西南地区草地群落灌木植物盖度对生态系统碳库的影响

中国西南地区草地主要为暖性及热性草丛、灌草丛, 约占全国草地面积的1/10, 分析灌木植物盖度与草地碳库及其构成的关系对于准确评估尚处于次生演替阶段的南方草地碳储量具有重要意义。该研究基于野外实地调查, 将西南地区不同地貌类型的41个代表性草地样地依据灌木植物盖度划分为3种类型: 无灌木植物草地群落(灌木植物盖度为0)、低灌木植物盖度草地群落(灌木植物盖度0-10%)和高灌木植物盖度草地群落(灌木植物盖度10%-30%), 测定了群落地上、地下生物量和凋落物生物量以及植物和土壤碳含量, 计算碳密度。结果表明: 随着草地群落灌木植物盖度增大, 生态系统植被碳密度从0.304 kg·m ^-2增加到1.574 kg·m ^-2, 其中根系和凋落物碳库也呈增长趋势; 土壤碳密度从7.215 kg·m ^-2增加到9.735 kg·m ^-2, 生态系统碳密度从7.519 kg·m ^-2增加到11.309 kg·m ^-2。草地碳库构成中, 低灌木植物盖度草地群落的土壤碳库占生态系统碳库比例最小。草地群落灌木植物盖度增加改变了草地生态系统碳库构成并导致生态系统碳库增加, 建议在估算草地生态系统碳库时, 需要统筹考虑并兼顾南方地区草地群落灌木植物盖度变化。     

A new ecological-wind erosion model to simulate the impacts of aeolian transport on dryland vegetation patterns

Drylands cover more than 40% of the land surface of the Earth and are characterized by patchy vegetation and that permits erosion of the surface. Vegetation-aeolian transport is an important feedback in drylands, particularly those undergoing shrub encroachment. Although one side of the feedback, the influence of vegetation loss on aeolian transport, has been well studied, the other, the influence of aeolian transport on existing vegetation, has been never studied in detail. In this study, a new ecological-wind erosion model (ECO-WEMO) that contains an aeolian transport component was created to simulate how aeolian transport impacts vegetation pattern and causes the state change. Two modeling scenarios were investigated: 1) stable grass and shrub communities without/with aeolian transport and 2) unstable shrub and grass communities without/with aeolian transport disturbed by different drought conditions. The first scenario focuses on the simulation of the influence of aeolian transport on vegetation communities and the second scenario focuses on the simulation of the state change of vegetation communities. The results from the first scenario show that: First, the mean biomasses of grass and shrub become consistent in the case of no wind in both shrub-dominated and grass-dominated communities. Second, the mean biomass of shrub becomes higher than the grass in the case of wind in shrub-dominated communities and the mean biomass of grass becomes higher than the shrub in the case of wind in grass-dominated communities. Third, the dust flux of shrub-dominated communities is higher than the grass-dominated communities. Fourth, the net change in surface height in shrub-dominated communities has a considerably higher range than in grass-dominated communities. Fifth, the spatial pattern of shrub-dominated communities is sparser than the spatial pattern of grass in the vegetation communities in the case of wind. The results from the second scenario show that: First, the state change only took place from grass-dominated communities to shrub-dominated communities in the condition of drought. Second, the state change only took place in the case of wind. Third, the state change didn't take place after the slight and moderate droughts but only took place after the drought. Fourth, large vegetation biomass reduction only took place in the case of wind after the severe drought. Our results confirm, in a modeling context, the important role that aeolian transport can play in vegetation dynamics and state change in deserts.     

Control of Exotic Annual Grasses to Restore Native Forbs in Abandoned Agricultural Land

Exotic annual grasses are a major challenge to successful restoration in temperate and Mediterranean climates. Experiments to restore abandoned agricultural fields from exotic grassland to coastal sage scrub habitat were conducted over two years in southern California, U.S.A. Grass control methods were tested in 5 m² plots using soil and vegetation treatments seeded with a mix of natives. The treatments compared grass‐specific herbicide, mowing, and black plastic winter solarization with disking and a control. In year two, herbicide and mowing treatments were repeated on the first‐year plots, plus new control and solarization plots were added. Treatments were evaluated using percent cover, richness and biomass of native and exotic plants. Disking alone reduced exotic grasses, but solarization was the most effective control in both years even without soil sterilization, and produced the highest cover of natives. Native richness was greatest in solarization and herbicide plots. Herbicide application reduced exotics and increased natives more than disking or mowing, but produced higher exotic forb biomass than solarization in the second year. Mowing reduced grass biomass and cover in both years, but did not improve native establishment more than disking. Solarization was the most effective restoration method, but grass‐specific herbicide may be a valuable addition or alternative. Solarization using black plastic could improve restoration in regions with cool, wet summers or winter growing seasons by managing exotic seedbanks prior to seeding. While solarization may be impractical at very large scales, it will be useful for rapid establishment of annual assemblages on small scales.     

Spectral feature differences between shrub and grass communities and shrub coverage retrieval in shrub-encroached grassland in Xianghuang Banner,Nei Mongol,China北大核心CSCD

Aims: Shrub-encroached grassland has become an important vegetation type in China's arid and semi-arid region. Our study objective is to explore the spectral features of shrub and grass communities, as well as their empirical relationships with shrub coverage. The quantitative estimation of shrub cover based on medium-resolution Landsat satellite imagery provides the practical basis for long term retrieval of large areas of shrub expansion in the grassland region. Methods: Linear models and Multiple Endmember Spectral Analysis Model (MESMA) based on medium resolution Landsat satellite imagery were developed to quantify the shrub coverage in a shrub-encroached grassland region in Xianghuang Banner, Nei Mongol using the spectral features and their seasonal differences between the shrub and grass communities. Important findings: Compared to Leymus chinensis and Stipa krylovii dominated grass communities, Caragana microphylla community had a higher normalized difference vegetation index (NDVI), modified red edge normalized difference vegetation index (mNDVI705), and red edge slope. The red edge position of C. microphylla community shifted to longer wavelengths. The average and the maximum shrub coverage was 13% and 25%, respectively, in the shrub-encroached grassland based on both models. The correlation coefficient of determination (R2) and root mean square error (RMSE) of the linear model was 0.31 and 0.05, respectively. We found that the linear model based on seasonal differences of shrub and grass community was more suitable for retrieving shrub coverage in the study area from medium resolution imagery than the MESMA model that is based on mid-summer images.     

An Ecohydrological Cellular Automata Model Investigation of Juniper Tree Encroachment in a Western North American Landscape

Woody plant encroachment over the past 140 years has substantially changed grasslands in western North American. We studied encroachment of western juniper (Juniperus occidentalis var. occidentalis) into a previously mixed shrub–grassland site in central Oregon (USA) using a modified version of Cellular Automata Tree–Grass–Shrub Simulator (CATGraSS) ecohydrological model. We developed simple algorithms to simulate three encroachment factors (grazing, fire frequency reduction, and seed dispersal by herbivores) in CATGraSS. Local ecohydrological dynamics represented by the model were first evaluated using satellite-derived leaf area index and measured evapotranspiration data. Reconstructed pre-encroachment vegetation cover percentages and the National Land Cover Database (NLCD 2006) vegetation map were used to estimate parameters for encroachment factors to represent juniper encroachment in CATGraSS. Model sensitivity experiments examined the influence of each encroachment factor and their combinations on trajectories of modeled percent cover of each plant functional type and emergent spatial vegetation patterns in the modeled domain. Simulation results identified grazing as the key factor leading to juniper encroachment, by reducing shrub and grass cover and promoting the formation of juniper tree clusters. Reduced fire frequency and increased seed dispersal by grazers further amplified juniper encroachment into grassland patches between clusters of juniper trees. Each encroachment factor showed different consequences on modeled vegetation patterns. Time series of modeled plant cover and spatial patterns of plant functional types were found to be consistent with an existing conceptual model described in the literature. The proposed model provides a tool that can be used to improve our understanding of the drivers and processes of woody plant encroachment and vegetation response to global change.     

Spectral feature differences between shrub and grass communities and shrub coverage retri- eval in shrub-encroached grassland in Xianghuang Banner,Nei Mongol,China

Aims Shrub-encroached grassland has become an important vegetation type in China’s arid and semi-arid region. Our study objective is to explore the spectral features of shrub and grass communities, as well as their empirical relationships with shrub coverage. The quantitative estimation of shrub cover based on medium-resolution Landsat satellite imagery provides the practical basis for long term retrieval of large areas of shrub expansion in the grassland region. Methods Linear models and Multiple Endmember Spectral Analysis Model (MESMA) based on medium resolution Landsat satellite imagery were developed to quantify the shrub coverage in a shrub-encroached grassland region in Xianghuang Banner, Nei Mongol using the spectral features and their seasonal differences between the shrub and grass communities. Important findings Compared to Leymus chinensis and Stipa krylovii dominated grass communities, Caragana microphylla community had a higher normalized difference vegetation index (NDVI), modified red edge normalized difference vegetation index (mNDVI₇₀₅), and red edge slope. The red edge position of C. microphylla community shifted to longer wavelengths. The average and the maximum shrub coverage was 13% and 25%, respectively, in the shrub-encroached grassland based on both models. The correlation coefficient of determination (R²) and root mean square error (RMSE) of the linear model was 0.31 and 0.05, respectively. We found that the linear model based on seasonal differences of shrub and grass community was more suitable for retrieving shrub coverage in the study area from medium resolution imagery than the MESMA model that is based on mid-summer images.     

库布齐沙漠南缘风沙-植被相互作用及其景观效应

干旱区尤其沙漠边缘地区的风沙与植被相互作用对塑造地表景观具有重要意义。选择库布齐沙漠南缘的油蒿灌丛地为研究区,开展了植被调查、风沙流观测和表层沉积物粒度采样测试,分析了顺风向植被盖度、风沙流结构与沉积物特征的沿程变化,探讨了风沙-植被相互作用及其对地表景观格局的影响。结果表明,风沙流与植被相互作用方式的改变使植物生长状况与地表蚀积模式发生变化,进而导致顺风向景观表现出明显的空间异质性。自上风向裸地过渡到均匀分布的新生油蒿和油蒿灌丛再至斑块状分布的灌丛沙堆,植被盖度与覆沙厚度先增大后减小,空气动力学粗糙度沿程不断增加且在过渡时其增幅最大,输沙率与沉积物粒度呈先减小后增大趋势,并在植被盖度与覆沙厚度最大处出现最小值。在沙漠边缘剥蚀高原上,起初适量风沙堆积促进油蒿定植与生长,均匀分布的油蒿灌丛进一步促进沙物质堆积,但当堆积厚度超过油蒿耐沙埋深度时发生退化,灌丛出现斑块状分布且风沙流在丘间地处侵蚀。据此,可理解为剥蚀高原风沙区景观异质性是风沙与植被相互协同与抑制作用的结果。     

The influence of subterranean termites on the hydrological characteristics of a Chihuahuan desert ecosystem

Summary Rainfall simulation at an average intensity of 124 mm·h^-1 was used to compare infiltration and run off on arid areas where subterranean termites had been eliminated four years prior to the initiation of the study (termite free) with adjacent areas populated by subterranean termites (termites present). Infiltration rates on termite free plots with less than 5% perennial plant cover were significantly lower 51.3±6.8 mm·h^-1 than rates on comparable termites present plots 88.4±5.6 mm·h^-1. On plots centered on Larrea tridentata shrubs, there were no differences in infiltration rates with or without termites. Plots with shrub cover had the highest infiltration rates 101±6 mm·h^-1. Highest run-off volumes were recorded from termite free <5% grass cover plots and the lowest from plots with shrubs. There were no differences in suspended sediment concentrations from termites present and termite free plots. Average bed load concentration was more than three times greater from termite free, <5% cover plots than from termites present, <5% cover plots.The reduction in infiltration, high run-off volumes and high bedloads from termite free areas without shrub cover is related to increased soil bulk density resulting from the collapse of subterranean galleries of the termites that provide avenues of bulk flow into the soil. Subterranean termites affect the hydrology of Chihuahuan desert systems by enhancing water infiltration and retention of top soil. The presence of a shrub canopy and litter layer cancels any effect of subterranean termites on hydrological parameters. Since approximately 2/3 of the area is not under shrub canopies, subterranean termites are considered to be essential for the maintenance of the soil water characteristics that support the present vegetation.     

Habitat use and movement patterns by dependent and independent juvenile Grasshopper Sparrows during the post‐fledging period

The post‐fledging period is a critical life stage for young grassland birds. Habitat selection by recently fledged birds may differ from that of adults and may change as juveniles transition from the care and protection of parents to independence. To describe patterns of habitat selection during these important life stages, we studied habitat use by juvenile Grasshopper Sparrows (Ammodramus savannarum) in a Conservation Reserve Program grassland in Maryland. We used radio‐telemetry to track daily movement patterns of two age classes of Grasshopper Sparrows during the post‐fledging period. Sparrows were classified as either dependent (<32‐d‐old) or independent (≥32‐d‐old). We characterized the vegetation at 780 vegetation plots (390 plots where birds were located and 390 paired random plots). Microhabitats where dependent birds were found had significantly more bare ground, litter, and plant species richness than paired random plots. In addition, dependent birds were found in plots with less bare ground, more warm‐season grass cover, more total vegetation cover, and more forb cover than plots used by independent birds. Plots where independent birds were located also had significantly more bare ground than random plots. Dependent birds are less able to escape from predators because their flight feathers are not fully grown so they may benefit from remaining in areas of greater vegetation cover. However, juveniles transitioning from dependence to independence must forage on their own, possibly explaining their increased use of more open areas where foraging may be easier. To properly manage habitat for grassland birds, management strategies must consider the changing needs of birds during different stages of development. Our results highlight the importance of diverse grassland ecosystems for juvenile grassland birds during the transition to independence.     

Barriers to shrub reestablishment following fire in the seasonal submontane zone of Hawai'i

Introduced grass species have invaded extensive areas of Hawaii Volcanoes National Park and increased the size and frequency of fire. Following fire, grass cover is enhanced while native shrub cover is reduced; the reduction in most shrubs persists for at least 20 years even in the absence of fire. Shrub seedlings were planted in burned and unburned plots with and without grass cover. Biomass of 14 month old shrub seedlings was generally highest in recently burned/grass removed plots, intermediate in old burn/grass removed plots, and lowest in unburned/grass removed plots. In contrast, shrub biomass in plots with grass cover was low and did not differ significantly among burn treatments. Light competition is likely to be responsible for differences in shrub growth rates; grass cover reduced light to 1–10% of background levels. In addition, pool sizes of available soil N were highest in recently burned, intermediate in old burn, and lowest in unburned areas.     

Shrub expansion may reduce summer permafrost thaw in Siberian tundra

Climate change is expected to cause extensive vegetation changes in the Arctic: deciduous shrubs are already expanding, in response to climate warming. The results from transect studies suggest that increasing shrub cover will impact significantly on the surface energy balance. However, little is known about the direct effects of shrub cover on permafrost thaw during summer. We experimentally quantified the influence of Betula nana cover on permafrost thaw in a moist tundra site in northeast Siberia with continuous permafrost. We measured the thaw depth of the soil, also called the active layer thickness (ALT), ground heat flux and net radiation in 10 m diameter plots with natural B. nana cover (control plots) and in plots in which B. nana was removed (removal plots). Removal of B. nana increased ALT by 9% on average late in the growing season, compared with control plots. Differences in ALT correlated well with differences in ground heat flux between the control plots and B. nana removal plots. In the undisturbed control plots, we found an inverse correlation between B. nana cover and late growing season ALT. These results suggest that the expected expansion of deciduous shrubs in the Arctic region, triggered by climate warming, may reduce summer permafrost thaw. Increased shrub growth may thus partially offset further permafrost degradation by future temperature increases. Permafrost models need to include a dynamic vegetation component to accurately predict future permafrost thaw.     

基于TM影像的白云岩与石灰岩上喀斯特植被时空变化差异研究

喀斯特土壤主要由白云岩和石灰岩风化而来,植被生长及其分布究竟怎样响应这一特殊地质背景?以人为干扰影响较小的喀斯特自然保护区为研究对象,采用监督分类法对1990年和2011年两期TM影像进行植被分类,并利用景观格局分析方法研究两种岩性上植被变化差异。结果表明,1990年和2011年研究区内两种母岩上均以乔木林和乔灌为主,草灌和草丛分布少,白云岩上乔木林的面积比例大于石灰岩上的比例,而草灌和草丛小于石灰岩上的比例;近20年来白云岩与石灰岩上草丛、草灌、灌丛和乔灌均以正向演替为主,但白云岩上正向演替比例大于石灰岩上的比例;两种岩性上植被斑块连接性均增强、破碎程度均降低,白云岩上植被斑块的破碎化程度和多样性指数较石灰岩上低,内部连接性强。由此可见,喀斯特白云岩较石灰岩有利于草丛、草灌的自然恢复,岩性引起的水土资源配置和养分地球化学循环过程差异制约着喀斯特地区植被的时空格局。     

Quantitative effects of vegetation cover on wind erosion and soil nutrient loss in a desert grassland of southern New Mexico,USA

Wind is a key abiotic factor that influences the dynamics of arid and semiarid systems. We investigated two basic relationships on vegetation manipulation (grass cover reduction) plots at the Jornada Experimental Range in southern New Mexico: (1) wind erosion rates (horizontal mass flux and dust emission) versus vegetative cover, and (2) nutrient loss versus vegetative cover. The results indicate that wind erosion rates and nutrient loss by dust emission are strongly affected by plant cover; however, the importance of shrubs and grasses in reducing dust flux may not be equal. The dramatic increase of wind erosion between 75% grass cover reduction and 100% grass cover reduction suggests that sparsely distributed mesquites are relatively ineffective at reducing wind erosion and nutrient loss compared to grasses. Comparisons of nutrients between surface soils and wind blown dust indicate that aeolian transport is a major cause for the loss of soil nutrients in susceptible environments. We found that increased aeolian flux over three windy seasons (March 2004–July 2006) removed up to 25% of total organic carbon (TOC) and total nitrogen (TN) from the top 5 cm of soil, and about 60% of TOC and TN loss occurred in the first windy season (March–July 2004). The balance between net loss of nutrients by aeolian processes and the addition of nutrients by biotic processes changed from negative (net loss) to positive (net accumulation) between 50% grass cover reduction and 25% grass cover reduction. The estimated lifetime of surface soil TOC and TN of about 10 years on the plot with 100% grass cover reduction indicates that impacts of wind erosion on soil resources can occur on very short timescales.     

Effects of Repeated Fires on the Structure,Composition, and Dynamics of Mediterranean Maquis: Short- and Long-Term Perspectives

Wildfires are an important agent in driving ecosystem function by altering vegetation structure and geomorphic processes. In recent decades, the number of wildfires and the total area burned has increased around the world, causing changes to natural regimes. In this study, we compared south- and north-facing slopes, their vegetation structure and dynamics, and the sediment yield generated in areas burned a number of times at the Carmel Mountain ridge in northern Israel. Our underlying hypothesis was that repeated and frequent fires significantly alter eco-geomorphic processes, including prolonged periods of soil erosion and delayed recovery of tree species. We tested whether these phenomenon are characterized by different rates on opposing aspects. To study the long-term changes of the vegetation we analyzed a 21-year (1985–2006) chrono-sequence of satellite images, in areas burned once, twice, or three times. Additionally, we estimated vegetation structure and cover at high resolutions in monitoring plots following a fire in 2005 in areas burned once or twice during the last two decades. To evaluate the long-term dynamics of the system, specific transition probabilities among the vegetation types, as a function of the number of times each site was burned, were used to construct Markov-based transition matrices. Additionally, runoff and sediment have been collected after precipitation events from the plots. The satellite image classifications revealed changes in the composition of tree, shrub, and herbaceous vegetation cover following wildfire events. Satellite image analyses suggest that recurring fires within short-time intervals may significantly alter the long-term structure of the vegetation communities, and may eliminate woody vegetation from the landscape (both trees and shrubs). Consequently, this results in the establishment and dominance of herbaceous vegetation communities. Similar trends were observed in the high-resolution monitoring plots. Sediment yields differed significantly in areas burned twice on south-facing slopes, compared to lower values obtained in areas burned once, or located on north-facing slopes. Thus, we demonstrate that repeated fires may dramatically alter long-term trajectories of Mediterranean-type vegetation communities and ecosystems. This pattern, in turn, may have significant implications for the associated geo-morphological processes, especially runoff and erosion, and should be of particular concern given recent changes of fire regimes.     

Restoration Practices Have Positive Effects on Breeding Bird Species of Concern in the Chihuahuan Desert

Woody plant encroachment into grasslands is a global concern. Efforts to restore grasslands often assume that removal of woody plants benefits biodiversity but assumptions are rarely tested. In the Chihuahuan Desert of the Southwestern United States, we tested whether abundances of grassland specialist bird species would be greater in plant communities resulting from treatment with herbicides to remove encroaching shrubs compared with untreated shrub‐dominated areas that represented pre‐treatment conditions. In 2010, we surveyed breeding birds and vegetation at 16 treated–untreated pairs. In 2011, we expanded the survey effort to 21 treated–untreated pairs, seven unpaired treatment areas, and five reference grassland areas. Vegetation in treatment areas had higher perennial grass foliar and basal cover and lower shrub foliar cover compared with untreated areas. Several regionally declining grassland specialists exhibited higher occurrence and relative abundance in treated areas. A shrubland specialist, however, was associated with untreated areas and may be negatively impacted by shrub removal. Bird community composition differed between treated and untreated areas in both years. Our results indicate that shrub removal can have positive effects on grassland specialist bird species, but that a mosaic of treated and untreated areas might be most beneficial for regional biodiversity .     

Soil Moisture, Native Revegetation, and Pinus lambertiana Seedling Survival, Growth, and Mycorrhiza Formation Following Wildfire and Grass Seeding

Grass seeding is widely used for erosion control, but its consequences for soil and regeneration following fire have been measured only infrequently. This study investigates the effect of grass seeding on the type and extent of plant cover; soil moisture percentage; and moisture stress, survival, growth, and root-tip and mycorrhiza formation of Pinus lambertiana (sugar pine) seedlings in a clearcut intensely burned by wildfire. One-year-old containerized sugar pine seedlings were planted in seeded and nonseeded areas in Spring 1988 and 1989 in the Longwood Fire area of southwest Oregon. In 1988, tree seedlings in grass-seeded plots experienced intense competition from the grass, reduced root-tip and mycorrhiza formation, low levels of soil moisture to meet evapotranspirational demand, high levels of mortality, and reduced growth. In 1989, however, the opposite was true: tree seedlings in nonseeded plots experienced competition from invading native annuals and perennials, low levels of soil moisture in summer, and higher levels of mortality. The studies we report here further indicate that, in an area characterized by extended summer drought, annual ryegrass impeded regeneration of sugar pine during the first season following the fire. Native species cover and richness have been significantly reduced in the seeded area and may affect long-term soil stability, productivity, and conifer restoration. Seeding of annual ryegrass at high rates under these conditions would seem ill advised.     

Feral horses dung piles as potential invasion windows for alien plant species in natural grasslands

Small scale disturbances could act as patches that provide sites for the colonization of competitively inferior species, promoting the establishment of non-native species in some cases. We analyzed the vegetation associated with feral horse dung piles in montane pampas grasslands in Mid-East Argentina and described the changes following their abandonment, evaluating whether dung piles act as invasion windows, allowing the entrance of alien plant species. We estimated the portion of the study area directly covered by horse manure and dung height was used to estimate the time elapsed after the abandonment of each pile. Vegetation replacement on dung piles of different ages was assessed and compared with grassland controls using discriminant analysis. We used regression analysis to look for changes in vegetation cover, species richness, species diversity and evenness in response to height (age) of the dung piles, and principal component analyses (PCA) to identify groups of plants associated with different successional stages. We compared cover of alien plant species on dung piles with grassland controls using one-way ANOVA. On average, 2.5% of the study area was covered by horse dung. Total vegetation cover, species richness, diversity and evenness increased after the piles were abandoned. Characteristic plant groups were associated with initial, middle and last phases of the studied succession. Vegetation on the dung piles significantly differed from that in grassland controls and two species were consistently associated with dung piles: the invasive Red Star Thistle, Centaurea calcitrapa, and a native grazing-intolerant grass, Nassella clarazii. Non-native species cover was also higher in dung piles than in control plots. Dung piles cover a significant portion of grassland area in our study site, produce significant changes in the vegetation and are associated with some invasive alien plants that could eventually colonize more pristine areas in the vicinity. On the other hand, they might represent refuges for palatable species, since horses seem to avoid them for grazing.     

Distribution and effects of tree leaf litter on vegetation composition and biomass in a forest-grassland ecotone

Aims After abandonment of grasslands, secondary succession leads to the invasion by woody species. This process begins with the accumulation of tree litter in the forest–grassland ecotone. Our objectives were to determine the relationships between litter amounts and vegetation composition and cover along natural forest–grassland ecotones and to experimentally study the initial effects of tree litter accumulation on grassland vegetation and on microsite conditions.Methods We established 11 transects varying from 12 to 15 m in length in different forest–grassland ecotones in the Lahn-Dill highlands, Germany, and measured the mass and cover of tree litter and the cover and composition of vegetation at five sequential positions along each transect by using 1 m 2 plots with five replications. In a field experiment, we established plots subjected to different litter amounts (0, 200 and 600g m ?2) and evaluated changes in grassland vegetation, soil temperature and soil nutrient availability below the litter layer.Important findings Tree litter amounts decrease from 650 to 65g m ?2 across the forest–grassland ecotone. Vegetation changed from shrubs and annual species (adapted to more stressful conditions) in the forests edge to grasses, rosettes and hemirosette species (with higher competitive abilities) in the grassland. These anthropogenic forest–grassland ecotones showed abrupt edges, and the two adjacent ecosystems were characterized by different species pools and functional groups. In the field experiment, the presence of a litter layer reduced vegetation biomass and cover; the species richness was only reduced in the treatment with high litter (600g m ?2). Additionally, adding litter on top of vegetation also reduced thermal amplitude and the number of frost days, while increasing the availability of some nutrients, such as nitrogen and aluminium, the latter being an indicator of soil acidification. Adding a tree litter layer of 600g m ?2 in grassland areas had strong effects on the composition and diversity of grassland vegetation by reducing the cover of several key grassland species. In, or near, forest edges, litter accumulation rapidly changes established vegetation, microsite conditions and soil nutrients.     

Grass invasion causes rapid increases in ecosystem carbon and nitrogen storage in a semiarid shrubland

Accurately predicting terrestrial carbon (C) and nitrogen (N) storage requires understanding how plant invasions alter cycling and storage. A common, highly successful type of plant invasion occurs when the invasive species is of a distinctly different functional type than the native dominant plant, such as shrub encroachment throughout the western United States and annual grass invasions in Mediterranean shrublands, as studied here. Such invasions can dramatically transform landscapes and have large potential to alter C and N cycling by influencing storage in multiple pools. We used a manipulation of non‐native annual grass litter within a shrub‐dominated habitat in southern California (coastal sage scrub, CSS) to study how grass invasion alters ecosystem C and N storage. We added, removed, or left unchanged grass litter in areas of high and low invasion, then followed soil and vegetation changes. Grass litter greatly increased C and N storage in soil, aboveground native and non‐native biomass. Aboveground litter storage increased due to the greater inputs and slower decomposition of grass litter relative to shrub litter; shading by grass litter further reduced decomposition of both non‐native and native litter, which may be due to reduced photodegradation. Soil C and N pools in areas of high litter increased ～20% relative to low litter areas in the two years following manipulation and were generally sinks for C and N, while areas with low litter were sources. We synthesize our results into a C cycle of invaded and uninvaded areas of CSS and link changes in storage to increases in the soil fungi : bacteria ratio, increased plant inputs, and decreased litter loss. Overall, we show that grasses, especially through their litter, control important abiotic and biotic mechanisms governing C and N storage, with widespread implications for C sequestration and N storage in semiarid systems undergoing grass or shrub invasions.     

基于SNIC-CNN-SVM模型的京津风沙源二期工程区土地利用/土地覆盖遥感识别研究

稀疏植被覆盖(草地、沙地、戈壁)演变能够直接表征区域生态环境和人类活动的动态影响变化。但由于大尺度稀疏植被区一般都具有地理跨度大,景观结构复杂多样,破碎化程度高,现有地表覆盖分类产品针对性不足等问题,使得该区域内林草沙的遥感提取难度较大,精度普遍偏低,直接制约生态效应评价模型的应用效果。因此,以典型大尺度稀疏植被区——京津风沙源治理二期工程区为研究区,研建了SNIC-CNN-SVM(SCS)模型,实现了大尺度稀疏植被区林草沙典型要素的信息自动提取和主要土地利用/土地覆盖类型识别。研究结果表明：1)引入惩罚性机制优化后的SNIC分割算法,有效提升了稀疏植被区与沙地区的边界区分度,有助于分类精度的提升;2)基于改进SNIC-CNN-SVM模型方案的研究区总体分类精度达89.41%,较优化前提高了11.17%,特别是乔、灌、草、沙地和戈壁的分类识别精度显著提升,表明该优化方案在以研究区为代表的稀疏植被区域分类中具有较好的应用效果和推广价值;3)分类结果显示,2020年工程区草地面积最大,占到了一半以上(51.52%),沙地占比11.96%,稀疏植被覆盖(草地、沙地、戈壁)区域占比68.68%...