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In the Southwestern United States, increasing demand for firewood has quickly promoted pinyon-juniper woodlands to commercial status. Slow recovery rates for pinyon and juniper and inadequate mensuration data present significant obstacles to predictive management efforts. Many National Forest districts have witnessed continued fuel use for at least the past 100 years. To explore the need for long-term analysis at the district level, we have developed FORMAN I, a computer simulation written in FORTRAN IV that models prolonged fuel harvesting and its impact on pinyon-juniper woodlands. The technique is well-suited for historical analyses and we comply with an initial application that involves the suggested impact of prehistoric peoples on a marginal woodland in Chaco Canyon, New Mexico. To accentuate the significance of the simulation, we have deliberately overestimated woodland parameters while maintaining conservative annual rates for firewood procurement. A low-density woodland (less than 14.8 cords/ha) is completely depleted within 200 years when subjected to tenth-through-twelfth century estimates of human demography for the canyon. Interesting conclusions are drawn from the lack of pinyon-juniper recovery at Chaco over the past millennium. Traditional assumptions, such as the pristine state of Southwestern vegetation prior to historic settlement and subsequent invasion of marginal grasslands by pinyon and juniper, are questioned. 相似文献
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Although long-lived tree species experience considerable environmental variation over their life spans, their geographical distributions reflect sensitivity mainly to mean monthly climatic conditions. We introduce an approach that incorporates a physiologically based growth model to illustrate how a half-dozen tree species differ in their responses to monthly variation in four climatic-related variables: water availability, deviations from an optimum temperature, atmospheric humidity deficits, and the frequency of frost. Rather than use climatic data directly to correlate with a species’ distribution, we assess the relative constraints of each of the four variables as they affect predicted monthly photosynthesis for Douglas-fir, the most widely distributed species in the region. We apply an automated regression-tree analysis to create a suite of rules, which differentially rank the relative importance of the four climatic modifiers for each species, and provide a basis for predicting a species’ presence or absence on 3737 uniformly distributed U.S. Forest Services’ Forest Inventory and Analysis (FIA) field survey plots. Results of this generalized rule-based approach were encouraging, with weighted accuracy, which combines the correct prediction of both presence and absence on FIA survey plots, averaging 87%. A wider sampling of climatic conditions throughout the full range of a species’ distribution should improve the basis for creating rules and the possibility of predicting future shifts in the geographic distribution of species. 相似文献
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Woodlands of the Mediterranean speciesJuniperus oxycedrus ssp.macrocarpa (maritime juniper) are both vulnerable and ecologically important. Their ecology and biological status along the SW coast
of Spain are not well known; this, the first major study of these juniper populations is a basis for future research and restoration
policies. These communities are subject to harsh conditions, the plant composition being controlled by several factors at
different scales. On a large scale, climate and soil texture play an important role in controlling the soil water availability
to plants, and in separating xerophytic from mesic communities. On a small scale, coastal physiography, and substrate composition
are related to differences in the floristic composition. Coastal plantations modify environmental conditions, such as sand
mobility and salt spray deposition, inducing important changes in plant communities. The population of maritime juniper on
this coast was estimated in ca. 25 000 individuals, of which 93.6% are concentrated in three locations. Large proportions
of young individuals were found in extensive and protected populations. Howerver, adult individuals dominated the smaller
populations located under pine plantations. This limitation of recruitment may be imposed by several factors. A male biased
ratio was detected on the southern coast of Cádiz, which I hypothesize is due to the lower cost of pollen production in a
stressful habitat. Preservation of suitable habitats, the recovery of abandoned pine plantations, and the connection between
juniper populations, seem to be important requisites for the conservation of maritime juniper in the southwestern coast of
Spain. 相似文献
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Coppedge BR Engle DM Masters RE Gregory MS 《Environmental monitoring and assessment》2006,118(1-3):125-145
We utilized landscape and breeding bird assemblage data from three Breeding Bird Survey (BBS) routes sampled from 1965–1995
to develop and test a grassland integrity index (GII) in a mixed-grass prairie area of Oklahoma. The overall study region
is extensively fragmented from long-term agricultural activity, and native habitat remnants have been degraded by recent encroachment
of woody vegetation, namely eastern redcedar (Juniperus virginiana L.). The 50 individual bird survey points along the BBS routes, known as stops, were used as sample sites. Our process first
focused on developing a grassland disturbance index (GDI) as a measure of cumulative landscape disturbances for these sites.
The GDI was based on five key landscape variables identified in an earlier species-level study of long-term avian community
dynamics: total tree, shrub, and herbaceous vegetation cover indices, overall mean landscape patch size, and grassland patch
core size. The GII was then developed based on breeding bird assemblage data. Assemblages were based on commonly used response
guilds reflective of five avian life history parameters: foraging mode/location, nesting location, habitat specificity, migratory
pattern, and dietary guild. We tested the response of 78 candidate assemblage metrics to the GDI, and eliminated those with
no or poor response or with high correlations (redundant), resulting in 13 metrics for use in the final index. Individual
metric scores were scaled to fall between 0 and 10, and the cumulative index to range from 0 to 100. Although broader application
and refinement are possible, the avian-based GII has an advantage over labor-intensive, habitat-based monitoring in that the
GII is derived from readily available long-term BBS data. Therefore, the GII shows promise as an inexpensive tool that could
easily be applied over other areas to monitor changes in regional grassland conditions. 相似文献
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Mortality Gradients within and among Dominant Plant Populations as Barometers of Ecosystem Change During Extreme Drought 总被引:7,自引:0,他引:7
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/ Mechanized military maneuvers are an intensive form of disturbance to plant communities in large areas throughout the world. Tracking by heavy vehicles can cause direct mortality and indirectly affect plant communities through soil compaction and by altering competitive relationships. We assessed the long-term condition of structural attributes of open woodland, grassland, and shrubland communities at Fort Carson, Colorado, in relation to levels of disturbance and soil texture. Covariate analyses were used to help separate the directional forcings by the chronic disturbance from the regenerative capacities in order to assess the relative resistance and resilience of the communities and to determine whether the continual disturbance-recovery processes balanced under current levels of utilization. All three communities responded differently to disturbance. In open woodlands, altered understory/overstory relationships were suggested by increased grass, forb, shrub, and total vegetation cover and smaller decreases in shorter than taller woody species with increasing levels of disturbance. Grassland communities generally displayed greater responses to disturbance than other communities, but temporal dynamics were often similar, indicating relatively less resistance but greater resilience of this community. Weed and exotic species increased both temporally and in relation to levels of disturbance in all three community types. Temporal trends in community-level indices of dissimilarity and diversity also indicate that rates of disturbance were greater than rates of recovery. Few variables were related to within-community differences in soil texture. While total aerial cover was temporally stable, changes in species composition and in basal cover in grasslands and shrublands suggest increasing erosion potential. 相似文献
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