Gradient analysis

Figure 16.6 shows a variety of ways of describing the distribution of vegetation used in a classic study in the Great Smoky Mountains (Tennessee), USA, where tree species give the vegetation its main character. Figure 16.6a shows the characteristic associations of the dominant trees on the mountainside, drawn as if the communities had sharp boundaries. The mountainside itself provides a range of conditions for plant growth, and two of these, altitude and moisture, may be particularly important in determining the distribution of the various tree species. Figure 16.6b shows the dominant associations graphed in terms of these two environmental dimensions. Finally, Figure 16.6c shows the abundance of each individual tree species (expressed as a percentage of all tree stems present) plotted against the single gradient of moisture.

Figure 16.6a is a subjective analysis that acknowledges that the vegetation of particular areas differs in a characteristic way from that of other areas. It could be taken to imply that the various communities are sharply delimited. Figure 16.6b gives the same impression. Note that both Figure 16.6a and b are based on descriptions of the vegetation.

However, Figure 16.6c sharpens the focus by concentrating on the pattern of distribution of the individual species. It is then immediately obvious that there is considerable overlap in their abundance - there are no sharp boundaries. The various tree species are now revealed as being strung out along the gradient with the tails of their distributions overlapping. The results of this 'gradient analysis' show that the limits of the distributions of each species 'end not with a bang but with a whimper'. Many other gradient studies have produced similar results.

Perhaps the major criticism of gradient analysis as a way of detecting pattern in communities is that the choice of the gradient is almost always subjective. The investigator searches for some feature of the environment that appears to matter to the organisms and then organizes the data about the species concerned along a gradient of that factor. It is not necessarily the most appropriate factor to have chosen. The fact that the species from a community can be arranged in a sequence along a gradient of some environmental factor does not prove that this factor is the most important one. It may only imply that the factor chosen is more or less loosely correlated with whatever really matters in the lives of the species involved. Gradient analysis is only a small step on the way to the objective description of communities.

species distributions along gradients end not with a bang but with a whimper choice of gradient is almost always subjective

5500

5000

4500

o 4000 at v le

El 3500

3000 2500 2000 1500

(Boreal forests)

Beech ¡ forests Meslc type ¡ Sedge type

(Heath bald)

Coves Canyons

Table mountain pine heath

Pitch pine heath

Virginia pine forest

Flats Draws Ravines

Sheltered slopes

Open slopes NE E W S N NWSESW

Ridges and peaks

tem30

tem30

Moisture level Sheltered slopes NE

Valley Dryer -

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Moisture level Sheltered slopes NE

Open slopes

Wetter

Figure 16.6 Three contrasting descriptions of distributions of the characteristic dominant tree species of the Great Smoky Mountains, Tennessee. (a) Topographic distribution of vegetation types on an idealized west-facing mountain and valley. (b) Idealized graphic arrangement of vegetation types according to elevation and aspect. (c) Distributions of individual tree populations (percentage of stems present) along the moisture gradient. Vegetation types: BG, beech gap; CF, cove forest; F, Fraser fir forest; GB, grassy bald; H, hemlock forest; HB, heath bald; OCF, chestnut oak-chestnut forest; OCH, chestnut oak-chestnut heath; OH, oak-hickory; P, pine forest and heath; ROC, red oak-chestnut forest; S, spruce forest; SF, spruce-fir forest; WOC, white oak-chestnut forest. Major species: 1, Halesia monticola; 2, Aesculus octandra; 3, Tilia heterophyUa; 4, Betula alleghaniensis; 5, Liriodendron tulipifera; 6, Tsuga canadensis; 7, B. lenta; 8, Acer rubrum; 9, Cornus florida; 10, Carya alba; 11, Hamamelis virginiana; 12, Quercus montana; 13, Q. alba; 14, Oxydendrum arboreum; 15, Pinus strobus; 16, Q. coccinea; 17, P. virginiana; 18, P. rigida. (After Whittaker, 1956.)

F. terminalis

- C. dossuarius T longiseta b T.

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ousilla H H

H. intermedia h C C ^^^ S. oblonga H

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50 60 70 80 90 Bray-Curtis similarity measure

Conochilus unicornis

Ascomorpha ovalis

Keratella tecta

Keratella tropica

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