biomass model was developed by employing empirical spatialdistribution models of the study region's precipitation,air temperature and soil temperature.This model was then successfully used to simulate the spatio-temporal variations in annual alpine ecosystem biomass production under climate change.For a 0.44°C decade-1 rise in air temperature,the model predicted that the biomasses of alpine meadow and alpine steppe remained roughly the same if annual precipitation increased by 8 mm per decade-1,but the biomasses were decreased by 2.7% and 2.4%,respectively if precipitation was constant.For a 2.2°C decade-1 rise in air temperature coupled with a 12 mm decade-1 rise in precipitation,the model predicted that the biomass of alpine meadow was unchanged or slightly increased,while that of alpine steppe was increased by 5.2%.However,in the absence of any rise in precipitation,the model predicted 6.8% and 4.6% declines in alpine meadow and alpine steppe biomasses,respectively.The response of alpine steppe biomass to the rising air temperatures and precipitation was significantly lesser and greater,respectively than that of alpine meadow biomass.A better understanding of the difference in alpine ecosystem biomass production under climate change is greatly significant with respect to the influence of climate change on the carbon and water cycles in the permafrost regions of the Qinghai-Tibet Plateau.

Diurnal Variation of Soil CO2 Efflux and Its Optimal Measuring Time-window of Temperate Meadow Steppes in Western Songnen Plain, China

In order to study the diurnal variation of soil CO2 effiux from temperate meadow steppes in Northeast China, and determine the best time for observation, a field experiment was conducted with a LI-6400 soil CO2 flux system under five typical plant communi- ties （Suaeda glauca （Sg）, Chloris virgata （Cv）, Puecinellia distans （Pd）, Leymus chinensis （Lc） and Phragmites australis （Pa）） and an alkali-spot land （As） at the meadow steppe of western Songnen Plain. The results showed that the diurnal variation of soil CO2 effiux exhibited a single peak curve in the growing season. Diurnal maximum soil respiration （Rs） often appeared between 1 1：00 and 13：00, while the minimum occurred at 21：00-23：00 or before dawn. Air temperature near the soil surface （Ta） and soil temperature at 10 cm depth （Tlo） exerted dominant control on the diurnal variations of soil respiration. The time-windows 7：00-9：00 could be used as the optimal measuring time to represent the daily mean soil CO2 effiux at the Cv, Pd, Lc and Pa sites. The daily mean soil CO2 effiux was close to the soil COz effiux from 15：00 to 17：00 and the mean of 2 individual soil CO2 effiux from 15：00 to 19：00 at the As and Sg sites, respectively. During nocturnal hours, negative soil CO2 fluxes （CO2 downwards into the soil） were frequently observed at the As and Sg sites, the magnitude of the negative CO2 fluxes were 0.10-1.55 gmol/（m2.s） and 0.10-0.69 gmol/（m2.s）at the two sites. The results im- plied that alkaline soils could absorb CO2 under natural condition, which might have significant implications to the global carbon budget accounting.

Soil and Vegetation Seasonal Changes in the Grazing Andean Mountain Grasslands

Andean grasslands ecosystems are fragile environments with rigorous climatologic conditions and low and variable food for the grazing. The Apolobamba area is located in the Bolivian Andean Mountains. Its high grasslands provide a natural habitat for wild and domestic camelids such as vicuna(Vicugna vicugna) and alpaca(Lama pacos). The botanical diversity plays an essential role in maintaining vital ecosystem functions. The objectives of this research were to determine the seasonal changes in soil properties, to study the vegetation changes during the wet and dry seasons and the influence of soil properties and camelid densities on the vegetation in the Apolobamba grasslands. Four zones with different vicuna populations were selected to be studied. The following soil parameters were determined: total organic carbon, total nitrogen, available phosphorous, cation exchange capacity, exchangeable cations, pH and texture. The vegetation season changes were studied through botanical identification, above-ground biomass, plant cover and species richness. Results showed that some soil properties such as C/N ratio, CEC, silt and clay percentages kept stable against the seasonal changes. Generally, soil nutrients were relatively higher during the dry season in the surface and subsurface. The results did not point out the predominant vegetation growth during the wet season. The seasonal vegetation growth depended on each species. Thegood soil fertility corresponded to the highest plant cover. Soil fertility presented no influence on the above-ground biomass of the collected species. The negative influence of camelid grazing on soil properties could not be assessed. However, overgrazing could affect some plant species. Therefore, protection is needed in order to preserve the biodiversity in the Andean mountain grasslands.

Warming and increased precipitation alter soil carbon cycling in a temperate desert steppe of Inner Mongolia

Warming and precipitation are key global change factors driving soil carbon(C)dynamics in terrestrial ecosystems.However,the effects of warming and altered precipitation on soil microbial diversity and functional genes involved in soil C cycling remain largely unknown.We investigated the effects of warming and increased precipitation on soil C cycling in a temperate desert steppe of Inner Mongolia using metagenomic sequencing.We found that warming reduced plant richness,Shannon-Wiener and Simpson index.In contrast,increased precipitation signifcantly infuenced Shannon-Wiener and Simpson index.Warming reduced soil microbial species by 5.4%while increased precipitation and warming combined with increased precipitation led to increases in soil microbial species by 23.3%and 2.7%,respectively.The relative abundance of Proteobacteria,which involve C cycling genes,was signifcantly increased by warming and increased precipitation.Warming signifcantly reduced the abundance of GAPDH(Calvin cycle)and celF(cellulose degradation)while it enhanced the abundance of glxR(lignin degradation).Increased precipitation signifcantly enhanced the abundance of pgk(Calvin cycle),coxL(carbon monoxide oxidation),malZ(starch degradation),and mttB(methane production).Moreover,a wide range of correlations among soil properties and C cycling functional genes was detected,suggesting the synergistic and/or antagonistic relationships under scenario of global change.These results may suggest that warming is benefcial to soil C storage while increased precipitation negatively affects soil C sequestration.These fndings provide a new perspective for understanding the response of microbial communities to warming and increased precipitation in the temperate desert steppe.

Energy exchange of an alpine grassland on the eastern Qinghai-Tibetan Plateau

The seasonal variability in the surface energy exchange of an alpine grassland on the eastern Qinghai- Tibetan Plateau was investigated using eddy covariance measurements. Based on the change of air temperature and the seasonal distribution of precipitation, a winter season and wet season were identified, which were separated by transitional periods. The annual mean net radiation （Rn） was about 39 % of the annual mean solar radiation （Rs）. Rn was relatively low during the winter season （21% of Rs） compared with the wet season （54 % of Rs）, which can be explained by the difference in surface albedo and moisture condition between the two seasons. Annually, the main consumer of net radiation was latent heat flux （LE）. During the winter season, sensible heat flux （H） was dominant because of the frozen soil condition and lack of precipita- tion. During the wet season, LE expended 66 % of Rn due to relatively high temperature and sufficient rainfall cou- pled with vegetation growth. Leaf area index （LAI） had important influence on energy partitioning during wet season. The high LAI due to high soil water content （θv） contributed to high surface conductance （go） and LE, and thus low Bowen ratio （β）. LE was strongly controlled by Rn from June to August when gc and θv were high. During the transitional periods, H and LE were nearly equally parti- tioned in the energy balance. The results also suggested that the freeze-thaw condition of soil and the seasonal distribution of precipitation had important impacts on the energy exchange in this alpine grassland.

Carbon and Nitrogen Storage in Inner Mongolian Grasslands: Relationships with Climate and Soil Texture

Understanding the spatial variability of soil carbon （C） storage and its relationship with climate and soil texture is critical for developing regional C models and for predicting the potential impact of climate change on soil C storage. On the basis of soil data from a transect across the Inner Mongolian grasslands, we determined the quantitative relationships of C and nitrogen （N） in bulk soil and particle-size fractions （sand, silt, and clay） with climate and soil texture to evaluate the major factors controlling soil C and N storage and to predict the effect of climate changes on soil C and N storage. The contents of C and N in the bulk soil and the different fractions in the 0 20 and 20 40 cm soil layers were positively correlated with the mean annum precipitation （MAP） and negatively correlated with the mean annual temperature （MAT）. The responses of C storage in the soil and particle-size fractions to MAP and MAT were more sensitive in the 0-20 cm than in the 2（~40 cm soil layer. Although MAP and MAT were both important factors influencing soil C storage, the models that include only MAP could well explain the variation in soil C storage in the Inner Mongolian grasslands. Because of the high correlation between MAP and MAT in the region, the models including MAT did not significantly enhance the model precision. Moreover, the contribution of the fine fraction （silt and clay） to the variation in soil C storage was rather small because of the very low fine fraction content in the Inner Mongolian grasslands.

Abrupt vegetation shifts caused by gradual climate changes in central Asia during the Holocene

Understanding the response of ecosystems to past climate is critical for evaluating the impacts of future climate changes.A large-scale abrupt shift of vegetation in response to the Holocene gradual climate changes has been well documented for the Sahara-Sahel ecosystem. Whether such a non-linear response is of universal significance remains to be further addressed. Here,we examine the vegetation-climate relationships in central Asia based on a compilation of 38 high-quality pollen records. The results show that the Holocene vegetation experienced two major abrupt shifts, one in the early Holocene(Shift I, establishing shift) and another in the late Holocene(Shift II, collapsing shift), while the mid-Holocene vegetation remained rather stable. The timings of these shifts in different regions are asynchronous, which are not readily linkable with any known abrupt climate shifts,but are highly correlated with the local rainfalls. These new findings suggest that the observed vegetation shifts are attributable to the threshold effects of the orbital-induced gradual climate changes. During the early Holocene, the orbital-induced precipitation increase would have first reached the threshold for vegetation "establishment" for moister areas, but significantly later for drier areas. In contrast, the orbital-induced precipitation decrease during the late Holocene would have first reached the threshold, and led to the vegetation "collapse" for drier areas, but delayed for moister areas. The well-known 4.2 kyr BP drought event and human intervention would have also helped the vegetation collapses at some sites. These interpretations are strongly supported by our surface pollen-climate analyses and ecosystem simulations. These results also imply that future climate changes may cause abrupt changes in the dry ecosystem once the threshold is reached.

Small-scale switch in cover-perimeter relationships of patches indicates shift of dominant species during grassland degradation

Aims Grasslands are globally threatened by climate changes and unsustainable land-use,which often cause transitions among alternative stable states,and even catastrophic transition to desertification.Spatial vegetation patch configurations have been shown to signify such transitions at large spatial scale.Here,we demonstrate how small-scale patch configurations can also indicate state transitions.Methods The whole spatial series of degradation successions were chosen in alpine grasslands characterized as seven typical communities.Patch numbers,and perimeter and cover of each patch were recorded using adjacent quadrats along transects in each type of the communities.Species abundance within each patch was measured.Important Findings Across seven grazing-induced degradation stages in the world’s largest expanse of grassland,from dense ungrazed turf to bare black-soil crust,patch numbers and perimeters first increased as patch cover decreased.Numbers and perimeters then decreased rapidly beyond an intersection point at 68%of initial continuous vegetation cover.Around this point,the vegetation fluctuated back and forth between the sedge-dominated grassland breaking-up phase and the forb-dominated phase,suggesting impending shift of grassland state.This study thus demonstrates how ground-based small-scale vegetation surveys can provide a quantitative,easy-to-use signals for vegetation degradation,with promise for detecting the catastrophic transition to desertification.

Neutral responses of plant community Ca concentration to nitrogen enrichment in a semiarid grassland

Calcium(Ca)is an essential nutrient for plant growth and Ca concentrations in forage have important implications for ruminant diet and health.It remains an open question whether forage Ca concentration would be decreased by increasing nitrogen(N)deposition.We manipulated the increasing rates of N addition(2008–2015)in a semiarid grassland,northern China.Plant Ca concentrations for all species were examined in each plot under N treatment.The Ca concentrations at functional group and community levels were calculated based on the concentration of each species presented and their relative biomass in each plot.We found that community-level Ca concentration remained stable across a gradient of wide-ranged N addition rates,although Ca concentration at both species and functional group levels showed negative responses to N enrichment.Given that forbs had higher Ca concentration than grasses,the increasing relative biomass of forbs canceled out the negative responses of species-level and functional group-level Ca concentration.Our results further showed that community Ca pool showed a positive but saturating response to N addition,with a threshold at the rate of 10 g N m^(−2)yr^(−1).Our findings highlight the role of changes in plant relative biomass in controlling the responses of forage Ca concentration and stock to N enrichment.

Responses of aboveground biomass of alpine grasslands to climate changes on the Qinghai-Tibet Plateau

Aboveground biomass in grasslands of the Qinghai-Tibet Plateau has displayed an overall increasing trend during 2003–2016, which is profoundly influenced by climate change. However, the responses of different biomes show large discrepancies, in both size and magnitude. By applying partial least squares regression, we calculated the correlation between peak aboveground biomass and mean monthly temperature and monthly total precipitation in the preceding 12 months for three different grassland types(alpine steppe, alpine meadow, and temperate steppe) on the central and eastern Qinghai-Tibet Plateau. The results showed that mean temperature in most preceding months was positively correlated with peak aboveground biomass of alpine meadow and alpine steppe, while mean temperature in the preceding October and February to June was significantly negatively correlated with peak aboveground biomass of temperate steppe. Precipitation in all months had a promoting effect on biomass of alpine meadow, but its correlations with biomass of alpine steppe and temperate steppe were inconsistent. It is worth noting that, in a warmer, wetter climate, peak aboveground biomass of alpine meadow would increase more than that of alpine steppe, while that of temperate steppe would decrease significantly, providing support for the hypothesis of conservative growth strategies by vegetation in stressed ecosystems.