Shrub Encroachment In Grasslands Research Articles

Is Spectral Unmixing Model or Nonlinear Statistical Model More Suitable for Shrub Coverage Estimation in Shrub-Encroached Grasslands Based on Earth Observation Data? A Case Study in Xilingol Grassland, China

Due to the effects of global climate change and altered human land-use patterns, typical shrub encroachment in grasslands has become one of the most prominent ecological problems in grassland ecosystems. Shrub coverage can quantitatively indicate the degree of shrub encroachment in grasslands; therefore, real-time and accurate monitoring of shrub coverage in large areas has important scientific significance for the protection and restoration of grassland ecosystems. As shrub-encroached grasslands (SEGs) are a type of grassland with continuous and alternating growth of shrubs and grasses, estimating shrub coverage is different from estimating vegetation coverage. It is not only necessary to consider the differences in the characteristics of vegetation and non-vegetation variables but also the differences in characteristics of shrubs and herbs, which can be a challenging estimation. There is a scientific need to estimate shrub coverage in SEGs to improve our understanding of the process of shrub encroachment in grasslands. This article discusses the spectral differences between herbs and shrubs and further points out the possibility of distinguishing between herbs and shrubs. We use Sentinel-2 and Gao Fen-6 (GF-6) Wide Field of View (WFV) as data sources to build a linear spectral mixture model and a random forest (RF) model via space–air–ground collaboration and investigate the effectiveness of different data sources, features and methods in estimating shrub coverage in SEGs, which provide promising ways to monitor the dynamics of SEGs. The results showed that (1) the linear spectral mixture model can hardly distinguish between shrubs and herbs from medium-resolution images in the SEG. (2) The RF model showed high estimation accuracy for shrub coverage in the SEG; the estimation accuracy (R2) of the Sentinel-2 image was 0.81, and the root-mean-square error (RMSE) was 0.03. The R2 of the GF6-WFV image was 0.72, and the RMSE was 0.03. (3) Texture feature introduced in RF models are helpful to estimate shrub coverage in SEGs. (4) Regardless of the linear spectral mixture model or the RF model being employed, the Sentinel-2 image presented a better estimation than the GF6-WFV image; thus, this data has great potential to monitor shrub encroachment in grasslands. This research aims to provide a scientific basis and reference for remote sensing-based monitoring of SEGs.

Sample plot design can affect the efficiency and accuracy of shrub coverage measurements in shrub-encroached grasslands

Shrub encroachment has become a global concern. The fraction of shrub coverage (FSC) is an important indicator that reflects the distribution of shrubs and the degree of shrub encroachment in grasslands. The line-point intercept (LPI) method is commonly used for FSC measurement in field surveys, but it’s often associated with issues of poor accuracy and low measurement efficiency. Here, we focus on Caragana microphylla shrub-encroached grassland as a case study. We used normalized difference vegetation index (NDVI) data obtained from unmanned aerial vehicle (UAV) to derive the actual values of FSC (FSCT) and to simulate measurements using the LPI method. We compared the results for measurements based on different sample plot designs, including variation in plot shape and line distribution, the number of sample lines, and the sample point spacings. The aim was to investigate the influence of these main parameters of sample plot design upon measurement results, in order to provide a reference for FSC measurements in field experiments. We first calculated FSCT and stratified the plots according to coverage levels. Based on this, we further evaluated the measurement accuracy of the LPI method under different parameter settings. The results revealed significant systematic errors in the measurement from circular plots. For square plots, the minimum number of lines required for the “high,” “medium,” and “low” coverage groups are 48, 16, and 8 (for 80% accuracy), and likewise, 108, 24, and 16 (for 90% accuracy), respectively. Futuremore, point spacings of 0.1 m and 0.5 m can achieve the same accuracy as the original spacing (0.02 m). We conclude that the systematic errors in circular plots are caused by the radial distribution of lines, whereas in square plots, achieving an efficient measurement requires considering different coverage levels, ensuring an adequate number of lines, and setting a relatively smaller point spacing.

Sexual Reproduction Is Not Responsible for Caragana Shrub Encroachment in Grasslands

Shrubs tended to increase their abundance as climatic aridity and grazing intensity increased in the Inner Mongolian grassland. Increasing shrub abundance was believed to be due to enhanced reproduction. However, the effects of climatic aridity and grazing on the sexual reproduction of shrubs in grassland remain largely unclear. In this study, we conducted field experiments with Caragana microphylla to examine the variation of sexual reproduction aspects (seed production, seed vigor, and sapling establishment) along a climatic aridity gradient (subhumid, semiarid, arid, and dry arid zones) and a grazing intensity gradient (fenced, mildly grazed, and severely grazed). We then quantified the population growth rate based on seed production and sapling establishment rates. Our objective was to evaluate whether sexual reproduction is the main mechanism for Caragana encroachment into grasslands. We found that climatic aridity decreased seed quantity and seed vigor but increased the sapling establishment rate of Caragana shrubs. Under ungrazed conditions, climatic aridity did not affect population growth rates, while under grazing conditions, increased aridity stresses reduced population growth rates. Grazing reduced seed production, sapling establishment, and population growth rates. Climatic aridity enhanced the negative effects of grazing on sexual reproduction, while grazing intensified the negative effects of aridity on the population growth of Caragana shrubs. In conclusion, climatic aridity, grazing, and their combined effects had negative effects on the sexual reproduction of Caragana shrubs. Therefore, sexual reproduction could not fully explain the increased abundance of shrubs with increasing aridity and grazing. Clonal reproduction might be of considerable importance for understanding the mechanism of shrub encroachment in grasslands.

Sandy Habitats Play an Important Role in Shrub Encroachment in Grasslands

Shrub species have increased in density and cover in desertification areas, however, the role of sandy habitats in contributing to the expansion of shrubs is poorly understood. Although the effect of sandy habitats on plant growth and reproduction have been demonstrated, most existing studies lack either experimental demonstration or an integrated study during the whole shrub life cycle. We performed field and laboratory experiments to examine the responses of four stages in the life cycle of shrubs (seed germination, plant growth, seed reproduction, clonal reproduction) to sandy habitats (including sand substrate, sand burial and wind erosion) for Caragana shrubs. Results showed that both sand substrate and sand burial facilitated seed germination, seedling biomass, sapling establishment, plant growth, and root-shoot ratio of Caragana. Meanwhile, they both strongly increased seed number and seed preservation, and thus enhanced sexual reproduction. Sand burial favored clonal reproduction of Caragana by promoting the formation of branch-derived ramets, while wind erosion benefited clonal reproduction by facilitating the formation of root-derived ramets. These results suggested that sandy habitats facilitated seed germination, plant growth, sexual reproduction, and clonal reproduction of Caragana, which could explain why shrub abundance, shrub area and shrub height of Caragana in sandy areas was higher than in grasslands. Our study provided an experimental demonstration that sandy habitats promoted the population growth of Caragana shrubs during the whole life cycle and highlighted the significant role of sandy habitats in facilitating shrub encroachment in grasslands.

Effects of shrub encroachment on soil aggregates and organic carbon vary in different grasslands in Inner Mongolia, China

AbstractWidespread shrub encroachment in grasslands can lead to changes in soil aggregates and soil organic carbon (SOC), especially in deep soils. Soil aggregates physicochemically protect SOC and thus affect soil carbon sequestration. Characterizing the changes in soil aggregates and their organic carbon associated with shrub encroachment is critical for evaluating the ecological consequences of shrub encroachment in different grasslands. In this study, we investigated soil aggregates of various sizes and their associated organic carbon at maximum soil depths of 5 m in shrub patches and neighboring grass matrix in two types (desert and typical) of grasslands in Inner Mongolia, China. The mean weight diameter (MWD) of the soil aggregates was similar between the shrub patches and grass matrix in both grasslands. However, the proportion of each aggregate size fraction to whole soil in the shrub patches was significantly lower than that in the grass matrix in the desert grasslands, while there was no significant difference between the shrub patches and grass matrix in the typical grasslands. In addition, the organic carbon content of aggregates in the shrub patches was greater than that in the grass matrix in deep soil layers (>50 cm) but was lower in the topsoil (0–10 cm depth) in the desert grasslands. However, the organic carbon content of aggregates at all soil depths was greater in shrub patches than in the grass matrix in typical grasslands. Our results suggest that the effects of shrub encroachment on the soil aggregate proportion and associated organic carbon vary at different soil depths and in different grassland types. The results of this study also suggest that it is necessary to determine the size structure and its relation to carbon content for soil aggregates to accurately predict SOC dynamics with shrub encroachment.

Using consecutive prescribed fires to reduce shrub encroachment in grassland by increasing shrub mortality

Woody plant encroachment into open grasslands occurs worldwide and causes multiple ecological and management impacts. Prescribed fire could be used to conserve grassland habitat but often has limited efficacy because many woody plants resprout after fire and rapidly reestablish abundance. If fire‐induced mortality could be increased, prescribed fire would be a more effective management tool. In California's central coast, shrub encroachment, especially of Baccharis pilularis (coyote brush), is converting coastal prairie into shrub‐dominated communities, with a consequent loss of native herbaceous species and open grassland habitat. B. pilularis has not been successfully controlled with single prescribed fire events because the shrub resprouts and reestablishes cover within a few years. We investigated whether two consecutive annual burns would control B. pilularis by killing resprouting shrubs, without reducing native herbaceous species or encouraging invasive plants. As expected, resprouting did occur; however, 2 years after the second burn, B. pilularis cover on burned plots was only 41% of the cover on unburned plots. Mortality of B. pilularis more than doubled following the second burn, likely maintaining a reduction in B. pilularis cover for longer than a single burn would have. Three native coastal prairie perennial grasses did not appear to be adversely affected by the two burns, nor did the burns result in increased cover of invasive species. Managers wanting to restore coastal prairie following B. pilularis encroachment should consider two consecutive annual burns, especially if moderate fire intensity is achievable.

Is prescribed fire a suitable management tool to reduce shrub encroachment in palm savannas?

Shrub encroachment in grasslands is a worldwide problem that has many ecological consequences, transforming previously open environments into dense forests. Disruption of natural fire regimes is one of the main causes of shrub encroachment, and the use of prescribed fire is a common strategy used to restore these ecosystems. In this study, we provide information about how a palm tree savanna under a process of shrub encroachment responds to the reintroduction of fire. We describe the effects of a first fire event on vegetation composition and structure using an experimental approach. We examine a species‐specific response to the fire. After one prescribed fire event applied to four study areas of 16 ha each, we analyzed the change in vegetation physiognomy and composition in burned and control plots for 1 year. Low‐intensity prescribed fire decreased height and cover of most shrub species and increased herbaceous vegetation cover over time. We classified shrub and herbaceous species response to fire according to the time they became present and their phenological characteristics. Our results can help stakeholders to determine if prescribed fire is helpful at reducing shrub encroachment in short term in similar ecosystems, considering how plant community responds to the reintroduction of fire after decades of fire suppression.

Soil microbial biomass and functional diversity in shrub-encroached grasslands along a precipitation gradient

The use of soil microbial and biochemical parameters could play an important role in monitoring effects of shrub encroachment in grasslands due to their rapid reaction to environmental fluctuations. We compared total organic carbon (Corg), soil total nitrogen (TON), microbial biomass, basal respiration, soil microbial quotient (Cmic/Corg) species and functional diversity between pairs of neighbouring, leguminous shrub-encroached and open grassland plots along a rainfall gradient (300mm to 1500mm mean annual precipitation (MAP)) in South Africa. "In semi-arid regions, soil Corg was higher in shrub-encroached grasslands than in open grasslands while the reverse was true in humid regions (1500 mm MAP)". Soil total nitrogen was generally higher in the shrub-encroached grasslands compared to open grasslands. Soil microbial biomass and basal respiration was higher in shrub-encroached grassland across the precipitation gradient. There was also a difference in microbial functional diversity between the encroached and adjacent open grasslands, which was most evident in the semi-arid regions. Our results suggest that land-cover change might influence microbial properties along the rainfall gradient.

Climate and native grassland vegetation as drivers of the community structures of shrub-encroached grasslands in Inner Mongolia, China

Shrub encroachment in grasslands will lead to a mosaic landscape with shrub patches interspaced with grass patches and thus alters local and regional water and heat balance. Syntheses on effects of the climate and native grassland community on the shrub-encroached grassland (SEG) are very limited at a large scale. In this study we examined the variation of their community characteristics in the Inner Mongolian grasslands, using 192 SEG plots data from field survey during 2012–2013. Our results indicated that shrub cover and patch size exhibit a significant increase with longitude, but showed a hump-shaped latitudinal pattern. Shrub patch density and herb abundance were negative associated with latitude and longitude, whereas plant height showed an opposite trend. Such geographical patterns were likely co-determined by climate and native vegetation. Precipitation was the major controller for shrub cover and patch size, and temperature was the primary factor influencing shrub height and patch density. Consequently, shrub cover and patch size were greater at dry and warm habitats than at moist and cool sites. In addtion, native vegetation also significantly influenced the variations in that of shrub patches. Diverse habitats with tall vegetation were likely more resistant to shrub encroachment. Furthermore, although we did not fully evaluate the effects of soils on SEGs, the discussion on the soil-SEG relationship suggests an important role of soils in characterizing SEG structures. Our study provides the first regional assessment on the variations of SEG structures, and has an implication in the regional shrub expansion management and control.

Changes in intrasystem N cycling from N2-fixing shrub encroachment in grassland: multiple positive feedbacks

Nitrogen-fixing species can increase both the availability and cycling of nitrogen (N) in ecosystems. Autumn olive (Elaeagnus umbellata Thunb.) is an exotic woody shrub associated with N2-fixing actinomycetes that forms dense patches in disturbed landscapes (i.e., riparian zones adjacent to crop systems, old fields and agricultural grasslands) throughout the midwestern United States. We used paired plots dominated by either E. umbellata or C3 grassland to test whether the shrub encroachment altered pools and fluxes of nitrogen (N) and carbon (C) in the soil. Annual mean of NO3‐N concentrations in soil water collected from porous cup tension lysimeters every 2 weeks for 1 year was 20 times higher in soil beneath E. umbellata compared to grassland vegetation. Temporal variation in NO3‐N leaching occurred in the shrubencroached plots, with more nitrate leaching in the dormant season relative to the growing season. Potential net N mineralization, nitrification rates, and extractable N in the surface 10 cm of soil were also higher below E. umbellata. Following establishment of N2-fixing shrub patches for 7‐13 years, the soil C:N ratio showed a declining trend due to lower total soil C rather than an increase in N. Labile carbon pools (i.e., microbial biomass C (MBC) and soil respiration rates) were lower in surface soil below E. umbellata, which demonstrated an additional positive feedback between encroachment of E. umbellata and N export. Less demand for mineralized N due to associated N2 fixation, coupled with higher rates of nitrification and lower microbial demand for N collectively contributed to higher export of N below the E. umbellata patched relative to the grassland system. Thus, areas invaded by this exotic N2-fixing species may function as N sources rather than the N conserving systems typically expected early successional communities following agricultural abandonment. # 2006 Elsevier B.V. All rights reserved.

Kemira broadens Ecocat operations

The use of soil microbial and biochemical parameters could play an important role in monitoring effects of shrub encroachment in grasslands due to their rapid reaction to environmental fluctuations. We compared total organic carbon (Corg), soil total nitrogen (TON), microbial biomass, basal respiration, soil microbial quotient (Cmic/Corg) species and functional diversity between pairs of neighbouring, leguminous shrub-encroached and open grassland plots along a rainfall gradient (300 mm to 1500 mm mean annual precipitation (MAP)) in South Africa. "In semi-arid regions, soil Corg was higher in shrub-encroached grasslands than in open grasslands while the reverse was true in humid regions (1500 mm MAP)". Soil total nitrogen was generally higher in the shrub-encroached grasslands compared to open grasslands. Soil microbial biomass and basal respiration was higher in shrub-encroached grassland across the precipitation gradient. There was also a difference in microbial functional diversity between the encroached and adjacent open grasslands, which was most evident in the semi-arid regions. Our results suggest that land-cover change might influence microbial properties along the rainfall gradient.

Historical shrub–grass transitions in the northern Chihuahuan Desert: modeling the effects of shifting rainfall seasonality and event size over a landscape gradient

AbstractWe use a spatially explicit landscape model to investigate the potential role of rainfall on shrub–grass transitions in the Jornada Basin of southern New Mexico during the past century. In long‐term simulations (1915–1998) along a 2700 m transect running from a dry lake bed to the foothills of a small mountain, we test two hypotheses: (i) that wetter winters and drier summers may have facilitated shrub encroachment in grasslands, and (ii) that increases in large precipitation events may have increased soil water recharge at deeper layers, thus favoring shrub establishment and growth. Our model simulations generally support the hypothesis that wetter winters and drier summers may have played a key role, but we are unable to reproduce the major shifts from grass‐ to shrub‐domination that occurred in this landscape during the early part of the 1900s; furthermore, the positive shrub response to wetter winters and drier summers was only realized subsequent to the drought of 1951–1956, which was a relatively short ‘window of opportunity’ for increased shrub establishment and growth. Our simulations also generally support the hypothesis that an increase in the number of large precipitation events may also have favored shrub establishment and growth, although these results are equivocal, depending upon what constitutes a ‘large’ event and the timing of such events. We found complex interactions among (i) the amount/seasonality of rainfall, (ii) its redistribution in the landscape via run‐on and runoff, (iii) the depth of the soil water recharge, and (iv) subsequent water availability for the growth and reproduction of shrubs vs. herbaceous plants at various landscape positions. Our results suggest that only a mechanistic understanding of these interactions, plus the role of domestic cattle grazing, will enable us to elucidate fully the relative importance of biotic vs. abiotic factors in vegetation dynamics in this semiarid landscape.