Gradient analysis of vegetation

PATTERNS AND THEIR MEANING The pattern of a landscape is, in its full detail, exceedingly complex. It is generally impossible to interpret adequately the relations of species and stands to one another and the landscape by observation alone. It is consequently necessary to develop abstract representations of the pattern, representations which show some relations of communities and environments which are most significant in the landscape pattern, but show these in a form more easily comprehended and apart from the complexity of the whole. The most familiar such abstract representation is the ecological series. In the complexity of the landscape paetern, certain main-directions of vegetational and environmental change may be recognized (cf. Meusel, 1940). Recognition of major correlations of properties of vegetation with differences in environment is originally direct and intuitive, but is later influenced also by means of measurement and interpretations of the significance of factors which, like those of the soil, are not so easily observed. When a single gradient is chosen for study, stand 110 THE BOTANICAL REVIEW samples may be arranged in sequence along this gradient to form an ecological series and interpreted as a gradient of environments and communities, an ecocline. Stands may be chosen and arranged in relation to a single factor-gradient, but the ecological series shows their relation not to a factor-gradient alone, but to a complex-gradient of many correlated factor-gradients, or of characteristics of environmental complexes (Whittaker, 1956). Within the ecocline one may choose to distinguish the complex-gradient of environments and the corresponding coenocline or gradient of communities (Whittaker, 1960). Although the ecological series is an approach toward isolation of a factor and its effects, it represents the variation in certain observed properties of ecosystems as most or all of these change along the gradient chosen for study. By the ecological series, characteristics of communities may be correlated with factors of environment, but the relation need not be assumed to be one of effect and cause. Environments and communities are coupled and interacting aspects of the ecosystem; environment acts not simply on the community, but in and through the function of the ecosystem to produce observed differences in community characteristics (Whittaker, 1954b). The relation between environments and communities in an ecological series may, however, have these characteristics: (1) The environmental gradient exists and can be measured apart from the presence of the communities along it. The gradient may thus be in a sense external to or separate from the community, although the gradient as it affects organisms may be modified by the community and the function of the ecosystem. (2) The relation between the gradient and communities is consistent; similar communities are observed to occur in habitats having similar levels or intensities of the gradient. (3) The normal complexities of ecological relations, effects of other environmental factors, chance differences in communities at similar levels of the gradient, and effects of communities in modifying the gradient not correlated with the gradient, may reasonably be neglected or controlled by choice of area or stands to be studied. (4) There is reason in present ecological understanding to think that the environmental gradient has significance in relation to the functions of ecosystems, such that differences in the functions of ecosystems that develop at different levels of the gradient are expressed in observable differences in communities. When these conditions occur, the relation between the environmental gradient and the gradient of community characteristics partially approaches the ideal of the "cause CLASSIFICATION OF NATURAL COMMUNITIES 111 and effect" relation (cf. Bunge, 1961). The synecologist in this area of study is concerned in general not with cause and effect but with correlations--variables which change together through an ecological series and which are often interrelated in the functions of the ecosystems along the gradient. To some degree some of these correlations approach the special circumstances to which designation of one gradient as cause or independent variable and others as effects or dependent variables may be appropriate (cf. Major, 1951; Whittaker, 1954b). When several major gradients influencing community characteristics are recognized in a landscape, stands may be arranged into ecological series in relation to each of these. An abstract representation of the landscape pattern as a multi-dimensional coordinate system of intersecting ecological series results (Ramensky, 1930; Sukatschew, 1932; Ellenberg, i950a, 1952a; Goodall, 1954a, 1954b; Whittaker, 1956, 1960; Bray and Curtis, 1957; Curtis, 1959). This general approach to study of landscape patterns and other relations of ecosystems through ecological series and abstract patterns (or by formal statistics of correlations and factor analysis) has been termed gradient analysis (Whittaker, 1951, 1952, 1956). The term expresses the fact that this is an analytic approach to ecosystems through measurable isolates as variables, and that the basis of relating stands to one another and a principal objective of the approach is the study of interrelations of gradients of environment, species populations, and community properties. For the techniques of arranging stands in ecological series or coordinate systems, and by extension for the approach itself, the term ordination (Goodall, 1954a; from Ordnung, Ramensky, 1930) is also current. Implications of such research for problems of classification may be clarified through study of an abstract pattern based on two major complex-gradients, using these gradients as axes of a chart (Fig. 1). Properties of the pattern represented by such a chart cannot be directly identified with those of the landscape pattern. The chart is a simplification of the landscape pattern; it omits from consideration factors not fitting into the complex-gradients studied. Points in the chart may represent, not particular stands, but average or most probable stand properties at a given combination of the gradients studied. The gradients represented as continuous on the chart are frequently interrupted by edaphic and topographic discontinuity and disturbance in the field. The chart summarizes changes of stands along the full extents of gradients which may be somewhere observed in the field by walking 112 THE BOTANICAL REVIEW VEGETATION OF GREAT SlVIOKY MOUNTAINS PRTERN OF EASTERN FOREST SYSTEM

Classification of natural communities

Little is known of the factors that engendered the classic pine forests of Michigan in the period before settlement by Europeans. Some of the forests were relatively open savannas of jack pine and red pine. Other stands were composed of large red and white pines, occasionally with an admixture of hemlock and hardwoods. The early Government Land Office Survey records of Ros- common and Crawford Counties were used to determine the composition and the disturbance history of the pine forests of northern lower Michigan. The abundance of pine (mainly Pinus banksiana, P. resinosa, and P. strobus) in this area that was between lobes of the Wisconsin Laurentide ice sheet was correlated with the presence of coarse-textured soils derived from outwash and ice-contact deposits. These soils promoted a vegetation type which was extremely susceptible to fire. Records suggest the average return time for severe crown fires ranged from 80 yr in the case of the jack pine forest type to 120-240 yr for the mixed pine type to 1200 yr for the hemlock-white pine-northern hardwoods type. Blowdowns had a much longer return time and reached their greatest frequency in the swamp conifer and hemlock-white pine-northern hardwoods types. A three-dimensional diagram summarizes the role of fire, windthrow, substrate, and topography in the formation of Michigan's pine forests.

Relation of Michigan's Presettlement Pine Forests to Substrate and Disturbance History

Introduction 13 0 The individualistic hypothesis .. ......... 133 The continuum 1 13 5 Evidence bearing on the continuum concept . 140 Interspecific association 142 Ordination, gradient analysis, and continuum analysis 146 Spatial gradients 147 Environmental gradients 147 Species position index values 148 Community index values ......10 1 Interpretations of unidimensional ordinations 1 ...5 .. 152 M ultidim ensional ordinations 15 3 The current state of the problem 159 Geometric models ..... . . . . . . .. . . . . . . . . . . . . . . . . . 161 Continuity and homogeneity 14 164 Classification and ordination 167 Assessment ....170 Literature Cited .- 173

The continuum concept of vegetation

We developed a quantitative and replicable classification system to improve understanding of historical com- position and structure within northern Wisconsin's forests The classification system was based on statistical cluster analysis and two forest metrics, relative dominance (% basal area) and relative importance (mean of relative dominance and relative density), as computed from the original US Public Land Survey (PLS) bearing-tree data Broad forest pat- terns are consistent between the two metrics; yet, detailed inspection highlights different aspects of historical structure Maps produced characterize vegetation at regional scales and reveal patterns that can be interpreted in the context of environmental constraints Our classifications have a fairly coarse spatial grain (26 km2) and fine-scale, patchily dis- tributed ecosystems types are not represented This resolution, however, is consistent with that of the PLS bearing-tree data, and maintaining it allowed retention of other beneficial map qualities, including quantitative representation of the data, replicability, flexibility, and an assessment of robustness and confidence Our classifications are broadly applicable for regional-scale scientific and forest-management uses, including (i) assessing natural variability, (ii) determining the potential distribution of species, (iii) setting goals for ecological restoration, and (iv) calculating landscape change RCsumC! : Nous avons mis au point un systkme quantitatif et reproductible de classification pour arnkliorer la compre-

https://www.nrs.fs.fed.us/pubs/jrnl/2002/nc_2002_schulte_001.pdf

Quantitative classification of a historic northern Wisconsin (U.S.A.) landscape: mapping forests at regional scales

land forests. The composition and floristic relationships of the upland forests have been described by Brown and Curtis (1952). The present study is concerned with the second type, with the forests which are found on poorly drained sites which receive runoff waters from adjacent uplands or which are subjected in other ways to temporary or permanent high levels of soil moisture. Such sites are a prominent feature of the heavily glaciated terrain in northern Wisconsin. They are found in partially filled lake basins, along stream terraces, or on flat lands of glacio-lacustrine origin. They support forests which vary from stands of pure conifers in the wettest situations to stands of nearly pure hardwoods on the most mesic sites.- The substrate in these lowland forests is rich in organic matter, frequently in the form of peat which may be present in beds many feet in thickness. The climate of the area is the Type I of Bochert (1950); it is characterized by cold snowy winters, reliable summer rains, and low rates of evaporation. The snow cover is usually deep enough to protect the soils from frost penetration to any great depth and often prevents any freezing of the soil. Cold air drainage into the lowland forest basins serves to increase the severity of the temperature microclimate and may result in frost on any day of the year. The importance of these lowland forests is greater than their areal extent might indicate. Perhaps less than 10 percent of the land area in northern Wisconsin is occupied by them, but they have received far less disturbance than any of the upland types. Their direct economic potential as sources of lumber or other forest products is low, but they are of great importance in stabilizing the water supply of the region. Their role in providing wintering grounds for the white-tailed deer is also of high significance in the over-all conservation program (Christensen, 1954).

E. M. Christensen

Papers

Phytosociology of the Lowland Forests of Northern Wisconsin