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1.
Deadwood is an important resource commonly used by inhabitants in arid lands. However, the low wood productivity and the presence of multi-stemmed trees restrict the use. Prosopis flexuosa woodlands are protected and inhabited by pastoralists who have land rights to use natural resources. As in other forests in the world, dead branches are the most commonly used. The factors causing the death of branches these trees are unknown. As P. flexuosa is a highly heliophilous species, branch mortality may depend on the growth habit and orientation of dry branches under the tree crown. With the participation of inhabitants, we assessed the present availability of deadwood in two Prosopis woodlands of different structure (semi-closed and open woodland), and evaluated the formation of deadwood in terms of shape and cardinal location of dry branches under the crown. We developed and compared regression models to estimate the amount of deadwood for erect, semi-erect and decumbent trees, and for the north and south areas under the crown (n = 120 trees). In addition, to determine the period of growth decline and the factors determining branch mortality, we compared annual radial increment between live and dead branches (n = 30 trees; 10 for each tree shape). The total amount of deadwood in adult Prosopis trees is higher in the semi-closed than in the open woodland (8.6 and 4.4 Tn ha−1, respectively). Only tree size determined the amount of deadwood present in each Prosopis tree, since we found no evidence related to the shape of the tree or the position of dry branches in the canopy. Branch decay was a large process of 18-20 years, and branch death appears to be the result of the action of climatic factors (dry period). The results suggest that the use of deadwood by the desert inhabitants is a tool that can potentially be used; however, the use of this resource taking into account the generation rates of deadwood has not been developed in arid lands. These practices at appropriate sites can contribute to a sustainable management of these woodlands, including the removal of deadwood in a model of local management on a site where potential productivity is relatively low.  相似文献   

2.
This paper discusses determinants of the historical and current spatial extent of the floodplain forest in Leipzig as well as its tree species composition using a GIS-data based delineation model and historical forest inventories for the floodplain forest in the district of Leipzig in Germany from the 19th to the 20th century. We found that the spatial extent of the floodplain forest remained considerably stable in spite of an overall decline in the entire floodplain area from the period where the city first experienced industrialisation in the 19th century to now. However, with river regulations and the alteration of forest management from coppice-with-standards forest to high forest in the 19th century, major changes can be found in the tree species composition of the floodplain forest. Comparing these findings with references from other European floodplain forests we discuss the impact of historical and current forest management as well as the city location's influence on the extent and tree species composition of urban floodplain forests. For urban forest management in particular there is a great need to integrate biophysical, historical and forestry knowledge when predicting future developmental trends.  相似文献   

3.
Hurricane disturbance has the potential to markedly affect coastal forest structure and ecosystem processes. This study focused on the impacts of Hurricane Katrina in Louisiana's Pearl River basin, which lies just west of Katrina's final landfall at the Louisiana–Mississippi border. Prior to landfall, composition and structure of bottomland hardwood forests in this region were studied with permanent forest inventory plots sampled in 1989, 1998, 2005 and following the storm in 2006. This enabled a direct comparison of forest structure and dynamics before and after the disturbance, including species-specific tree mortality and damage rates, biomass production, and differences among forest types having varied hydrologic regimes. Background tree mortality rate before Hurricane Katrina was 1.9%, while average annual mortality was 20.5% for the census interval including the disturbance. Change in live tree biomass estimated from allometric models demonstrated a shift from an average annual production of 3.5 Mg ha−1 before the disturbance, to an average loss of 77.6 Mg ha−1 from the storm. Damage associated with Hurricane Katrina varied significantly with tree species but not tree size. Flooded cypress-tupelo swamp forests sustained the least damage and frequently flooded bottomland hardwood forests sustained the highest damage. Hurricane disturbance influenced the structure and composition of these coastal forests through species-specific differences in damage and mortality rates, and varied impacts dependent on forest flooding regime.  相似文献   

4.
In this study, we quantified the effects of local neighbourhood competition, light availability, and proximity to skid trails on the growth of sugar maple (Acer saccharum Marsh.) trees following selection harvest. We hypothesized that growth would increase with decreasing competition and increasing light availability, but that proximity to skid trails would negatively affect growth. A total of 300 sugar maples were sampled ∼10 years after selection harvesting in 18 stands in Témiscamingue (Québec, Canada). Detailed tree and skid trail maps were obtained in one 0.4 ha plot per stand. Square-root transformed radial growth data were fitted to a linear mixed model that included tree diameter, crown position, a neighbourhood competition index, light availability (estimated using the SORTIE light model), and distance to the nearest skid trail as explanatory variables. We considered various distance-dependent or -independent indices based on neighbourhood radii ranging from 6 to 12 m. The competition index that provided the best fit to the data was a distance-dependent index computed in a 6 m search radius, but a distance-independent version of the competition index provided an almost equivalent fit to data. Models corresponding to all combinations of main effects were fit to data using maximum likelihood, and weighted averages of parameter estimates were obtained using multimodel inference. All predictors had an influence on growth, with the exception of light. Radial growth decreased with increasing tree diameter, level of competition and proximity to skid trails, and varied among crown positions with trees in suppressed and intermediate positions having lower growth rates than codominants and dominants. Our results indicate that in selection managed stands, the radial growth of sugar maple trees depends on competition from close (≤6 m) conspecific neighbours, and is still affected by proximity to skid trails ∼10 years after harvesting. Such results underscore the importance of minimizing the extent of skid trail networks by careful pre-harvest planning of trail layout. We also conclude that the impact of heterogeneity among individual-tree neighbourhoods, such as those resulting from alternative spatial patterns of harvest, can usefully be integrated into models of post-harvest tree growth.  相似文献   

5.
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6.
We examined the effects of fire recurrence on a mosaic structure of Quercus suber and Erica-Cistus shrubland communities of Southeastern France in order to improve the understanding of fire-vegetation interplay. Plant communities that were similar in 1959 (woodlands on shrublands called maquis on acidic soils) were compared along a gradient of fire recurrence, from 0 to 4 fires, with different time intervals between fires. The results showed that understory cover increased roughly with fire recurrence, whereas tree height, cover, density, stand basal area and litter depth decreased. Different fire recurrences along the past decades led to different vegetation types. High fire recurrence corresponded to maquis and sparse cork-oak woodlands while pure oak woodlands established in the absence of fire during the same period. In all, the diversity of tree diameter decreased with fire recurrence. High fire recurrence (3 or 4 fires in 50 years) led to a simple vertical structure of vegetation with a mono-layered shrub cover and few Quercus suber trees. In contrast, spatial connections between plants were maximal at longer time intervals, leading to a multi-layered vegetation. We finally discuss the potential implications of past fires on the behavior of future fires in the perspective of a sustainable management of these Mediterranean ecosystems.  相似文献   

7.
We examined spatial aspects of harvesting impacts on aspen regeneration at 25 sites in northern Minnesota. These sites had been clearcut or partially harvested 4–11 years ago. At each site, residual overstory, which was composed of trees other than aspen, soil disturbance, and tree regeneration were determined along transects leading away from skid trails into the neighboring stand. We characterized spatial extent of soil disturbance as soil strength using an Eijkelkamp soil cone penetrometer. Soil disturbance dropped off very quickly at the edge of skid trails, suggesting that the impact of harvesting traffic on areas adjacent to skid trails is minor. On skid trails, disturbance levels were higher on sites harvested in summer than on sites harvested in winter. Even after adjustment for differences in soil disturbance, stands harvested in winter had higher regeneration densities and greater aspen height growth than stands harvested in summer, suggesting that aspen regeneration was more sensitive to a given level of soil disturbance on summer-harvested sites versus on winter-harvested sites. Soil disturbance and residual overstory interactively reduced aspen regeneration densities and height growth, indicating that avoidance of soil disturbance is even more critical in partially harvested stands. Predictions based in the spatial patterns of impact found in this study indicated that harvesting conditions may have a great impact in future productivity of a site.  相似文献   

8.
Red-shouldered hawks (Buteo lineatus) are threatened in Wisconsin and when nest sites are found during the cruising or marking stage of timber harvesting, the harvest is altered to accommodate the hawks. If nest site locations are known before initiation of timber harvest, foresters can employ a proactive approach to manage red-shouldered hawks while maintaining timber production. We searched for red-shouldered hawks nest sites on Marinette County Forest (MCF) which encompasses 94,000 ha in northeastern Wisconsin and is the second largest county forest in the state. We used a comparative modeling approach to evaluate distribution and habitat relationships of red-shouldered hawk nest sites in relation to a suite of environmental variables in MCF. Models were used to develop forest management recommendations for red-shouldered hawks in Wisconsin. During the spring of 2006 and 2007, we broadcasted conspecific calls to survey 1121 calling stations along forest roads and trails. We located 20 and 25 active nesting territories in 2006 and 2007, respectively (11 of which were active in both years). To understand nest site selection, we measured 22 habitat variables within 0.04-ha plots at active nest sites (n = 34) and at stratified random sites (n = 61). Logistic regression with information-theoretic model selection identified a model including greater tree species richness and closeness to forested wetland as the best-approximating model. Variable selection with Discriminant Function Analysis (DFA) indicated that nest selection was best explained by greater number of tree species, closer distance to forested wetlands, greater volume of downed woody debris, fewer small sawlogs, and increased proximity to streams. Univariate comparisons identified four of the five aforementioned variables in the DFA model as significant. Red-shouldered hawks are likely more common in Wisconsin than their state status suggests. Forest management for red-shouldered hawk nest sites should focus on increasing tree species richness, increasing down woody debris volume, and protecting forested wetlands. These recommendations may assist property managers to locate and plan for continued persistence of this species on MCF.  相似文献   

9.
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10.
Species richness and evenness have greatly declined in oak–hickory forests in the central hardwood region in the U.S.A. in the past 100 years due to the rapid population growth of Acer saccharum. This study used a 50-year record of spatial dynamics to examine how demographic processes, particularly recruitment, may have contributed to this increase in an old-growth forest remnant, Brownfield Woods, Urbana, Illinois, U.S.A. The impact of canopy disturbance, including the outbreak of Dutch elm disease, on this increase was also evaluated. Historical maps of trees (≥7.6 cm DBH) from 1951, 1988, and 2001 in a 180 m × 280 m area were used to develop a series of univariate Ripley's L(d) functions to study changes in spatial patterns of three size classes of A. saccharum over time. Bivariate Ripley's L(d) functions were also utilized to evaluate spatial associations between recruitment and canopy disturbance. Our results indicated that A. saccharum was aggregated at most spatial scales up to 80 m during 1951–2001. Such aggregation arose mainly from small individuals. Furthermore, newly recruited individuals were aggregated at multiple spatial scales, and were significantly associated with canopy disturbance in general, as well as gaps created by Ulmus trees killed by Dutch elm disease. The aggregation of the 1951 initial group of small individuals changed via mortality to a random distribution over time. The results indicate that tree deaths caused by disturbances of different scales and types were the main cause of increased recruitment of A. saccharum in Brownfield Woods. The occurrence of Dutch elm disease further accelerated its population increase. This study demonstrated a direct spatial link between recruitment of A. saccharum and disturbance, and provided a long-term case study of a population explosion.  相似文献   

11.
Quercus eduardii and Q. potosina are the most abundant tree species of the temperate forests of Sierra Fría, Aguascalientes, Mexico, in an area ranging between 2200 and 2600 m above see level. During the last century these clonal oaks were intensively exploited to obtain charcoal and firewood for local use; in addition, sections of the forest were transformed into grasslands for livestock. The effects of disturbance on the population dynamics of these species are poorly known. Therefore, a demographic study was carried out in order to: (a) evaluate the effects of disturbance on the population growth rate of these species; (b) assess the effects of inter-annual environmental variability on the long term population dynamics; (c) evaluate the relative importance of sexual reproduction and clonal propagation on population growth rate; (d) simulate the impact of different levels of tree harvesting on the population dynamics of these species, and (e) recommend a harvesting intensity, based on the information obtained above. Annual, mean and periodic matrices as well as stochastic simulations were used. These size-classified population matrix models also were employed to devise a schedule that maximizes the percentage of individual plants that can be harvested without affecting their population growth rates. For this reason specific entries of the matrices were modified to simulate different harvesting intensities. The study was carried out in two disturbed and two undisturbed sites during a 4-years period. Two plots were established inside each site; one plot was excluded from livestock and the other was left intact. Annual population growth rates were above or equal to unity in all sites, species and years, whereas mean, periodic and stochastic simulation matrix models showed no significant differences between species, years, sites or from unity, suggesting that logging and grazing did not have a negative effect on the population growth rate of these species. Both species produced clonal offspring during the 4 years of the study, but reproduced sexually only once, suggesting a masting reproduction. Elasticity analyses showed that the contribution of clonal propagation is more important than fecundity to the population growth rate of both species. Mean annual and periodic matrix models showed that extractions as low as 5% cause a population decline, while with stochastic simulations extractions of up to 5% are possible for both species; however, the environmental stochasticity will drive populations to local extinctions.  相似文献   

12.
Pathogens and bark beetles are integral disturbance agents in many forest and woodland ecosystems. Over the past decade, pinyon Ips (Ips confusus) has interacted with regional drought to cause widespread mortality in singleleaf pinyon (Pinus monophylla) and Colorado pinyon (Pinus edulis) woodlands of western North America. We used mixed-effects and logistic regression modeling approaches to explore biotic and abiotic influences on the spatial heterogeneity of tree mortality. Stand structure, cumulative canopy and stem mortality, and occurrence of specific mortality agents were measured for 217, 0.1-ha plots, established in 2005 across 11 mountain ranges in central Nevada.  相似文献   

13.
Selective logging is the most widely employed method of commercial timber production in Asia, and its impact on forest structure, composition, and regeneration dynamics is considerable. However, the successional processes in forest communities after logging in semiarid mountains are poorly understood. To provide more information on these processes, we used data from tree rings, direct and indirect age determinations, and field measurements of stand structure to reconstruct the historical disturbance regime, stand development patterns, and successional processes in a natural Picea crassifolia forest community in the Qilian Mountains of northwestern China. The results showed that the density of P. crassifolia forest increased significantly after logging. The densities of second growth forests 30 and 70 years after logging disturbance had increased to 2874% and 294% of primary forest's density, respectively. Logging disturbance did not alter tree species composition of logged stands. However, the diversity of understory species changed significantly among the successional phases. Logging disturbance decreased the spatial heterogeneity of second growth forest. The spatial distributions of recruitment were affected by the location of the remaining trees. There was less recruitment near the remaining trees than near forest that had been cut. In addition, logging disturbance also induced a growth release for the trees on the sites sampled. Our results imply that the succession and regeneration of P. crassifolia forest may be improved if the remaining trees could be retained relative uniform distribution pattern, thinning or selective logging could be performed to height density, exotic shrubs could be removed or the shrubs cover could be reduced during the earlier successional stages.  相似文献   

14.
Concern over uncontrollable wildfire in pinyon–juniper woodlands has led public land managers in the southwestern United States to seek approaches for mitigating wildfire hazard, yet little information is available concerning effectiveness and ecological responses of alternative treatments. We established a randomized block experiment at a pinyon–juniper site in northern Arizona and tested effects of no treatment (Control), thinning only (Thin), prescribed fire only (Burn), and thinning followed by prescribed fire (Thin + Burn) on overstory structure, hazardous fuels reduction, and woody understory responses. One year after implementation, mean trees per hectare (TPH) of Utah juniper (Juniperus osteosperma) and pinyon pine (Pinus edulis), and basal area (BA) of pinyon, were significantly (P < 0.05) less in Thin and Thin + Burn treatments than Control. Additionally, pinyon TPH was less in Burn than Control. Quadratic mean diameter was significantly greater in Thin and Thin + Burn than in Control and Burn treatments. Thinning shifted diameter distributions from uneven- to even-sized. Crown fuel load (CFL) of both pinyon and juniper was significantly lower in Thin and Thin + Burn compared with Control and Burn treatments. Thin, Burn, and Thin + Burn treatments resulted in significantly greater 1-h surface fuel loads compared with the Control. The Thin treatment resulted in significantly greater mean load of the 1000-h fuel class compared with Burn and Control treatments, but did not differ from Thin + Burn. Forest floor Oi (litter) layer was not significantly affected by the treatments but Oe + Oa (duff) depth was significantly less in the Burn treatment compared with Thin and Control. Live shrubs and tree regeneration showed no differences among treatments. We concluded that thinning and thinning followed by prescribed fire were effective approaches for fuels reduction; however, resulting stand structures may be novel and outside the historical range of variability. Prescribed fire alone had minimal effects on structure and fuels reduction. Woody shrubs and tree regeneration in the understory suggested that these treatments may not have long-term deleterious ecological effects.  相似文献   

15.
Soil surface CO2 flux (Sflux) is the second largest terrestrial ecosystem carbon flux, and may be affected by forest harvest. The effects of clearcutting on Sflux have been studied, but little is known about the effect of alternative harvesting methods such as selective tree harvest on Sflux. We measured Sflux before and after (i) the creation of forest canopy gaps (simulating group tree selection harvests) and (ii) mechanized winter harvest but no tree removal (simulating ground disturbance associated with logging). The experiment was carried out in a sugar maple dominated forest in the Flambeau River State Forest, Wisconsin. Pre-treatment measurements of soil moisture, temperature and Sflux were measured throughout the growing season of 2006. In January–February 2007, a harvester created the canopy gaps (200–380 m2). The mechanization treatment consisted of the harvester traveling through the plots for a similar amount of time as the gap plots, but no trees were cut. Soil moisture and temperature and Sflux were measured throughout the growing season for 1 year prior to harvest and for 2 years after harvest. Soil moisture and temperature were significantly greater in the gap than mechanized and control treatments. Instantaneous Sflux was positively correlated to soil moisture and soil temperature at 2 and 10 cm, but temperature at 10 cm was the single best predictor. Annual Sflux was not significantly different among treatments prior to winter 2007 harvest, and was not significantly different among treatments after harvest. Annual (+1 std. err.) Sflux averaged 967 + 72, 1011 + 72, and 1012 + 72 g C m−2 year−1 in the control, mechanized and gap treatments, respectively, for the 2-year post-treatment period. The results from this study suggest selective group tree harvest significantly increases soil moisture and temperature but does not significantly influence Sflux.  相似文献   

16.
In the Pacific Northwest, USA, red-tree voles (Arborimus longicaudus) are of conservation and management interest owing to their apparent association with late-seral forests and the relatively small extent of such forests, largely a function of timber harvest, fire, and conversion of forests to non-forest uses during the past century. We created and evaluated a series of red-tree vole habitat association models, and applied the best model to evaluate tree vole habitat quality within and outside of reserves throughout most of their range in Oregon and northern California. We modeled presence and absence of tree vole nests across a gradient of biotic, abiotic, and spatial features; and within and outside of reserves. The best model included spatial coordinates, percent slope, basal area of trees with diameter at breast height (dbh) between 45 and 90 cm, maximum tree dbh, and standard deviation of conifer dbh. Plots with tree vole nests contained many late-seral/old-growth forest attributes such as large diameter, older, and variably sized trees. Evaluation of the best model, including rigorous cross-validation, showed the model to be statistically robust and to have very good/excellent predictive ability. Reserves had significantly higher mean habitat quality than non-reserved lands, and reserves had much more high quality habitat than non-reserves.  相似文献   

17.
In South Africa, non-timber forest products,especially mopane worms(Imbrasia belina) have become an important source of food and cash income for rural people living in or near the mopane woodlands. However,the continued harvesting has implications for long term management of the mopane woodlands in the northern parts of South Africa. In order to keep a balance between mopane worm harvesting and biodiversity, an understanding of socio-economic factors influencing the harvest at household level is vital to reducing depletion and overexploitation of the mopane woodland resources. This study evaluated the socio-economic factors influencing mopane worm harvesting decisions in rural households in Limpopo Province, South Africa using a binary logistic regression model based on a household survey of 120 respondents.Age, gender, education, income from mopane worm sales,social capital(group affiliation), household size, lack of institution/law to regulate use, distance to the nearest harvesting site, and regional locations were statisticallysignificant factors influencing household mopane worm harvest decisions. Implications for policy are discussed.  相似文献   

18.
Competition is a major determinant of plant growth and is often used in studies of tree growth and species coexistence. However, these approaches are usually temporally static, i.e., assessed at a single point or period in time. While constantly changing forest conditions due to natural and human-induced disturbances potentially alter competition among individuals, static approaches cannot qualify the temporal variability of competitive interactions. Here we present a longitudinal analysis of competitive interactions among trees and discuss the implication of our results for ecological interpretation.Spatially-explicit tree growth data were obtained from 18 study plots (0.4 ha each) in sugar maple (Acer saccharum Marsh.) stands in Quebec, Canada. During the studied period (1980-2003), these stands had been disturbed by insect outbreaks (forest tent caterpillar, Malacosoma disstria Hubner) and by commercial partial harvest. We analyzed radial growth rates (outcome of competition) on an annual basis and as a function of tree biology (bole diameter, crown position), competition (above- and belowground competition from neighbours) and environmental conditions (light availability, harvest disturbance).Competitive interactions changed throughout the studied period. Canopy disturbance from partial harvest interacted with defoliators and influenced competition symmetry by favoring smaller trees.Competitive interactions seemed to have switched from below- to above-ground following canopy recovery after harvest. Release from competition due to partial harvest increase neighbourhood size (radius of effective competition) and enhanced the competitive pressure from larger individuals.The temporal variability in parameter estimates may be used for setting confidence intervals on competitive success (growth rates), thereby yielding a more robust basis for ecological interpretation. Our results also show that temporal variability in competitive interactions could contribute to the maintenance of high tree species diversity and structural complexity in some ecosystems by temporally altering species-specific responses to environmental change and disturbance.  相似文献   

19.
《林业研究》2021,32(5)
Miombo woodlands near human settlements are under significant pressures from human activities,with negative consequences on their structure and composition.As studies are limited,we assessed the structure and species composition of a portion of miombo woodland along an increasing disturbance gradient from a national park,through a buffer zone to communal lands in northeast Zimbabwe.Five concentric plots of 2 and 11 m radii were established in each area to record woody species composition,diameter,height,basal area,density and volume as well as evidence of disturbance.Effects of site,growth stage and their interaction on vegetation diversity and structural parameters were tested using a general linear model(GLM).Principal component analysis(PCA) tested the association between species and site and ANOVA the differences in the level of disturbance across strata.Species diversity did not differ between sites except for evenness,which increased with disturbance.Evenness and richness were greater in seedlings and saplings than mature trees across sites,respectively.Sapling and mature tree diameters differed significantly between sites.Volume and density of mature trees declined with increasing disturbance while seedling densities peaked at intermediate disturbance levels(buffer zone).Tree harvesting,was more evident in the buffer zone and in the communal area relative to the national park.In contrast,fire frequency was greater in the national park and in the buffer zone relative to the communal area.The results of this study identify a true miombo woodland dominated by Brachystegia boehmii with a stable population,as illustrated by an inverse-J shape in diameter class distribution on all sites,and that these woodlands are generally resilient to disturbances,maintaining similar species composition and structure at various levels of disturbance.However,continued monitoring of disturbance levels and miombo woodland response is recommended to ensure sustainable utilisation of these resources.  相似文献   

20.
Tyynelä  Tapani M. 《New Forests》2001,22(3):239-257
The tree species diversity, groundvegetation species richness, and soilcharacteristics were studied forEucalyptus camaldulensis woodlots and thenatural miombo woodlands in the Mukarakate Areaof Northeastern Zimbabwe. The woodlandcharacteristics were measured and soil sampleswere taken at a total of thirty-four miombowoodland plots and twenty-seven eucalyptusplantations.The number of tree species was significantlyhigher for the miombo woodland sites than forthe eucalyptus woodlots. The Berger-Parker andShannon-Wiener Index values for speciesdiversity were higher for the tree speciesfound on the miombo woodland sites. There wereno significant differences between theeucalyptus woodlots and the miombo woodlands inthe percentage of grass cover and the number ofgrass and herb species. The same grass and herbspecies were common in both the woodlots andthe woodlands. The older woodlots hadsignificantly more tree and herb species thanthe younger woodlots.Soil nutrient analyses showed no significantdifferences between the woodlots and thewoodlands regarding their available phosphorusand total nitrogen, but the total phosphorusand total carbon were higher for the miombowoodlands. The results suggest that E.camaldulensis woodlots established in areasthat were formerly fields and grazing areas inthe savannah-like conditions pertaining inSouthern Africa do not cause the loss ofspecies richness or soil nutrients. In heavilydeforested areas eucalyptus woodlots may evencontribute positively to the protection andpreservation of indigenous trees species andother flora.  相似文献   

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