The crown-gap ratio (C) and crown cover: Derivation and simulation study

The crown-gap ratio C is defined as the mean distance between adjacent crowns divided by the mean crown diameter. Previous field studies have established that for a limited range of vegetation stands crown cover is related to C by a general function, viz. crown cover = k/(1 + C)², where k is a constant. In the present study we investigated the general derivation of the function which relates crown cover to C. The function was then used to determine values for k, a range of point distributions being simulated from semi-regular to clumped, with mixtures of crown sizes. The relationship between crown cover and 1/(1 + C)² was linear in all cases tested (R²= 1.0). The value of k was shown to depend on the sampling technique used, the degree of clumping, and the range in crown sizes permitted. The variation in k was reduced by using a specific sampling method and limiting the range of spatial distributions and crown sizes considered. The constraints imposed were:

• 1 Sampling followed a zig-zag transect (Delaunay two-sided model).
• 2 Point distributions were not overly clumped.
• 3 The range of crown diameters approximated a Gaussian distribution with a relative range of 1–4.

Given these constraints the value of k was determined to be 0.806 with a variance of less than 2%. A table is given to convert values for C into crown cover percentages and, if required, to calculate foliage cover per cent. The constraints imposed are considered to be within conditions normally encountered in the field, making the use of C an accurate and simple method by which to estimate crown or foliage cover per cent.     

Comparison of crown cover estimates for woody vegetation in arid rangelands

Three methods for estimating crown cover of woody vegetation were compared, in three mapped field sites and one theoretical map of randomly distributed individuals, by computer simulated sampling. The effect of plant size on the performance of the methods was investigated by varying the size of individuals in the theoretical map. Using the line-intercept and point-sampling techniques according to standard procedures, the net crown cover of a species is estimated and overlapping areas are not recognized. The variable plot or Bitterlich gauge technique estimates the total crown cover of individuals, and areas of overlap are thus recorded more than once. Consequently, Bitterlich gauge estimates will always be greater whenever individuals intermingle or overtop one another. Line-intercept and point-sampling techniques produced highly variable estimates when plants were small, but were not prone to greater variability when plant distribution departed from randomness. The Bitterlich gauge estimates were no more variable with small than with large plants, but were affected by the degree of plant aggregation. The problem is overcome by selecting a sample point spacing and a half-angle for the gauge that optimizes plant counting. The advantages of the variable plot method over the other two are discussed in terms of speed and operator differences, and it is recommended as a monitoring technique for woody vegetation in arid rangelands.     

Using aerial photographs to remotely assess tree hollow availability

Tree hollows provide critical habitat for many species worldwide. The conservation of hollow-bearing trees presents a particular challenge for forest managers, partly due to difficulties in predicting their occurrence across a landscape. We trialled a novel approach for assessing relative hollow availability, by remotely estimating mature crown cover and senescence from aerial photographs in Tasmania, Australia. These estimates were tested against plot-based field assessments of actual occurrence of hollow-bearing trees. In dry forest we conducted ground-based surveys of hollows for all mature trees (>50 cm dbh) in 37 half-hectare plots. In wet forest, we conducted helicopter-based surveys of hollows for all mature trees in 45 oldgrowth plots (0.29–4.63 ha). Aerial photographs (1:10,000–1:25,000) were used to classify the senescence and cover of mature crowns in each plot. Regression analysis showed that, in dry forest, hollow-bearing tree densities were strongly related to the remote assessment of mature crown cover, with an 8% increase in variability explained if senescence was also included (R ² = 0.50). In wet forest, mature crown cover alone was the best model (R ² = 0.53 when outliers were removed). Assessing senescence was less important in dense wet forests than dry forest because trees take longer to form mature-shaped crowns and so mature-shaped crowns are more likely to have hollows. These results suggest that, with skilled photo-interpretation, aerial photographs can be useful for remotely assessing the relative density of hollow-bearing trees. This approach has the potential to greatly improve conservation planning for hollows and hollow-dependent fauna.     

Shovelomics: high throughput phenotyping of maize (Zea mays L.) root architecture in the field

We present a method to visually score 10 root architectural traits of the root crown of an adult maize plant in the field in a few minutes. Phenotypic profiling of three recombinant inbred line (RIL) populations of maize (Zea mays L.; B73xMo17, Oh43xW64a, Ny821xH99) was conducted in 2008 in a silt loam soil in Pennsylvania and in a sandy soil in Wisconsin, and again in 2009 in Pennsylvania. Numbers, angles and branching pattern of crown and brace roots were assessed visually at flowering. Depending on the soil type in which plants were grown, sample processing took from three (sand) to 8 min (silt-loam). Visual measurement of the root crown required 2 min per sample irrespective of the environment. Visual scoring of root crowns gave a reliable estimation of values for root architectural traits as indicated by high correlations between measured and visually scored trait values for numbers (r ²?=?0.46?C0.97), angles (r ²?=?0.66?C0.76), and branching (r ²?=?0.54?C0.88) of brace and crown roots. Based on the visual evaluation of root crown traits it was possible to discriminate between populations. RILs derived from the cross NY821 x H99 generally had the greatest number of roots, the highest branching density and the most shallow root angles, while inbred lines from the cross between OH43 x W64a generally had the steepest root angles. The ranking of genotypes remained the same across environments, emphasizing the suitability of the method to evaluate genotypes across environments. Scoring of brace roots was better correlated with the actual measurements compared to crown roots. The visual evaluation of root architecture will be a valuable tool in tailoring crop root systems to specific environments.     

Net changes in regional woody vegetation cover and carbon storage in Texas Drylands, 1937–1999

Although local increases in woody plant cover have been documented in arid and semiarid ecosystems worldwide, there have been few long‐term, large‐scale analyses of changes in woody plant cover and aboveground carbon (C) stocks. We used historical aerial photography, contemporary Landsat satellite data, field observations, and image analysis techniques to assess spatially specific changes in woody vegetation cover and aboveground C stocks between 1937 and 1999 in a 400‐km² region of northern Texas, USA. Changes in land cover were then related to topo‐edaphic setting and historical land‐use practices. Mechanical or chemical brush management occurred over much of the region in the 1940–1950s. Rangelands not targeted for brush management experienced woody cover increases of up to 500% in 63 years. Areas managed with herbicides, mechanical treatments or fire exhibited a wide range of woody cover changes relative to 1937 (?75% to + 280%), depending on soil type and time since last management action. At the integrated regional scale, there was a net 30% increase in woody plant cover over the 63‐year period. Regional increases were greatest in riparian corridors (33%) and shallow clay uplands (26%) and least on upland clay loams (15%). Allometric relationships between canopy cover and aboveground biomass were used to estimate net aboveground C storage changes in upland (nonriparian) portions of regional landscapes. Carbon stocks increased from 380 g C m^?2 in 1937 to 500 g C m^?2 in 1999, a 32% net increase across the 400 km² region over the 63‐year period. These plant C storage change estimates are highly conservative in that they did not include the substantial increases in woody plant cover observed within riparian landscape elements. Results are discussed in terms of implications for ‘carbon accounting’ and the global C cycle.     

Soil C:N:P stoichiometry responds to vegetation change from grassland to woodland

Woody encroachment has been a major land cover change in dryland ecosystems during the past century. While numerous studies have demonstrated strong effects of woody encroachment on soil carbon (C), nitrogen (N), and phosphorus (P) storage, far less is known about the plasticity of soil C:N:P stoichiometry in response to woody encroachment. We assessed landscape-scale patterns of spatial heterogeneity in soil C:N:P ratios throughout a 1.2 m soil profile in a region where grassland is being replaced by a diverse assemblage of subtropical woody plants dominated by Prosopis glandulosa, an N₂-fixing tree. Woody species had leaf and fine root C:N:P ratios significantly different from grasses. Variation in soil C:N ratios in both horizontal and vertical planes was remarkably smaller than that of soil N:P and C:P ratios. Spatial patterns of soil C:N ratio throughout the profile were not strongly related to vegetation cover. In contrast, spatial patterns of soil N:P and C:P ratios displayed a strong resemblance to that of vegetation cover throughout the soil profile. Within the uppermost soil layer (0–5 cm), soil N:P and C:P ratios were higher underneath woody patches while lower within the grassland; however, this pattern was reversed in subsurface soils (15–120 cm). These results indicate a complex response of soil C:N:P stoichiometry to vegetation change, which could have important implications for understanding C, N, and P interactions and nutrient limitations in dryland ecosystems.     

Regional Estimate of Nitric Oxide Emissions Following Woody Encroachment: Linking Imaging Spectroscopy and Field Studies

Woody encroachment, a spatially explicit process of land-cover change, is known to affect the biophysical and biogeochemical properties of ecosystems. However, little information is available on the impacts of woody encroachment on N oxide emissions from savanna regions. We combined hyperspectral remote sensing and field measurements to quantify spatial patterns and estimate regional fluxes of soil N oxide emissions as they covary with vegetation cover and soil type across a semiarid rangeland in north Texas. Soil nitric oxide (NO) emissions were highly correlated with Prosopis canopy cover, allowing the extrapolation of NO fluxes from hyperspectral observations of woody cover. NO emissions were highly variable, ranging from 0 to 550 kg NO-N km^–2 y^–1 across the region, with the lowest emissions from shallow clay soils and highest from deeper upland clay loams. An estimate of annual NO emissions based on remotely derived Prosopis cover, temperature, and precipitation was 160 kg NO-N km^–2 y^–1, almost twice that of the value derived from traditional averaging of field measurements. We conclude that relationships between NO emissions and remotely sensed structure and composition are advantageous for quantifying NO emissions at the regional scale. This study also provides new insight into the role of woody encroachment on biogeochemical processes that are highly variable and otherwise difficult to measure at the regional scale.     

The decomposition of sugar beet residues: mineralization versus immobilization in contrasting soil types

The leaves and crowns from ¹⁵N-labelled sugar beets were incubated in either a silty clay loam or sand soil for almost one year. Four additions of fresh, chopped residues mixed with soil were tested: ¹⁵N-labelled leaves alone, ¹⁵N-labelled leaves plus unlabelled crowns, unlabelled leaves plus ¹⁵N-labelled crowns, and ¹⁵N-labelled crowns alone; a control with no addition was also incubated. The C:N ratio of the leaves was 11 and that of the crowns 40. Incubations were carried out in pots kept at 20 °C and optimal moisture conditions. The leaves mineralized N from the start of the experiment but the addition of crowns to soil at first caused immobilization of nitrogen followed eventually by mineralization after 6 or 12 weeks depending on soil type. The extra amounts of mineral N found in soil at the end of the experiment where additions were made corresponded to the sum of the background mineralization and the addition; no priming effects were encountered. Very slight differences only were found between the initial rates of mineralization of C in all of the treatments. Although there was also little difference between the sand and silty clay loam soils in the direct mineralization of nitrogen from the sugar beet leaves, where N was first immobilized (i.e. from crowns or a mixture) re-release of N took place more quickly in the sand soil. The total recovery of¹⁵ N found in soils after 24 weeks incubation ranged from 70% to 90% with least being lost from the sugar-rich but N-deficient crowns. Where leaves plus crowns were incubated together both residues contributed to the microbial biomass N.In practice, immobilization of this magnitude and duration (expressed as a temperature sum) could exceed the growth period of a spring sown crop. The actual immobilization found in any one field is likely to depend on the C:N ratio of the residues and could account for much of the variation in the residual benefit of sugar beet residues reported in the literature.     

Multi-Scale Associations between Vegetation Cover and Woodland Bird Communities across a Large Agricultural Region

Improving biodiversity conservation in fragmented agricultural landscapes has become an important global issue. Vegetation at the patch and landscape-scale is important for species occupancy and diversity, yet few previous studies have explored multi-scale associations between vegetation and community assemblages. Here, we investigated how patch and landscape-scale vegetation cover structure woodland bird communities. We asked: (1) How is the bird community associated with the vegetation structure of woodland patches and the amount of vegetation cover in the surrounding landscape? (2) Do species of conservation concern respond to woodland vegetation structure and surrounding vegetation cover differently to other species in the community? And (3) Can the relationships between the bird community and the woodland vegetation structure and surrounding vegetation cover be explained by the ecological traits of the species comprising the bird community? We studied 103 woodland patches (0.5 - 53.8 ha) over two time periods across a large (6,800 km²) agricultural region in southeastern Australia. We found that both patch vegetation and surrounding woody vegetation cover were important for structuring the bird community, and that these relationships were consistent over time. In particular, the occurrence of mistletoe within the patches and high values of woody vegetation cover within 1,000 ha and 10,000 ha were important, especially for bird species of conservation concern. We found that the majority of these species displayed similar, positive responses to patch and landscape vegetation attributes. We also found that these relationships were related to the foraging and nesting traits of the bird community. Our findings suggest that management strategies to increase both remnant vegetation quality and the cover of surrounding woody vegetation in fragmented agricultural landscapes may lead to improved conservation of bird communities.     

Deforestation and Reforestation of Latin America and the Caribbean (2001–2010)

Forest cover change directly affects biodiversity, the global carbon budget, and ecosystem function. Within Latin American and the Caribbean region (LAC), many studies have documented extensive deforestation, but there are also many local studies reporting forest recovery. These contrasting dynamics have been largely attributed to demographic and socio‐economic change. For example, local population change due to migration can stimulate forest recovery, while the increasing global demand for food can drive agriculture expansion. However, as no analysis has simultaneously evaluated deforestation and reforestation from the municipal to continental scale, we lack a comprehensive assessment of the spatial distribution of these processes. We overcame this limitation by producing wall‐to‐wall, annual maps of change in woody vegetation and other land‐cover classes between 2001 and 2010 for each of the 16,050 municipalities in LAC, and we used nonparametric Random Forest regression analyses to determine which environmental or population variables best explained the variation in woody vegetation change. Woody vegetation change was dominated by deforestation (?541,835 km²), particularly in the moist forest, dry forest, and savannas/shrublands biomes in South America. Extensive areas also recovered woody vegetation (+362,430 km²), particularly in regions too dry or too steep for modern agriculture. Deforestation in moist forests tended to occur in lowland areas with low population density, but woody cover change was not related to municipality‐scale population change. These results emphasize the importance of quantitating deforestation and reforestation at multiple spatial scales and linking these changes with global drivers such as the global demand for food.     

Mapping gains and losses in woody vegetation across global tropical drylands

Woody vegetation in global tropical drylands is of significant importance for both the interannual variability of the carbon cycle and local livelihoods. Satellite observations over the past decades provide a unique way to assess the vegetation long‐term dynamics across biomes worldwide. Yet, the actual changes in the woody vegetation are always hidden by interannual fluctuations of the leaf density, because the most widely used remote sensing data are primarily related to the photosynthetically active vegetation components. Here, we quantify the temporal trends of the nonphotosynthetic woody components (i.e., stems and branches) in global tropical drylands during 2000–2012 using the vegetation optical depth (VOD), retrieved from passive microwave observations. This is achieved by a novel method focusing on the dry season period to minimize the influence of herbaceous vegetation and using MODerate resolution Imaging Spectroradiometer (MODIS) Normalized Difference Vegetation Index (NDVI) data to remove the interannual fluctuations of the woody leaf component. We revealed significant trends (P < 0.05) in the woody component (VOD_wood) in 35% of the areas characterized by a nonsignificant trend in the leaf component (VOD_leaf modeled from NDVI), indicating pronounced gradual growth/decline in woody vegetation not captured by traditional assessments. The method is validated using a unique record of ground measurements from the semiarid Sahel and shows a strong agreement between changes in VOD_wood and changes in ground observed woody cover (r² = 0.78). Reliability of the obtained woody component trends is also supported by a review of relevant literatures for eight hot spot regions of change. The proposed approach is expected to contribute to an improved assessment of, for example, changes in dryland carbon pools.     

The use of functionally graded dental crowns to improve biocompatibility: a finite element analysis

In post-core crown restorations, the significant mismatch between stiffness of artificial crowns and dental tissues leads to stress concentration at the interfaces. The aim of the present study was to reduce the destructive stresses by using a class of inhomogeneous materials called functionally graded materials (FGMs). For the purpose of the study, a 3-dimentional computer model of a premolar tooth and its surrounding tissues were generated. A post-core crown restoration with various crown materials, homogenous and FGM materials, were simulated and analyzed by finite element method. Finite element and statistical analysis showed that, in case of oblique loading, a significant difference (p < 0.05) was found at the maximum von Mises stresses of the crown margin between FGM and homogeneous crowns. The maximum von Mises stresses of the crown margin generated by FGM crowns were lower than those generated by homogenous crowns (70.8 vs. 46.3 MPa) and alumina crown resulted in the highest von Mises stress at the crown margin (77.7 MPa). Crown materials of high modulus of elasticity produced high stresses at the cervical region. FGM crowns may reduce the stress concentration at the cervical margins and consequently reduce the possibility of fracture.     

Hidden crown jewels: the role of tree crowns for bryophyte and lichen species richness in sycamore maple wooded pastures

Tree crowns typically cover the vast majority of the surface area of trees, but they are rarely considered in diversity surveys of epiphytic bryophytes and lichens, especially in temperate Europe. Usually only stems are sampled. We assessed the number of bryophyte and lichen species on stems and in crowns of 80 solitary sycamore maple trees (Acer pseudoplatanus) at six sites in wooded pastures in the northern Alps. The total number of species detected per tree ranged from 13 to 60 for bryophytes, from 25 to 67 for lichens, and from 42 to 104 for bryophytes and lichens considered together. At the tree level, 29 % of bryophyte and 61 % of lichen species were recorded only in the crown. Considering all sampled trees together, only 4 % of bryophyte, compared to 34 % of lichen species, were never recorded on the stem. Five out of 10 red-listed bryophyte species and 29 out of 39 red-listed lichen species were more frequent in crowns. The species richness detected per tree was unexpectedly high, whereas the proportion of exclusive crown species was similar to studies from forest trees. For bryophytes, in contrast to lichens, sampling several stems can give a good estimation of the species present at a site. However, frequency estimates may be highly biased for lichens and bryophytes if crowns are not considered. Our study demonstrates that tree crowns need to be considered in research on these taxa, especially in biodiversity surveys and in conservation tasks involving lichens and to a lesser degree also bryophytes.     

基于中高分辨率遥感的植被覆盖度时相变换方法

植被覆盖度是衡量地表植被状况、指示生态环境变化的一个重要指标,也是许多学科的重要参数。传统的测量方法难以获取时间连续的面状数据,且耗时、耗力,很难大范围推广。遥感估算方法虽然可以弥补传统方法的不足,但由于云覆盖等天气条件的影响,获得同一时相覆盖整个研究区的遥感影像非常困难,时相的差异必然导致研究结果产生误差。针对植被覆盖度这一重要生态参数,结合低分辨率遥感数据的时间优势和中高分辨率遥感数据的空间优势,提出一种时相变换方法,将源于中高分辨率影像的植被覆盖度变换到研究需要的时相上。首先,利用像元二分模型计算MODIS尺度的时间序列植被覆盖度,并利用已经获得的SPOT影像计算其获取时相上的植被覆盖度;其次,利用土地利用图划分植被覆盖类型,并利用MODIS数据和土地利用数据之间的空间对应关系制作MODIS像元内各类植被覆盖的面积百分比数据;再次,利用面积百分比数据提取各类植被覆盖的纯像元,结合MODIS植被覆盖度时间序列,从而提取各类植被覆盖纯像元的植被覆盖度时间序列曲线;最后利用像元分解的方法提取MODIS像元内各类植被覆盖组分的植被覆盖度的变化规律,将其应用到该组分对应位置上SPOT像元的植被覆盖度上,从而将其变换到所需要的时相上。在密云水库上游进行试验,将覆盖研究区的10景SPOT5多光谱影像计算的植被覆盖度统一变换到7月上旬,结果显示:视觉效果上明显好转,且空间上连续一致;变换前后植被覆盖度的统计量对比结果也符合植被生长规律;利用外业样点数据与对应位置的植被覆盖度变换结果进行回归分析,结果发现各植被覆盖类型的R2均在0.8左右,表明变换结果与实测值非常接近,时相变换的效果较好,从而可以很好地促进相关研究精度的提高。     

Crown typology and the identification of rain forest trees on large-scale aerial photographs

This paper assesses the potential of large-scale aerial photographs for the identification of rain forest trees. Colour slides at 1: 3000 scale were acquired in French Guiana, above the canopy of a 25 ha study site where trees had already been identified. Firstly, the stereoscopic analysis of photographic prints served to establish a typology of the crowns, based on seven main classes of criteria: crown size, crown status, crown contour, crown architecture, foliage cover, foliage texture and colour, completed where possible by information on phenology. The terminology chosen was based on those proposed in previous studies. Secondly, a smaller area of 5 ha was delimited in the field, on which 15 tree categories (vernacular names) represented by 5 crowns or more on the photographs were selected. For each category, a standard crown was described using terminology previously defined. Twelve tree categories, including first and second class commercial timbers, displayed specific characteristics allowing them to be identified on aerial photographs. Further analyses will be undertaken in the future to measure the success of this identification method. This method may be applied for the recognition of particular species of interest: commercial, rare, endemic or key-stone species. Phenological data, as well as information on crown architectural development, can also be retrieved from aerial photographs, bringing new prospects for a better knowledge of crowns biology and their functional role in the forest ecosystem.     

Simulating vegetation processes along the Kalahari transect

The Sheffield Dynamic Global Vegetation Model has simulated the structure and net carbon exchange of vegetation at five sites along the Kalahari transect where there is a strong gradient in precipitation from 299 to 918 mm yr^?1. There has been a decline in precipitation of 8 mm yr^?1 along the whole of the transect since about 1970. Simulations of vegetation dynamics and structure indicate that this decline has exerted a notable effect on the vegetation, with reductions in woody plant cover at the dry end of the transect and reductions in tree density at the wetter end. These changes were driven primarily by reductions in the net primary production and increased rates of mortality, with rather small impacts of fire.     

Intratree Variation in Fruit Production and Implications for Primate Foraging

We tested the hypothesis that fruit quantity and quality vary vertically within trees. We quantified intratree fruit production before exploitation by frugivores at different heights in 89 trees from 17 species fed on by primates in Kibale National Park, Uganda. We also conducted a pilot study to determine if the nutritional value of fruit varied within tree crowns. Depending on the species and crown size, we divided tree canopies into 2 or 3 vertical layers. In 2-layered trees, upper crowns produced fruits that were 9.6–30.1% bigger and 0.52–140 times the densities of those from lower crowns, with one exception. Among 2-layered trees, upper crowns produced a mean of 46.9 fruits/m³ (median 12.1), while lower crowns produced a mean of 14.1 fruits/m³ (median 2.5). Among 3-layered trees, upper crowns produced a mean density of 49.9 fruits/m³ (median 12.5), middle crowns a mean of 16.8 fruits/m³ (median 6.6), and lower crowns a mean of 12.8 fruits/m³ (median 1.8). Dry pulp and moisture were systematically greater per fruit in the highest compared to the lowest canopy layers (22.4% and 16.4% respectively in 2-layered trees, 49.7% and 21.8% respectively in 3-layered trees). In 1 tree of Diospyros abyssinica, a pilot nutritional study showed that upper crown ripe fruit contained 41.9% more sugar, 8.4% more crude proteins, and 1.8 times less of the potentially toxic saponin than lower crown ripe fruit, but the result needs to be verified with more individuals and species of trees. We discuss the consequences of intratree variations in fruit production with respect to competition among frugivorous primates.     

Estimating woody and herbaceous vegetation cover from time series satellite observations

In this paper we test a method to estimate the tree and grass vegetation cover over Australia from satellite-derived normalized difference vegetation index (NDVI) time series (monthly 1981–91, ≈5 km pixels) observations. The evergreen cover is assumed to track along the base of the NDVI time series, which is assumed to be equivalent to the woody vegetation cover. The base of the NDVI time series is estimated using modifications to a classical econometric model (i.e. time series is the sum of trend, seasonal and random components). Estimates of the average evergreen component during 1982–85 and 1986–89 were generally consistent with known vegetation distributions. Changes in evergreen cover were largely restricted to the south-west and south-east of Australia. Those changes were largely the result of differences in rainfall between the two periods. The proposed method for estimating woody vegetation cover is found to be generally robust. However, there are some regions where the grass (or pasture) is mostly evergreen. Some possible refinements are proposed to handle such cases.     

Impact of off-road vehicles on soil and vegetation in a desert rangeland in Saudi Arabia

Off-road vehicle driving is considered as main contributor to land degradation in arid regions. This study examined the impact of off-road vehicles (ORV) on soil and vegetation in a natural recreational desert meadow of Raudhat Khuraim, Saudi Arabia. Vegetation canopy cover and plant height away from road tracks were assessed. Also, species density and canopy cover, bare ground cover and soil attributes were assessed in four microhabitats; tracks, inter-tracks, verges, and away from vehicle tracks (undisturbed natural areas). Results show that the cover of forbs and grasses was negatively associated with distance from road verges. It was observed that the height of woody species responded negatively to distance away from tracks. Cover of native species decreased under verge, inter-track and track microhabitats giving more opportunity for weeds to flourish. Bare ground was highest (60.7%) in tracks. ORV impact on soil bulk density was clear with an increase of 38% under tracks compared to soils of undisturbed natural vegetation and a similar decrease in porosity was observed. On the other hand, soil electrical conductivity was significantly higher (5.45 mS cm^?1) under disturbance compared to 1.32 mS cm^?1 in undisturbed natural vegetation. Organic matter and nitrogen were not affected significantly by ORV disturbance. The results emphasize that managing off-road vehicle driving is essential for conserving native vegetation.     

Effects of grazing and topography on long-term vegetation changes in a Mediterranean ecosystem in Israel

The dynamics of Mediterranean vegetation over 28 years was studied in the Northern Galilee Mountains, Israel, in order to identify and quantify the major factors affecting it at the landscape scale. Image analysis of historical and current aerial photographs was used to produce high resolution digital vegetation maps (pixel size = 30 cm) for an area of 4 km² in the Galilee Mountains, northern Israel. GIS tools were used to produce corresponding maps of grazing regime, topographic indices and other relevant environmental factors. The effects of those factors were quantified using a multiple regression analyses. Major changes in the vegetation occurred during the period studied (1964–1992); tree cover increased from 2% in 1964 to 41% in 1992, while herbaceous vegetation cover decreased from 56% in 1964 to 24% in 1992. Grazing, topography and initial vegetation cover were found to significantly affect present vegetation patterns. Both cattle grazing and goat grazing reduced the rate of increase in tree cover, yet even intensive grazing did not halt the process. Grazing affected also the woody-herbaceous vegetation dynamics, reducing the expansion of woody vegetation. Slope, aspect, and the interaction term between these two factors, significantly affected vegetation pattern. Altogether, 56% and 72% of the variability in herbaceous and tree cover, respectively, was explained by the regression models. This study indicates that spatially explicit Mediterranean vegetation dynamics can be predicted with fair accuracy using few biologically important environmental variables.