ISSN:1051-0761    

DOI:10.2307/1942044

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Spatially explicit population models are becoming increasingly useful tools for population ecologists, conservation biologists, and land managers. Models are spatially explicit when they combine a population simulator with a landscape map that describes the spatial distribution of landscape features. With this map, the locations of habitat patches, individuals, and other items of interest are explicitly incorporated into the model, and the effect of changing landscape features on population dynamics can be studied. In this paper we describe the structure of some spatially explicit models under development and provide examples of current and future research using these models. Spatially explicit models are important tools for investigating scale‐related questions in population ecology, especially the response of organisms to habitat change occurring at a variety of spatial and temporal scales. Simulation models that incorporate real‐world landscapes, as portrayed by landscape maps created with geographic information systems, are also proving to be crucial in the development of management strategies in response to regional land‐use and other global change processes. Spatially explicit population models will increase our ability to accurately model complex landscapes, and therefore should improve both basic ecological knowledge of landscape phenomena and applications of landscape ecology to conservation and management.

ISSN:1051-0761    

DOI:10.2307/1942045

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Land managers need new tools, such as spatial models, to aid them in their decision‐making processes because managing for biodiversity, water quality, or natural disturbance is challenging, and landscapes are complex and dynamic. Spatially explicit population models are helpful to managers because these models consider both species‐habitat relationships and the arrangement of habitats in space and time. The visualizations that typically accompany spatially explicit models also permit managers to "see" the effects of alternative management strategies on populations of interest. However, the expense entailed in developing the data bases required for spatially explicit models may limit widespread implementation. In addition, many of the models are developed for one or a few species, and dealing with multiple species in a landscape remains a significant challenge. To be most useful to land managers, spatially explicit population models should be user friendly, easily portable, operate on spatial and temporal scales appropriate to management decisions, and use input and output variables that can be measured affordably.

ISSN:1051-0761    

DOI:10.2307/1942046

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

We address model specification, parameter estimation, and model reliability for spatially explicit population models (SEPMs). We assume that these models have the complementary goals of understanding the processes that influence the number and distribution of animals in space and time, and forecasting the effect of management or other human activities on population abundance and distribution. Incorrect model structure, parameter estimates, or both will result in unreliable model output. Spatially explicit models require knowledge of population spatial structure, dispersal, and movement rates, in addition to the usual demographic parameters and structural assumptions such as density‐dependence, and are thus potentially very vulnerable to propagation of model uncertainty. Sensitivity analysis and validation can both be used to evaluate the reliability of SEPMs, but the level of spatiotemporal resolution at which the model should be evaluated is often not clear. Many SEPMs are very complex, and validation may only be possible or meaningful on a sub‐model basis. Forecasting, that is, prediction under a different set of conditions than that under which the model was built, will provide a stronger test of model reliability. Forecasts from SEPMs can be used to generate hypotheses that can then be tested as parts of large‐scale adaptive management experiments. In this way resource management goals can be achieved, while providing enhanced understanding of systems and improved predictability of future scenarios.

ISSN:1051-0761    

DOI:10.2307/1942047

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Spatially explicit models for animal populations (SEPMs) necessarily embody assumptions about plant community structure and dynamics. This paper explores the advantages and limitations of directly linking animal SEPMs with models for vegetation dynamics. Such linkages may often be unnecessary. For instance, in research focussed on questions with short time horizons, the spatial patterning of vegetation can be reasonably approximated as a fixed landscape templet for animal population dynamics. But if one needs to consider longer time scales (e.g., decades to centuries), landscapes will be dynamic. Models of vegetation dynamics provide useful tools for predicting landscape dynamics. We outline the sorts of output from vegetation models that might be useful in animal SEPMs. We discuss as a concrete example recent forest simulators, which predict with reasonable accuracy some variables (e.g., tree species composition), but which, to date, are quite poor for others (e.g., seed production). Moreover, because vegetation models target a restricted range of temporal and spatial scales, they may be more useful for certain consumer groups than for others. Despite these cautionary observations, we believe that the time is ripe for fruitful linkages between models of vegetation dynamics and animal SEPMs.

ISSN:1051-0761    

DOI:10.2307/1942048

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

In a range of plant species from three Californian vegetation types, we examined the widely used "normalized difference vegetation index" (NDVI) and "simple ratio" (SR) as indicators of canopy structure, light absorption, and photosynthetic activity. These indices, which are derived from canopy reflectance in the red and near‐infrared wavebands, highlighted phenological differences between evergreen and deciduous canopies. They were poor indicators of total canopy biomass due to the varying abundance of non‐green standing biomass in these vegetation types. However, in sparse canopies (leaf area index [LAI]°0‐2), NDVI was a sensitive indicator of canopy structure and chemical content (green biomass, green leaf area index, chlorophyll content, and foliar nitrogen content). At higher canopy green LAI values (>2; typical of dense shrubs and trees), NDVI was relatively insensitive to changes in canopy structure. Compared to SR, NDVI was better correlated with indicators of canopy structure and chemical content, but was equivalent to the logarithm of SR. In agreement with theoretical expectations, both NDVI and SR exhibited near‐linear correlations with fractional PAR intercepted by green leaves over a wide range of canopy densities. Maximum daily photosynthetic rates were positively correlated with NDVI and SR in annual grassland and semideciduous shrubs where canopy development and photosynthetic activity were in synchrony. The indices were also correlated with peak springtime canopy photosynthetic rates in evergreens. However, over most of the year, these indices were poor predictors of photosynthetic performance in evergreen species due to seasonal reductions in photosynthetic radiation‐use efficiency that occurred without substantial declines in canopy greenness. Our results support the use of these vegetation indices as remote indicators of PAR absorption, and thus potential photosynthetic activity, even in heterogeneous landscapes. To provide accurate estimates of vegetation‐atmosphere gas fluxes, remote NDVI and SR measurements need to be coupled with careful estimates of canopy photosynthetic radiation‐use efficiency.

ISSN:1051-0761    

DOI:10.2307/1942049

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

To investigate the risk of frost damage to Scots pine (Pinus sylvestris L.) in northern regions under climatic warming, a submodel for such damage to trees was included in a forest ecosystem model of the gap type. An annual growth multiplier describing the effects of frost was calculated with the help of simulated daily frost hardiness and daily minimum temperature. The annual growth multiplier was used in the main ecosystem model when simulating the development of a tree stand using a time step of one year. Simulations of the growth and development of Scots pine stands in southern Finland (60°N) under an elevating temperature indicated that climatic warming could increase the risk of frost damage due to premature onset of growth during warm spells in the late winter and early spring. Risk of frost damage implies uncertainty in yield expectations from boreal forest ecosystems in the event of climatic warming.

ISSN:1051-0761    

DOI:10.2307/1942050

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

A model representing the nitrogen dynamics of a northern hardwood forest is amended to include a mass balance and to predict percolating nitrate‐nitrogen concentrations. A component for including land application of municipal sewage sludge is added and Monte Carlo simulation used to predict the long‐term impact of land application on stand development, soil humus dynamics, nitrogen saturation, and nitrate losses at Hubbard Brook Experimental Forest in New Hampshire. Simulations suggest that applying 4 Mg/ha (dry mass) of anaerobically digested sludge to a 30‐yr‐old forest at 3‐yr intervals for 50 yr may increase production of harvestable boles and branches by an average of 24.6% over controls. An application rate of 8 Mg/ha results in little additional biomass increase but significantly threatens groundwater and surface water quality. Repeated applications significantly increase the nitrogen in soil humus. Species representation is not affected significantly unless sludge is applied after harvest during forest reorganization. Sustainable loading rates are on the order of 3‐4 Mg/ha if sludge is applied at 3‐yr intervals.

ISSN:1051-0761    

DOI:10.2307/1942051

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Vine maple (Acer circinatum Pursh.) clone development, expansion, and regeneration by seedling establishment were studied in 5‐240 yr old managed and unmanaged Douglas‐fir (Pseudotsuga menziesii (Mirb.) Franco) stands in coastal Oregon. Stem length, number of stems, and crown area were all significantly (P ≤ 0.01) related to stand age, and clone development was most rapid during the first 50 yr of stand development. Following clear‐cutting, clones were clumps of 1‐3 m long upright stems. Fifty years after disturbance, clones consisted of 5‐15 drooping aerial stems>10 m long and basal sprouts 1‐2 m long; some stems had been pinned to the forest floor by fallen trees or branches and had layered. In stands>120 yr in age, clones were often quite complex, composed of several decumbent stems each of which connected the ramets of 1‐10 new aerial stems. Vine maple clone expansion occurs by the layering of long aerial stems. Over 95% of the layered stems we observed had been pinned to the forest floor by fallen debris. Unsevered stems that we artificially pinned to the forest floor initiated roots within 1 yr. Thinning may favor clonal expansion because fallen slash from thinning often causes entire clones to layer, not just individual stems. Clonal vine maple seed production and seedling establishment occurred in all stages of stand development except dense, young stands following crown closure. There were more seedlings in thinned stands than in unthinned stands and in unburned clearcuts than in burned clearcuts.

ISSN:1051-0761    

DOI:10.2307/1942052

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Edge is an important landscape feature of fragmented forest landscapes in the Pacific Northwest, USA. Our primary objective of this study is to characterize the changes in microclimatic variables from recent clearcut edges into the old‐growth Douglas‐fir forests as influenced by edge exposures and local weather conditions. Microclimatic gradients are described along transects extending from recently clearcut edges 240 m into stands of old‐growth Douglas‐fir (Pseudotsuga menziesii (Mirb.) Franco) forest west of the Cascade Range in the U.S. Pacific Northwest. Data for air temperature, soil temperature, relative humidity, short‐wave radiation, and wind speed were collected over the course of the day from 16 different edges representing a range of edge orientations and local weather conditions over two growing seasons (1989‐1990). Data for soil moisture were collected over three consecutive days in September 1990. Two indices, significance of edge influence (SEI) and depth of edge influence (DEI), were used to evaluate the effects of edges on microclimatic variables. Edge effects typically extended 30 to>240 m into the forest. From the edge into the forest, air temperatures decreased during the day and increased at night; the reversal produced mid‐morning and late‐afternoon periods when a gradient was absent. Changes in soil temperature from the edge into the forest were comparable to those for air temperature, except that edge effects did not extend as deeply into the forest. The gradient for relative humidity increased from the edge and was steepest in mid‐afternoon. Humidity effects sometimes extended>240 m into the forest. Short‐wave radiation decreased rapidly with distance from the edge, reaching interior forest levels by 30‐60 m. Wind speed decreased exponentially from the edge into the forest, depending on the relationship of edge orientation to wind direction; stronger winds influenced conditions deeper inside the forest, sometimes>240 m from the edge. Edge orientation played a critical role for all variables; for air and soil temperature and humidity, it affected the times of day at which maximum and minimum values peaked. Influence of local weather conditions on gradients was highly variable. Overall, however, gradients generally were longest and steepest on partially clear, warm, dry days, at southwest‐facing edges, and for air temperature, soil temperature, and relative humidity. SEI and DEI were found to be necessary measurements for evaluating edge effects on microclimatic variables, which responded differently depending on time of day, edge orientation, and local weather. No single value could be calculated for DEI. Because many ecological features near edges, such as tree stocking and regeneration, dispersal of flying insects, and decomposition of woody debris, seem related to microclimatic gradients, forest management to protect interior conditions should shift from the traditional charge ("create as much edge as possible") to a new charge in which the amount of edge is reduced at both the stand and landscape levels.

ISSN:1051-0761    

DOI:10.2307/1942053

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY