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1.
Mixed forest, containing a eucalypt overstorey and an understorey of rainforest tree species, accounts for approximately 20% (195,000 ha) of Tasmania's wet eucalypt forest. In wood production areas it is typically clearfelled, burnt and then sown with eucalypt seed. This management removes virtually all standing seed sources within the coupe, so that recolonisation of coupes by rainforest tree species depends largely on seed sources located at the coupe edges. We quantified the influence of mature mixed forest edges on rainforest regeneration following clearfelling by modelling the change in the density of the regeneration of the four dominant rainforest tree species (Nothofagus cunninghamii, Atherosperma moschatum, Phyllocladus aspleniifolius and Eucryphia lucida) with increasing distance from forest edge. We also assessed the influence on rainforest tree regeneration of prevailing wind direction, age of regeneration, characteristics of the mature edge vegetation and of the competing regenerating vegetation within the coupe. Distance from edge and age of coupe were highly significant (p < 0.01) effects in each of the species models. We found that the abundance of regeneration declined with increasing distance from edge for all four rainforest tree species, and with the exception of A. moschatum regeneration, increased with coupe age up to the age of 15 years. The abundance of N. cunninghamii and E. lucida, which are species with restricted seed dispersal, declined most steeply with increasing distance from the edge. A. moschatum, which is a species with the potential for long distance seed dispersal by wind, was more abundant than N. cunninghamii and E. lucida at distances greater than 20 m from coupe edges. More than 500 seedlings ha−1 were present at all distances from coupe edge for P. aspleniifolius, reflecting its capacity to germinate after disturbance from soil-stored and bird-dispersed seed. There were no significant differences in seedling density upwind or downwind of coupe edges, although the potential for dense regeneration of N. cunninghamii and E. lucida and for long distance dispersal of A. moschatum appeared to be greatest downwind of edges. Other variables that significantly affected the abundance of regeneration were the height of rainforest tree species in the edge vegetation (N. cunninghamii model), the cover of rainforest tree species in the edge vegetation (A. moschatum model) and the cover of competing eucalypt regeneration within the coupe (P. aspleniifolius model). The proportion of rainforest tree species that regenerated vegetatively was small (3.1%). We concluded that management which maintains mature mixed forest edges, or patches of mature forest within coupes, is likely to result in greater levels of rainforest regeneration and a more rapid shift towards pre-harvest composition following logging. We use our results to demonstrate that variable retention harvesting systems, such as aggregated retention or stripfelling, which reduce the distance to rainforest seed source, would result in a greater abundance of rainforest regeneration over a larger proportion of the coupe than current clearfell, burn and sow silviculture.  相似文献   

2.
动物取食和扩散森林植物种实研究进展   总被引：1，自引：0，他引：1  
动物取食扩散森林植物种实作为森林生态系统中植物和动物联系的-个重要方面, 不仅是联系母株与其子代幼苗的重要桥梁, 也是植物生活史中的-个重要阶段。文中综述了近年来关于动物取食森林植物种实及其与这些植物种子散布、萌发、幼苗生长和种群扩展等方面关系的研究成果。动物在取食和破坏森林植物种实的同时也将这些植物的种子扩散到-个新的生境斑块, 其消化道机械的或化学的作用提高了这些森林植物的种子萌发率和萌发进程, 促进了其早期定居和种群扩大。动物对森林植物的这种作用具有长期性, 形成特定动植物间或多物种间的协同进化。近年来, 森林生态系统受到了严重的干扰和破坏, 可能会因为干扰了森林植物种实的动物散布者而影响森林植物种群的扩展。  相似文献   

3.
Cultivation of the woody legume Sesbania sesban in agroforestry systems has been widely promoted for soil fertility improvement in Africa. Despite the existence of natural populations in eastern Zambia, considerable seed originating from Kenya has been introduced to the area, while further introductions from Malawi are planned. Large-scale anthropogenic seed dispersal raises concerns regarding genetic management, which were addressed here by random amplified polymorphic DNA (RAPD) analysis. Eleven populations of Sesbania from southern and East Africa were assessed, including S. sesban populations already or likely to be introduced into eastern Zambia and reference taxa. Seventeen primers revealed a very large number of polymorphisms (538) in surveyed material. Differentiation among S. sesban populations was unusually high for a woody perennial (70% of variation among populations), with all individuals stringently assigned to specific populations in cluster analysis, even for geographically proximate sites. High differentiation, combined with the unusually large number of polymorphisms determined, resulted in the identification of a number of population-diagnostic markers. High differentiation may reflect the restricted ecology of S. sesban, human disturbance and frequent natural fluctuations in niche availability. Hydrochory, posited as a mechanism for gene flow between populations located along the same drainage system, appears to be discounted. Geographic proximity was not always a good guide for genetic similarity among S. sesban populations. In particular, rather proximate populations from Zambia and Malawi appeared highly differentiated genetically, while Zambian populations were relatively similar to a geographically distant population from Namibia. Differentiation was consistent with the presence of two botanic varieties in the southern Africa region, vars. nubica and zambesiaca. Estimates of Nei's genetic diversity suggested that S. sesban populations from East Africa were most diverse. Overall, population diversity estimates were low, possibly due in part to frequent bottlenecks, short longevity and a short juvenile phase. The unusually large number of polymorphisms detected here provided an opportunity to test for data redundancy and primer selection methodologies during RAPD analysis. Combined data from a subset of eight primers selected according to three different approaches provided a very good overall fit to the complete data set (rM > 0.95). Different primer selection approaches influenced the reliability of clustering and diversity analysis, with a minimum of eight primers required for reliable ranking of diversity estimates. The implications of our data for conservation, cultivation and monitoring of S. sesban in the southern Africa region are discussed. Caution is advised during the introduction of germplasm into eastern Zambia from both distant (Kenya) and relatively proximate (Malawi) sources.  相似文献   

4.
The establishment of invasive plant populations is controlled by seed input, survival in the soil seed bank, and effects of soil surface disturbance on emergence, growth, and survival. We studied the invasive vine Celastrus orbiculatus Thunb. (Oriental bittersweet) to determine if seedlings in forest understory germinate from the seed bank or from seed rain. We also conducted a greenhouse experiment to investigate the role of leaf litter mass and physical texture on seedling survival, growth, and allocation. In the understory of an invaded mixed hardwood forest, we measured seed input, seedling emergence with seed rain, and seedling emergence without seed rain. Mean seed rain was 168 seeds m−2: mean seedling emergence was 107 m−2, and there was a strong correlation between seed rain and seedling emergence. The ratio of seedlings to seed input (0.61) was close to the seed viability (0.66) leaving very few seeds to enter the seed bank. Seed bank germination under field conditions was low (1 seedling m−2). Soil cores were incubated in a greenhouse to determine seed bank viability, and germination from these soil cores did not occur. To determine how litter affects seedling establishment and growth, we measured seedling emergence and biomass allocation in a greenhouse experiment. Seeds were placed below intact and fragmented deciduous leaf litter in amounts ranging from zero to the equivalent of 16 Mg ha−1. Seedling emergence was not affected by fragmented litter, but decreased to <20% as intact litter increased to 16 Mg ha−1. Increasing litter resulted in greater allocation to hypocotyl and less to cotyledon and radicle, and this effect was greater in intact litter. C. orbiculatus seedlings achieve emergence through forest floor litter through plasticity in allocation to hypocotyl growth. The low survival of C. orbiculatus in the seed bank suggests that eradication of seedling advance regeneration and adult plants prior to seed rain may be an effective control strategy. However, the intact forest floor litter of an undisturbed forest will not prevent seedling establishment.  相似文献   

5.
We investigated the population age structure and spatial patterning of a common understory perennial, Trillium erectum var. album, in primary and post-logging secondary (industrially logged in the early 1900s) cove forests of the southern Appalachian Mountains. Within each of 8 T. erectum populations (4 in primary forest and 4 in secondary forest), we mapped the locations of all T. erectum plants in a 10 m × 10 m plot located within the center of each population. We recorded the height and life stage of each plant and excavated a randomly selected subset for age determination. Trillium erectum populations in primary forests displayed a somewhat stronger and more consistent spatial structure than those found in secondary forests. However, aside from being on-average younger due to a higher concentration of juveniles, age structures of secondary forest populations were similar to those of primary forests and both forest types had similar maximum plant ages. Our results, when considered with the life history of the species, suggest that logging did not eliminate T. erectum from secondary forests. Rather, residual plants and patches of plants that survived logging provided the propagules needed to expand and reestablish populations following logging. This relatively rapid recovery stands in stark contrast to the slow recovery observed by others in post-agricultural secondary forest. When considered in the context of other studies of post-disturbance recovery in secondary forests, our results suggest that the full history of a site must be carefully considered when examining the impacts of disturbance on forest perennial herbs.  相似文献   

6.
We evaluated the impact of spatial heterogeneity resulting from disturbance and neighbor density on long-term population dynamics of sugar maple (Acer saccharum) in Brownfield Woods, an old-growth forest remnant in central Illinois. All trees with diameter at breast height (DBH) ≥ 7.6 cm in eight discrete quadrats (48 m × 68 m) and one large quadrat (216 m × 260 m) were mapped in Brownfield Woods in 1951, 1988, and 2001. Two sets of neighbor-specific transition matrices were constructed for two census periods (1951–1988 and 1988–2001) based upon historical maps of trees. Different neighborhoods defined by the occurrence of disturbance and neighbor density resulted in major differences in subsequent demography and population dynamics of A. saccharum in Brownfield Woods during 1951–2001. Disturbance subpopulations with high neighbor density demonstrated greater population growth than the other subpopulations and were characterized by relatively high recruitment and mortality. Dutch elm disease subpopulation shared similar characteristics. In contrast, non-disturbance subpopulations demonstrated census-specific demography. Using matrix modeling, we demonstrated the importance of spatial heterogeneity at the scale of neighboring trees to the dynamics at the scale of tree populations.  相似文献   

7.
Tree vegetation and size structure was sampled in a miombo woodland area in E Tanzania and related to environmental factors, particularly soil and disturbance history. A total of 86 tree species was found. Four plant communities were distinguished through multivariate classification. Community 1 was dominated by Brachystegia boehmii, Brachystegia bussei and Julbernardia globiflora, and community 2 by B. boehmii and Brachystegia spiciformis. Community 1 was found on grey, eroded soil and community 2 on red, residual soil, a fact that opens up possibilities to use soil signals of satellite data for vegetation mapping. Community 3 is heavily disturbed miombo woodland near villages and community 4 was found on more clayey soil where miombo woodland is not expected.

At our 42 sampled sites, density ranged from 74 to 1041 individuals ha−1 and basal area from 3.9 to 16.7 m2 ha−1. Regeneration is generally good but large sized trees are less prominent in communities 3 and 4 due to harvesting. With reduced disturbance miombo species may rapidly resume dominance in community 3. A higher than expected representation by the size class 30–40 (−50) cm dbh in community 2 is probably related to disturbance history. Prevalence of certain species (Pseudolachnostylis maprouneifolia, Pterocarpus angolensis and Diplorhynchos condylocarpon) may be related to frequent fires. Selective logging will soon cause extinction of Dalbergia melanoxylon, whereas Pterocarpus angolensis still has good regeneration, possibly because individuals below logging size have a good seed set.

A way to get an easy overview of size classes in all species in an area using PCA is discussed.  相似文献   


8.
Between 1987 and 2000, a spruce beetle (Dendroctonus rufipennis) outbreak infested 1.19 million ha of spruce (Picea spp.) forests in Alaska, killing most of the large diameter trees. We evaluated whether these forests would recover to their pre-outbreak density, and determined the site conditions on which spruce germinated and survived following the spruce beetle outbreak in forests of the Anchor River watershed, Kenai Peninsula, Alaska. White spruce (Picea glauca) and Lutz's spruce (Picea × lutzii), a hybrid between white and Sitka spruce (Picea sitchensis), dominate the study area. We measured the pre- and post-outbreak density of spruce in 108 3 m × 80 m plots across the study area by recording all live trees and all dead trees >1.5 m tall in each plot. To determine the fine scale site conditions on which spruce germinated and survived, we measured ground surface and substrate characteristics within 20 cm circular plots around a subset of post-outbreak spruce seedlings. The density of post-outbreak spruce (855/ha) was adequate to restock the stands to their pre-outbreak densities (643/ha) for trees >1.5 m tall. We could not accurately estimate recovery for pre-outbreak spruce seedlings because dead seedlings may have decayed in the 5–18 years since the beetle outbreak occurred. At the fine scale, spruce that germinated post-outbreak grew on a wide variety of substrates including downed log, stump, mesic organic mat, peat, hummocks and mineral soil. They exhibited a strong preference for downed logs (53%) and stumps (4%), and most (91%) of the downed logs and stumps that spruce rooted on were heavily decayed. This preference for heavily decayed logs and stumps was especially evident given that their combined mean cover was only 2% in the 3 m × 80 m plots. Within the 3 m × 80 m plots, spruce seedling survival was negatively correlated with bluejoint (Calamagrostis canadensis) litter cover.  相似文献   

9.
We examined the extent of soil disturbance associated with bulldozer yarding and the regrowth of woody vegetation on bulldozer paths (skid trails) in selectively logged dipterocarp forest. In an area logged in 1993, using conventional, i.e., uncontrolled, harvesting methods, about 17% of the area was covered by roads and skid trails. In contrast, in a 450-ha experimental area where reduced-impact logging guidelines were implemented, 6% of the area was similarly disturbed. Skid trails in the reduced-impact logging areas were less severely disturbed than those in conventional logging areas; the proportion of skid trails with subsoil disturbance was less than half that in conventional logging areas. Four years after logging, woody plant recovery on skid trails was greater in areas logged by reduced-impact than by conventional methods. Skid trails where topsoil had been bladed off had less woody vegetation than skid trails with intact topsoil. In a chronosequence of logging areas (3, 6, and 18 years after logging), species richness and stem densities of woody plants (>1 m tall, <5 cm dbh) were lower on skid trail tracks than on skid trail edges or in adjacent forest. Both richness and density increased with time since logging, but even 18 years after logging, abandoned skid trails were impoverished in small woody stems compared with adjacent forest. Minimizing soil and stand disturbance during logging appears to allow a more rapid recovery of vegetation on bulldozed soils, but the long-term fate of trees growing on compacted soils remains uncertain.  相似文献   

10.
We used data from a long-term (14–18 years) demographic study to infer the maximum longevity for populations of 93 relatively abundant tree species in central Amazonia. We also assessed the influence of several life-history features (wood density, growth form, mortality rate, recruitment rate, stem diameter, growth increment, population density) on tree longevity. Data on 3159 individual trees were collected in 24 permanent, 1 ha plots in undisturbed forest arrayed across a large (ca. 1000 km2) study area. For each species, three estimates of longevity were generated (by dividing the stem diameter of the largest tree by the median, upper quartile, and upper decile of observed diameter-growth rates), and the mean of these three values was used as a longevity estimate. Longevity values ranged from 48 years in the pioneer Pourouma bicolor (Cecropiaceae) to 981 years for the canopy tree Pouteria manaosensis (Sapotaceae), with an overall mean of 336±196 years. These growth-based estimates of maximum tree age were concordant with those derived from analyses of mean mortality rates. Tree longevity was positively correlated with wood density, maximum stem diameter, and population density, and negatively correlated with annual mortality, recruitment, and growth rates. On average, pioneer species had much lower longevity than did non-pioneers, whereas among old-growth trees, emergent species had greater longevity than did canopy species. Our results are consistent with radiocarbon-based studies that suggest that Amazonian trees can occasionally exceed 1000 years of age.  相似文献   

11.
We investigated how multiple-crop forestry has influenced the magnitude and variability of soil and plant phosphorus (P) fertility and site disturbance. Kinleith Forest, on Mamaku Plateau, covers >100,000 ha and comprises mainly plantation Pinus radiata. Three study areas in the forest were chosen to represent natural state (native forest), first crop of P. radiata (24 years growth), and second crop of P. radiata (4 years growth of second crop). The adjacent areas have similar relief and climate, and the soils are all the same age, being predominantly Andic Haplohumods developed in 1770 calendar-year-old non-welded tephra (Taupo Ignimbrite, ca. 0.5–0.8 m in thickness) and overlying a buried paleosol on earlier tephric material.

Soil properties were compared using a random geometric sampling scheme stratified in a 40-m grid. Soil samples (0–20 cm) were taken at 1.5, 4.5 and 13 m spatial intervals in random directions away from each primary node, providing 192 sample sites for each study area. Additionally at selected sites, samples of the current year's foliage from the upper crowns were collected, the thickness of Taupo Ignimbrite (i.e. depth to buried paleosol) was recorded by augering, and site disturbance was assessed using a new six-point scale based on change relative to a modal soil profile. Geostatistics and geographical information systems (GIS) were used to assess variability and effects of forest management on the measured properties. Soil Bray-2 P concentrations were below guidelines for satisfactory growth (12 mg kg−1) at all sites, and no differences were recorded between the different management areas. However, the amount of within-site variability in Bray-2 P increased with the number of crops. Foliar P concentrations were only marginally deficient in both the first and second crops, indicating that P is currently not significantly limiting growth. The lack of difference in foliar P between first and second crops indicates no crop-to-crop decline in foliar P status and suggests that no site P fertility decline has occurred. The soils have an unusual ability to continue releasing P through successive sequential extractions in the Bray-2 P test, indicating a strong buffering capacity, and this may explain the apparent lack of deficiency even with Bray-2 P values of <12 mg kg−1. The site disturbance index increased and the spatial distribution of P data became increasingly variable with crop rotation.

GIS, inverse-distance weighting and kriging proved useful in illustrating the trends between crops. The spatial variability of results indicated that there was no obvious pattern to the variability and that more site-specific forest management in the region would be difficult. However, there was some evidence that less disturbance during harvesting may minimise variability of soil P supply.  相似文献   


12.
Fire has been linked to the regeneration of giant sequoia (Sequoiadendron giganteum) [Lindley] Buchholz but no studies have directly investigated the effects of opening size and fuel treatment on giant sequoia establishment. Giant sequoia seedling density was analyzed in 36 group selection openings harvested in 1993 at Mountain Home Demonstration State Forest, CA. The experiment consisted of four replicates of a randomized 3 × 3 factorial design which investigated the effects of opening size and fuel treatment on giant sequoia regeneration. Small, medium, and large circular openings had average diameters of 15, 30, and 61 m, respectively. A total of 12 large, 12 medium, and 12 small openings were created. Three fuel treatments were randomly applied to the openings: tractor pile and burn, broadcast burn, and lop and scatter. Analysis of variance (ANOVA) detected no significant differences between treatments and seedling density (p < 0.05). All the treatments had low giant sequoia seedling density regardless of opening size or fuel treatment. Regeneration was completely absent in all openings with the lop and scatter fuel treatment while small openings had low giant sequoia regeneration density regardless of fuel treatment. Giant sequoia seedling density was low because of below average annual and summer precipitation following the creation of the openings and of low seed dispersal. The group selection silvicultural system attempts to simulate the structural complexity of the prehistoric, patchy, high intensity fire regime that once existed in the mixed-conifer forests of the southern Sierra Nevada but important ecosystem processes such as increased seed dispersal following patchy, high intensity fire and large scale nutrient cycling are not duplicated.  相似文献   

13.
Knowledge of SGS in plants is vital to understand the ecological and evolutionary dynamics of populations and to plan conservation strategies. Some of the major factors that can affect spatial genetic structure (SGS) in plants are the level of gene flow, spatial arrangement and life stages of individuals within populations. Applying six highly variable microsatellite markers, we investigated the effect of these factors on spatial genetic structure selecting two natural populations of sycamore maple, which is an insect-pollinated, autotetraploid and an indigenous hardwood species in Germany and in other central European countries. The two study populations had different shapes (“compact” and “elongated”) and tree densities. Significant SGS extended to ~180 m in the elongated population and to ~35 m in the compact population. Juvenile plants of the compact population showed significant SGS up to 40 m. Estimate of Sp statistic in high-density population was almost double of that in the population with low density. Gene dispersal distance in the low-density population was about 9 times higher than in the population with high density. The similar level of significant SGS in both adult and juvenile plants suggested minimal or no effect of life stages of individuals on SGS in the sycamore maple population. The data presented in this study can provide guidelines for seed collection and to establish populations for the conservation and management of genetic resources of the species.  相似文献   

14.
Three Alnus glutinosa populations from central Slovakia were investigated by means of isozyme analysis. The average expected heterozygosity ranged from 0.151 to 0.173. Despite significant differences in allele frequencies among populations, genetic differentiation was low (FST=0.022). Intrapopulation fixation indices indicate no or very slight deviation from panmictic expectations towards the heterozygote deficiency. No indications were found that the investigated populations have experienced recent bottlenecks in population size. A spatial autocorrelation analysis based on Moran’s I indicates the existence of a spatial genetic structuring in the population Král’ová. Patch size was estimated to be 70–100 m, which is more than observed in other broadleaved tree species. This difference is explained by the linear shape of alder populations and a higher mobility of seeds.  相似文献   

15.
The dwarf mistletoe, Arceuthobium oxycedri, is found on populations of Juniperus oxycedrus, in central Spain. This species can have negative effects on the physiology of its host, including mortality. Understanding the mechanisms that control its distribution and dispersal is critical to assessing its potential for spread. We assessed dwarf mistletoe distribution within a population of J. oxycedrus, including infected and uninfected host individuals. A new null model of parasitic dispersion was built using two dispersal kernel forms that were simulated with lower and upper envelopes for second‐order functions to summarize a point pattern, such as Ripley's K, nearest‐neighbour distribution and pair correlation functions. Nine dispersal scenarios were constructed with half‐bandwidth kernels (10, 20, 30 m) and initial population of infected trees (P0 = 05, 10 and 20). These scenarios were compared with the observed pattern and evaluated using the goodness‐of‐fit test. Significant differences at short distance (r < 10 m) were found between the observed pattern and simulated patterns, corresponding to the range of seed dispersal of the dwarf mistletoe. Interactions between infected and uninfected hosts patterns at all scales were identified, suggesting that A. oxycedri uses other mechanisms in addition to ballistic seed shooting as secondary dispersal agents to spread to distances greater than 20 m. Given that the seed characteristics facilitate dispersal by adhesion, we infer that spread between host individuals is amplified by seed transport by birds or small mammals.  相似文献   

16.
For sustainable forest management, it is important to know the response of timber species to the change in environment caused by logging. We performed a 2-year study on germination, survival and growth of four timber species, Cedrela odorata, Swietenia macrophylla, Hymenaea courbaril, and Cariniana micrantha, and one non-commercial species Tachigali vasquezii. We sowed seeds of these species in five microenvironments: log landing, gap-crown and gap-trunk, skidder trail and understory, in a tropical lowland moist rain forest in northern Bolivia. We related seed and seedling performance to light availability, soil compaction, and plant competition. Germination did not differ significantly between microenvironments but survival of germinated seeds for most species was significantly higher (P < 0.05) in the log landing (46–100%) than in the understory (0–7%). After 2 years, the tallest plants were always found in the log landing (119–190 cm) and the smallest in the understory (12–26 cm) caused by a higher relative height growth rate (RHGR) in the log landing (0.003–0.004 cm cm−1 per day) compared to the understory (0.000–0.001 cm cm−1 per day). During the first year RHGR was positively related to canopy openness for all species and negatively to the number of overtopping competitors for three species. During the second year also water infiltration explained observed variation to RHGR. These results show that abandoned log landings and logging gaps are suitable environments for the regeneration of timber species studied. This finding suggests that the removal of competitors in log landings and logging gaps combined with leaving seed trees near these microenvironments or sowing seeds, will improve regeneration of timber species in tropical forests.  相似文献   

17.
Deciduous fodder trees and shrubs are important feed resources for the critical summer period in semi-arid and sub-humid Mediterranean environments. Eleven such species were established in central Macedonia, Greece as 1-year old seedlings in 1987 and studied through 1994. Height and biomass were statistically different among species and tended to increase with the age of plants. Common Robinia pseudoacacia was the tallest and the most productive species, reaching more than 3 m height and 4 kg DM/plant 8 years since planting; it was followed by Ostrya carpinifolia, the variety monophylla of Robinia, Colutea arborescens and Amorpha fruticosa. Grazeable proportion of total biomass was about 50% and decreased as the age of plants increased. Repeated annual cutting resulted in significant reduction of both height and total biomass by 51% and 88%, respectively as compared with uncut plants at the end of the eighth year while frequent cutting tended to increase the grazeable proportion of biomass after the fourth year. To attain an optimum combination of quantity and quality of biomass in deciduous fodder tree and shrub plantations cutting or grazing should not start earlier than the third year since establishment.  相似文献   

18.
We studied the occurrence of 22 plant functional groups and the distribution patterns of 12 forest herbs in four clear-felled areas of different ages, and across ecotones between the forest and clearcuts in a 4383 ha beech forest in central Belgium. The main goal was to improve our knowledge on the clearcut harvesting system and its possible influence on herb vegetation. Within each study site, the herbaceous vegetation was sampled along a north–south and an east–west transect with points at 20 or at 10 m intervals. We studied a total of 82 vegetation relevés (4 m2) on 18 transects according to the Braun–Blanquet method. Around the study sites, the abundance of each species present was noted according to a grid-map with cells of 50 m × 50 m, up to a distance of 150 m from the clearcut edges. Differences in species’ cover percentage and frequency between the clearcuts and the surrounding matrix were tested. The colonisation rate for each species was calculated and possible effect of clearcut age on the plant composition was examined. Results of this study show that plant functional groups with a high conservation value, such as ancient-forest species, stress-tolerant species and autochorous species showed a progressive decrease in time after clearcutting. Most of the studied forest herbs had a lower frequency and/or cover in the clearcut areas than in the surrounding forest matrix. For some of these species, we found a progressive abundance decrease along a 150 m-gradient within the forest towards the clearcut (e.g. Dryopteris dilatata, Luzula pilosa, L. sylvatica, Oxalis acetosella). Others, such as Anemone nemorosa and Circaea lutetiana, are characterised by a regular cover in the forest which suddenly drops at the edge and sustains a very low level throughout the clearcut. If some forest herbs recover better than others, data collected so far indicate that most of the studied species are not able to recover such disturbed sites within a few years. We conclude that silvicultural systems should be adapted to the dispersal and recruitment limitations of forest herbs in order to enable their long-term conservation. To achieve this goal, management without large clear fellings and harvesting methods that never leave the ground completely bare are proposed as alternatives to the clearcutting system.  相似文献   

19.
A 2-year demographic field study was carried out on the understorey tree Garcinia lucida in the humid Atlantic forests of South Cameroon. The bark and the seeds of the species are exploited for medicinal purposes and palm wine processing in West and Central Africa. The bark is often removed over almost the entire circumference of the stem, leading to high mortality rates. The goal of the study was to assess the impact of bark harvesting on population structure and dynamics, in order to obtain an indication to what extent the actual harvesting practices can be sustained for longer periods, and to provide information on the future availability of the resource. Comparison of populations that had been subject to different harvest regimes allowed assessment of the effects of G. lucida bark harvesting on population stand structure. In lightly exploited populations, unharvested individuals represented more than the half of the trees (>10 cm DBH), while this was only one-third in heavily exploited stands. The percentage of dead exploited trees (>20 cm DBH) was more than 40 and 70%, for lightly and heavily exploited populations, respectively. This reflects the high intensity of bark harvesting, mainly directed at large trees. For the assessment of bark harvesting on population dynamics, we constructed matrix models for each of the three common bark harvest practices: (1) partial debarking of stem, (2) ring-barking trees and (3) felling trees at approximately 1 m height above ground level to extract bark. For all harvest practices, the asymptotic population growth rates (λ) were not significantly different from 1 indicating that populations were neither growing nor decreasing in size. Nevertheless, model projections showed that the stock of available trees decreased. Under an annual regime of partial debarking of all trees >10 cm DBH, the amount of harvestable trees gradually declined and reached 50% of the initial size after 20 years. For ring-barking and felling trees, the projections showed a sharp drop in the amount of harvestable trees directly after the start of the simulations, and the amount continued to decline below 4% of the initial size under an annual harvest regime. Even after a long time, populations did not recover to their pre-harvest numbers. The stable size of harvested populations predicted by λ values suggest that current harvest practices of bark do not jeopardize the future existence of G. lucida populations. However, as the frequently applied harvest practices of ring-barking and felling strongly reduced the stock in our simulations, commercial bark exploitation cannot be sustained over long periods. Nevertheless, as long as populations can recover from the loss of exploited individuals, by continuous recruitment, there is still prospect for sustained bark extraction.  相似文献   

20.
Widespread bark beetle outbreaks are currently affecting multiple conifer forest types throughout western North America, yet many ecosystem-level consequences of this disturbance are poorly understood. We quantified the effect of mountain pine beetle (Dendroctonus ponderosae) outbreak on nitrogen (N) cycling through litter, soil, and vegetation in lodgepole pine (Pinus contorta var. latifolia) forests of the Greater Yellowstone Ecosystem (WY, USA) across a 0-30 year chronosequence of time-since-beetle disturbance. Recent (1-4 years) bark beetle disturbance increased total litter depth and N concentration in needle litter relative to undisturbed stands, and soils in recently disturbed stands were cooler with greater rates of net N mineralization and nitrification than undisturbed sites. Thirty years after beetle outbreak, needle litter N concentration remained elevated; however total litter N concentration, total litter mass, and soil N pools and fluxes were not different from undisturbed stands. Canopy N pool size declined 58% in recent outbreaks, and remained 48% lower than undisturbed in 30-year old outbreaks. Foliar N concentrations in unattacked lodgepole pine trees and an understory sedge were positively correlated with net N mineralization in soils across the chronosequence. Bark beetle disturbance altered N cycling through the litter, soil, and vegetation of lodgepole pine forests, but changes in soil N cycling were less severe than those observed following stand replacing fire. Several lines of evidence suggest the potential for N leaching is low following bark beetle disturbance in lodgepole pine.  相似文献   

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