Leaf size, specific leaf area and microhabitat distribution of chaparral woody plants: contrasting patterns in species level and community level analyses

We examined variation in leaf size and specific leaf area (SLA) in relation to the distribution of 22 chaparral shrub species on small-scale gradients of aspect and elevation. Potential incident solar radiation (insolation) was estimated from a geographic information system to quantify microclimate affinities of these species across north- and south-facing slopes. At the community level, leaf size and SLA both declined with increasing insolation, based on average trait values for the species found in plots along the gradient. However, leaf size and SLA were not significantly correlated across species, suggesting that these two traits are decoupled and associated with different aspects of performance along this environmental gradient. For individual species, SLA was negatively correlated with species distributions along the insolation gradient, and was significantly lower in evergreen versus deciduous species. Leaf size exhibited a negative but non-significant trend in relation to insolation distribution of individual species. At the community level, variance in leaf size increased with increasing insolation. For individual species, there was a greater range of leaf size on south-facing slopes, while there was an absence of small-leaved species on north-facing slopes. These results demonstrate that analyses of plant functional traits along environmental gradients based on community level averages may obscure important aspects of trait variation and distribution among the constituent species.     

Vegetation convergence during early succession on coal wastes: a 6‐year permanent plot study

Question: Does the course of succession on a coal mine restored by hydroseeding converge with a reference community in terms of species composition and vegetation structure? What is the rate of succession on restored areas? How does the balance between local colonization and extinction rates change during succession? Which species group (native or hydroseeded) determines the successional process? Location: Large reclaimed coal mine in the north of Palencia province, northern Spain (42°50′N, 4°38′W). Methods: Between 2004 and 2009 we monitored annually vascular plant species cover in nine permanent plots (20 m² each) at a restored mine; these plots were structured to account for site aspect (north, south and flat). Three identical permanent plots were established in the surrounding reference community and monitored in 2004 and 2009. We used detrended correspondence analysis to assess successional trends and rates of succession, generalized linear mixed models to derive patterns of vegetation structural changes and turnover through time, and Huisman–Olff–Fresco modelling to illustrate response of individual species through time. Results: The three restored mine areas exhibited a successional trend towards the reference community through time, although speed of convergence differed. However, after 6 years the restored sites had diverged considerably and this was greater than the dissimilarity reduction with respect to the reference community. Richness, diversity and native species cover increased linearly through time, whereas hydroseeded species cover decreased. Success of hydroseeded species initially differed in the three areas, and this was negatively related with native species colonization rates. Response patterns through time of ten hydroseeded and 20 most common native species are described. Conclusions: Vegetation structural parameters rapidly converged with the reference community, whereas compositional convergence needed much longer. At the same time, successional composition trajectories and rates were related to site properties (here aspect).     

Early post-fire succession in California chaparral: Changes in diversity, density, cover and biomass

For four consecutive years, following the fires in November 1993, temporal variations in species richness, cover and biomass of component plant groups in early post-fire chaparral succession were monitored on different aspects at the Stunt Ranch Santa Monica Mountains Reserve, southern California. Plant groups were categorized based on growth form, life form, ability to fix nitrogen, geographic origin and regeneration strategies. North-facing slopes exhibited higher species richness, higher species turnover rate over time and faster vegetation recovery in terms of biomass accumulation and return to pre-fire species composition. This was probably due to higher species richness and biomass of nitrogen-fixing species found on north-facing slopes in comparison to south-facing slopes. On both north- and south-facing slopes, annuals had the highest species turnover rate, followed by herbaceous perennials and shrubs. In the first four post-fire years, annual species were the largest floristic group, but herbaceous perennials and shrubs were the major contributors to community biomass. Nitrogen-fixing species and exotics contributed significantly to early post-fire community structure. Although the general trends in post-fire succession are clear in terms of temporal changes in the relative proportions of different plant groups, environmental variation and the nature of plant life histories of component species, especially dominant species, could alter such trends significantly.     

Spatial patterns of species distributions in grazed subalpine grasslands

Spatial patterns of species diversity have important influences on the functioning of ecosystems, and the effect of livestock grazing on spatial heterogeneity can differ depending on the scale of the analysis. This study examined the effects of grazing on the spatial patterns of species distributions and whether the effects of grazing on the spatial distributions of a species vary with the scale of the analysis. Data were collected at three locations in the subalpine grasslands of Ordesa-Monte Perdido National Park and Aísa Valley, Central Pyrenees, Spain, which differed in mean stocking rates. Aspect explained about one-third of the environmental variation in species distributions. In flat areas, spatial variation in species composition varied with grazing intensity at two scales. At a coarse scale (among vegetation patches), grazing promoted patchiness, and among-transect variation in species diversity and grazing intensity were positively correlated. At a fine scale (within vegetation patches), the disruption of the self-organizing processes of the species spatial distributions resulted in a reduction in the long-range spatial autocorrelations of some of the characteristic species and in the homogenization of species spatial distributions. The presence of encroaching Echinospartum horridum had a significant influence on the effect of grazing on south-facing grassland slopes.     

The influence of aspect on the early growth dynamics of hydroseeded species in coal reclamation areas

Question: Does aspect affect hydroseeding success and the development of vegetation during early vegetation establishment on the steep slopes of coal wastes during the reclamation process? Location: Open‐pit coal mine near Villanueva de la Peña, northern Spain. Methods: In the first year after hydroseeding, we monitored the dynamics of hydroseeded species in three permanent plots of 20 m² on north‐ and south‐facing slopes every two months. Soil properties and weather conditions were also monitored. Results: Aspect was related to total plant cover during early revegetation, and south‐facing slopes had the lowest cover. Aspect also influenced the early dynamics of hydroseeded grasses and legumes establishing on these slopes. Grass cover was greater on the north slope throughout the study, but differences in plant cover between north and south slopes appeared later for the legumes. Aspect also affected the relative contribution of both of grasses and legumes to the total plant cover, with grasses dominant on both northern and southern slopes, except during the summer on the southern slope. The species with the greatest difference in cover between the north‐ and south‐facing slopes were Festuca spp., Lolium perenne and Trifolium repens. Conclusion: In coal mine reclamation areas of Mediterranean climates, differences in the development of hydroseeded species depended on the slope of the coal mine reclamation areas, and this information is of importance to managers in selecting species for use in reclamation.     

The importance of topography and climate on short-term revegetation of coal wastes in Spain

The initial colonization phase is the crucial start point for succession and therefore for restoration. However, little is known about abiotic factors that influence the early stages of revegetation dynamics on restored coal mines, particularly in a Mediterranean climate. This information is crucial for improving our ability to reclaim land despoiled by mining. Here, we characterized the short-term plant community development in the first 3 years after hydroseeding on a topographically diverse reclaimed open-pit coal mines in Spain. Topography influenced both community composition and diversity producing different trajectories between the three different aspects (north-facing, south-facing, flat). Hydroseeded species provided most of the initial vegetation cover, which brought about most of the compositional differences between these aspects. We also found that the changing climate through the growing season also influenced floristic composition and diversity. Summer drought reduced the cover of hydroseeded perennial species and allowed native species to colonize. These results emphasize the need to take topography into account when developing reclamation management plans in Mediterranean ecosystems, and an acknowledgement that climate might drive the succession in the desired direction, hence increasing success in restoring mining impacts.     

Short‐term bryoid and vascular vegetation response to reforestation alternatives following wildfire in conifer plantations

Question: How are dynamics of early‐seral post‐fire vascular plant and bryoid (terrestrial mosses, lichens, and fungi) vegetation impacted by reforestation activities, particularly manual vegetation removal and planting density? Does the relationship between vegetation dynamics and vegetation removal differ between harsh (west‐facing) and moderate (east‐facing) aspects? Location: Five high‐severity burn plantation forests of Pseudotsuga menziesii in southwestern Oregon, USA. Methods: Plantations severely burned in a recent wildfire were planted with conifer seedlings as a four‐species mixture or a monoculture, at two different densities, with and without manual vegetation removal. A subset of plots was also planted on a contrasting aspect within each plantation. The contrasting aspects differed in potential solar insolation and were indicative of moderate (eastern exposure) and harsh (western exposure) site conditions. Covers of shrub, herbaceous and bryoid vegetation layers were measured during reforestation activities 2–4 yr after the fire. Dynamics of structural layer cover and community composition were compared among treatments with analysis of variance and multivariate analyses (non‐metric multidimensional scaling and blocked multi‐response permutation procedure). Results: Structural layer cover and community composition differed between areas that received reforestation treatments and untreated areas. However, variability within treatments in a plantation was greater than variability within treatments across plantations. Effects of vegetation removal on composition and structure were more evident than effects of planting or altering planting density. Vegetation removal decreased cover of tall and low shrub and the bryoid layer, and increased herbaceous layer cover. Bryoid community and low shrub structural layer responses were more pronounced on moderate aspects than on harsh aspects. Vegetation removal shifted vascular plant community composition towards exotic and annual species. Conclusions: These reforestation treatments may be implemented without substantially altering early‐seral vegetation community composition dynamics, especially in areas with harsh site conditions. Site conditions, such as aspect, should be evaluated to determine need and potential effects of reforestation before implementation. Monitoring for exotic species establishment should follow reforestation activities.     

Influence of slope aspect on Mediterranean woody formations: Comparison of a semiarid and an arid site in Israel

This study investigates the effects of slope aspect on plant community characteristics such as plant cover, species composition and above-ground biomass production in Mediterranean trees and shrubs in two climatological regions. Two experimental sites were selected in a climatic gradient that runs from the foothills of the Judean Hills to the northern Negev desert in Israel. In each site, 16 quadrats of 10 m × 10 m (eight south-facing and eight north-facing slopes) were established and the vegetation was recorded. Dominant tree and shrub species were measured using allometric parameters of area and volume, and representative branches were cut and weighed. Species studied were Quercus calliprinos, Phillyrea latifolia, Pistacia lentiscus, Cistus creticus, Coridothymus capitatus, and Sarcopoterium spinosum. The results showed that slope aspect had significant effects on the composition, structure and density of the plant communities developing in both sites. Vegetation structure within a site changed significantly in the short distance separating the north and south-facing slopes, and that pattern remained generally constant when comparing the two sites along the rainfall gradient. The data collected here provides new insight into the slope aspect effects on biomass allocation of different woody life forms of eastern Mediterranean plant communities.     

Gradients in field-layer vegetation on an arid misty mountain plateau in the Sudan

Abstract. Earlier studies have described how moist, on-shore winds cause meso-scale vegetation patterns on arid mountains near the sea. However, all protruding objects such as trees, micro-relief, and hill slopes influence the distribution of sea-mist. The influence of the tree-canopy, aspect, and distance to the sea on the field-layer vegetation in montane savanna was investigated on 16 hills in the Red Sea Hills, at 34 - 38 km from the sea. At 32 sites, total field-layer cover, species cover, and species number were estimated in a sub-canopy plot and in a nearby open plot on seaward and leeward slopes. Cover and species number in the understorey are significantly higher than in the open. The difference is highest on seaward slopes. Detrended correspondence analysis reveals short species-axes of ca. 2 SD-units. Differences between plots are mainly in species cover. This fits a principal components ordination model. PCA and its constrained version RDA give concordant results. The explanatory variables, Tree-cover and Relative Radiation Index (aspect), have similar indirect influences on plants, and are significantly correlated with axis 1, which is interpreted as a moisture and temperature gradient. The moist seaward plots show an independent trend in species composition along axis 2, which correlates with distance to the sea. On a presence basis the variables, all representing different spatial separation, correlate on the first axis. Presumably, the species composition, at all spatial scales, is directly or indirectly related to the variation in temperature and moisture.     

Richness, diversity, and rate of primary succession over 20?year in tropical coastal dunes

The tropical coastal dunes in central Gulf of Mexico have been stabilizing over the last decades resulting in reduced substrate mobility, and promoting primary succession. We describe changes in species richness and diversity in dune vegetation during 20?years. Our questions: (a) Do species richness and diversity increase over time as predicted by models of ecological succession or do they show a hump-backed manner similar to the observations in temperate coastal dunes?, (b) What is the interaction between vegetation cover and diversity and species richness?, (c) Is there a relationship between species diversity and succession rate and does succession rate change over time?, and (d) How do plant functional types change during succession? In order to answer these questions, we set 140 4?×?4?m permanent plots in a mobile dune area and monitored vegetation cover and species richness from 1991 to 2011. In time, diversity increased in a logistic manner toward an asymptotic value once vegetation cover surpassed 60?%. Species richness increased in a humped-back shape, also reaching a maximum peak at 60?% vegetation cover. The succession rate of diversity was measured by the Euclidean distance, and showed a significant humped-back relation, meaning that it was slower in early and late successional stages. The study supports the intermediate disturbance theory. The conservation of coastal dunes vegetation should focus on all, species-poor and species-rich habitats that help to maintain the ecological integrity of these ecosystems. The understanding of community dynamics and diversity patterns becomes an essential component of coastal dune management and conservation.     

Limits to convergence of vegetation during early primary succession

Questions: Primary succession, measured by changes in species composition, is slow, usually forcing a chronose‐quence approach. A unique data set is used to explore spatial and temporal changes in vegetation structure after a 1980 volcanic eruption. On the basis of data from a transect of 20 permanent plots with an altitudinal range of 250 m sampled through 2005, two questions are asked: Do changes along the transect recapitulate succession? Do plots converge to similar composition over time? Location: A ridge between 1218 and 1468 m on Mount St. Helens, Washington, USA. Methods: Repeat sampling of plots for species cover along a 1‐km transect. Floristic changes were characterized by techniques including DCA, clustering and similarity. Results: Species richness and cover increased with time at rates that decreased with increasing elevation. The establishment of Lupinus lepidus accelerated the rate of succession and may control its trajectory. Diversity (H) at first increased with richness, then declined as dominance hierarchies developed. Primary succession was characterized by overlapping phases of species assembly (richness), vegetation maturation (diversity peaks, cover expands) and inhibition (diversity declines). Each plot passed through several community classes, but by 2005, only four classes persisted. Succession trajectories (measured by DCA) became shorter with elevation. Similarity between groups of plots defined by their classification in 2005 did not increase with time. Similarity within plot groups converged slightly at the lower elevations. Despite similarities between temporal and spatial trends in composition, trajectories of higher plots do not recapitulate those of lower plots, apparently because Lupinus was not an early colonist. Any vegetation convergence has been limited to plots that are in close proximity.     

Acceleration of vegetation succession on eroded land by reforestation in a subtropical zone

Based on field investigations in the East River basin in Guangdong Province, south China, the processes of vegetation development and vegetation succession on bare slopes with high erosion rates are studied. Different reforestation measures have been applied to the slopes, which have resulted in very different processes of vegetation development and succession. On an experimental plot with burned forest but surviving roots, the vegetation restored naturally and quickly because there was little soil erosion. However, in the plots suffering from long-term severe soil erosion, natural vegetation recovery on these barren slopes is very slow. After 26 years, a barren slope has only been partly colonized by poorly developed vegetation composed of heliophilous herbages and scattered shrubs. On the other hand, plantation of some selected wood species on the slope land has dramatically accelerated the vegetation recovery and succession. The plots were reforested with plantation of Acacia auriculiformis in the early 1980s. Twelve years later, the vegetation cover of the artificial forests reached 90% and an understory vegetation community consisting of local species had naturally developed. Reforestation with suitable strategies may control erosion and greatly accelerate vegetation succession in the eroded slope land in the subtropical zones.     

Aspen (<Emphasis Type="Italic">Populus tremuloides</Emphasis>) stands and their contribution to plant diversity in a semiarid coniferous landscape

We conducted a field study to determine the relative contributions of aspen (Populus tremuloides), meadow, and conifer communities to local and landscape-level plant species diversity in the Sierra Nevada and southern Cascade Range, northeastern California, USA. We surveyed plant assemblages at 30 sites that included adjacent aspen, conifer, and meadow communities across a 10,000-km² region. We statistically investigated patterns in local and landscape-scale plant diversity within and among the three vegetation types. Summing across sites, aspen stands supported more plant species overall and more unique plant species than either meadow or conifer communities. Local richness and diversity did not differ between aspen and meadow plots; conifer forest plots were significantly lower in both measures. Heterogeneity in species composition was higher for aspen forest than for meadows or conifer forest, both within sites and between sites. Plant communities in aspen stands shared less than 25% of their species with adjacent vegetation in conifer and meadow plots. Within aspen forest, we found a negative relationship between total canopy cover and plant diversity. Our results strongly support the idea that plant communities of aspen stands are compositionally distinct from adjacent meadows and conifer forest, and that aspen forests are a major contributor to plant species diversity in the study region. Current patterns of aspen stand succession to conifer forest on many sites in the semiarid western US are likely to reduce local and landscape-level plant species diversity, and may also have negative effects on other ecosystem functions and services provided by aspen forest.     

Factors Controlling Vegetation Establishment and Water Erosion on Motorway Slopes in Valencia, Spain

In semiarid Mediterranean areas, the widespread environmental impact caused by the construction of motorways, railways, and pipelines has created an increasing need for effective restoration. We examined the influence of slope characteristics on vegetation and water erosion on 71 motorway slopes in a semiarid Mediterranean region. Specifically, we studied the effect of slope angle, type (roadfill vs. roadcut) and aspect (north vs. south) on soil properties, vegetation cover, species richness, floristic composition, and water‐caused erosion. Temporal dynamics of soil water content was monitored and related to the soil water potential in order to explain possible differences in vegetation cover between slope types. The main factors influencing vegetation on motorway slopes were the angle, type, and aspect of the slope. Vegetation was almost completely lacking on roadcuts with slopes greater than 45°. On gentler slopes, vegetation cover was 44–78% on roadfills but did not reach 10% on roadcuts, regardless of aspect. The main soil properties affected by the slope type and aspect were the organic matter content, soil available P, and water content. Rill erosion, gully erosion, and mass movement were all significantly higher on roadcuts than roadfills. A total of 308 spontaneous colonizers and seeded species were recorded. The type and aspect of the slope also controlled species composition. The short duration of available water in the soil with respect to soil water potential proved to be a limiting factor to plant colonization on roadcuts and south‐facing slopes as well as the low soil fertility in the case of roadcuts. Our results underscore the difficulty of revegetating slopes with angles greater than 45°, where the probability of seeds moving downhill is high. Future efforts should focus on increasing the surface roughness or building terraces at regular intervals in order to reduce slope angle to less than 45° and favor seed trapping and germination. On gentler slopes, adjusting of seed mixes according to dominant species associated with each slope type and aspect should improve considerably the success of roadside revegetation.     

A comparison of the vegetation of forested and non-forested solution dolines in Hungary: a preliminary study

The present study compares the vegetation characteristics of two large forested and one large non-forested solution dolines in Hungary. We investigated the species composition and vegetation pattern along north to south transects (across the doline bottoms) and compared the richness of different species groups (dry and wet groups) on the doline slopes. We applied linear regression models for each slope to explore the effects of topography on species richness, and Detrended Correspondence Analysis (DCA) to detect the major gradients of floristic variation within each site. We found that the vegetation changed significantly along all transects; and, regardless of the vegetation cover, the doline bottoms contained several cool-adapted species. Variations within the two species groups were more pronounced on the south-facing slopes. The changes were similar in the forested dolines, indicating the role of forest cover in maintaining many cool-adapted species on the north-facing slopes as well. However, the number of cool-adapted species increased significantly along both slopes of the non-forested doline from the upper edge to the bottom. Contrary to our expectations, the species turnover along the slopes of the non-forested doline was lower than that along the slopes of the forested ones. We conclude that both the forested and non-forested dolines serve as refuges for many plant species adapted to different environmental conditions. Apart from providing an understanding of population patterns along environmental gradients, our results may also contribute to our understanding of an even more fundamental question for a future research agenda: the probable effects of climate change on vegetation characteristics in climatic islands with environmental conditions substantially different from the surrounding areas.     

Succession secondaire et perte de diversité végétale après réduction du broutage dans un paturage boisé des Alpes centrales suisses

Freléchoux F., Meisser M. and Gillet F. 2007. Secondary succession and loss in plant diversity following a grazing decrease in a wooded pasture of the central Swiss Alps. Bot. Helv. 117: 37 – 56. Reduced cattle grazing pressure in the Alps has caused the reforestation of many subalpine pastures during the last decades. To understand the dynamics of natural reforestation and to evaluate how this change affects plant species diversity, we described the vegetation of a wooded pasture in the central Swiss Alps (Sembrancher, Valais) using the integrated synusial method. Based on stratified vegetation relevés in 27 plots,we defined 11 community types at the synusial level (two tree-layer, five shrub-layer, and four herb-layer synusiae), and four community types at the phytocoenosis level (pasture, tall forbs and scrub, wooded pasture and forest). The spatial distribution of these four phytocoenoses suggests that they represent successional stages after abandonment, and that the pathway of vegetation succession depends on the aspect. We suppose that on northern oriented, cool and shady locations, abandoned pastures first develop towards tall-forb meadows and scrub with Alnus viridis, and then to a preforested stage with Picea abies and Larix decidua. In contrast, on western oriented, warm and sunny location, Larix decidua (mainly) and Picea abies directly colonize the abandoned pastures, but further succession finally leads to the same pre-forested stage as on northern slopes. Plant species richness was highest in open areas and decreased by 25% as tree cover increased from 6% to 65%. According to our successional model, plant species diversity is lost more rapidly on northern slopes (with species-poor green alder scrub) than on western slopes (with species-rich young larch forests), suggesting that northern slopes most urgently need an appropriate grazing management. Manuscrit accepté le 28 mars 2007     

How does size and isolation affect patches of steppe-like vegetation on slumping hills in Transylvania,Romania?

The region of Southern Transylvania in Romania contains large expanses of species-rich grassland and mixed farmland. Within these landscapes, clusters of small, steep-sided slumping hills support mosaics of herbaceous vegetation caused by the heterogeneity in temperature and water supply and maintained by mowing or grazing. These are of conservation interest not only for the high species density, but also the relict steppe-like vegetation types especially found on their south-facing slopes. Usually surrounded by more intensively used mesic grassland, these hills can be considered patches of species-rich grassland vegetation. We therefore surveyed the vascular plants in 10 m² plots on the south- and north-facing slopes of 50 hills in 12 clusters in order to investigate the influence of their size and isolation on the species richness and composition of the vegetation. We found that larger hills had higher plot-scale species richness, a greater proportion of competitive species and a lower proportion of species of steppe-like vegetation, but only on their south-facing slopes where the conditions are more extreme. Both large and small hills should be considered as important habitats and potential sources of propagules for plant communities in the surrounding areas.     

Soil fungal communities reflect aspect-driven environmental structuring and vegetation types in a Pannonian forest landscape

In temperate regions, slope aspect is one of the most influential drivers of environmental conditions at landscape level. The effect of aspect on vegetation has been well studied, but virtually nothing is known about how fungal communities are shaped by aspect-driven environmental conditions. I carried out DNA metabarcoding of fungi from soil samples taken in a selected study area of Pannonian forests in northern Hungary to compare richness and community composition of taxonomic and functional groups of fungi between slopes of predominantly southerly vs. northerly aspect. The deep sequence data presented here (i.e. 980 766 quality-filtered sequences) indicate that both niche (environmental filtering) and neutral (stochastic) processes shape fungal community composition at landscape level. Fungal community composition correlated strongly with aspect, with many fungi showing preference for either south-facing or north-facing slopes. Several taxonomic and functional groups showed significant differences in richness between north- and south-facing slopes and strong compositional differences were observed in all functional groups. The effect of aspect on fungal communities likely is mediated through contrasting mesoclimatic conditions, that in turn influence edaphic processes as well as vegetation. The compositional differences observed in fungi are largely consistent with the coenologically described forest types, which indicates the usefulness of these habitat types as a framework to better understand environmental differences that influence fungal community composition at landscape level. Finally, the data presented here provide unprecedented insights into the diversity and landscape-level community dynamics of fungi in the Pannonian forests.     

茂县土地岭植被恢复过程中物种多样性动态特征

植被恢复是退化生态系统重建的重要途径,植被恢复过程物种多样性的变化反映了植被的恢复程度.通过群落调查和多样性分析,研究了岷江上游土地岭植被恢复过程中群落物种多样性特征.结果表明： 恢复过程中6类不同类型群落分别表现其对于不同环境特征、干扰及更新方式等的响应;森林是较灌丛更适合当地环境状况的植被类型;人工恢复无干扰和轻度干扰群落的多样性相对较高,是较好的恢复模式.重度干扰使得1年生植物与地下芽植物比例增加,其它口食性较好的多年生草本减少.较强的干扰是群落无法更新、长期处于灌丛阶段且多样性较低的重要原因.本地区人工恢复群落在更新进程和多样性维持上优于自然更新群落,种植华山松加速了本地区植被演替进程.建议以适合恢复区域的多种恢复配置方式进行造林,并避免较强干扰,可以加速群落演替进程并保持恢复群落较高的物种丰富度与多样性.     

The influence of logging and topography on the distribution of spruce-fir forests near their Southern limits in Great Smoky Mountains National Park,USA

We studied the effects of logging history, topography, and potential insolation on the lower-elevation limit of existing spruce-fir forest in Great Smoky Mountains National Park (GRSM). Dummy-variable regression, analysis of variance, and classification trees were applied to environmental data within a geographic information system framework. The effect of logging history on the lower limit of spruce-fir depended on aspect. On north-facing slopes (270°– 90°), the presence of spruce-fir was independent of both logging history and potential insolation. On south-facing sites (90°–270°), the elevation of spruce-fir was significantly higher (by 122 m) in areas that had been logged historically. Classification-tree models suggested an even greater logging effect, indicating that both the lower limit and the upper dominance zone of spruce-fir forest are, on average, nearly 200 m higher in historically logged landscapes. Presence of spruce-fir on south aspects was also significantly related to potential insolation, but the strength of this effect was not dependent on logging history. Classification-tree models, developed separately using data from logged sites versus unlogged sites, were used to estimate the current area of spruce-fir forest in the park expected under the hypothetical scenario that no spruce-fir had been logged (38,675 ha) versus the alternate scenario that it had hall been logged (11,727 ha). At present the area of spruce-fir forest in the park is 21,242 ha. We found greater prevalence of spruce-fir on the Tennessee side of the divide on south aspects and historically logged sites, possibly due to greater occurrence of westerly winds and associated cloud cover.