Paper 31: Pattern and Process in the Plant Community (1947) by Alex S. Watt

Paper 31: Pattern and Process in the Plant Community (1947)

Alex S. Watt

Alexander Watt was a Scottish botanist and plant ecologist. When this talk was given, he was a professor of botany at University of Cambridge. This paper was originally an address given to the British Ecological Society on January 11, 1947.

The Plant Community as a Working Mechanism

Watt begins his talk classifying the plant community as a "working mechanism". He continues by describing his aim of applying dynamic principles to the plant community to formulate laws by which it maintains and regenerates itself. He says it is relatively unknown how individuals and species are put together, what determines their relative proportions and their spatial and temporal relations to each other. Ultimately he wants a qualitative statement of the nature of the relations between the compounds of the community.

He presents a record of seven communities, the first of which their features make for orderliness, and the latter which specifically depart from that order. He assumes with these case studies essential uniformity in the fundamental factors of habitat (separate from context of whole environment) and essential stability of the community.

The Evidence from Seven Communities

The regeneration complex

This is a mosaic of patches that form an intergrading series that can be assigned to a few types of phases. He presents Tregaron Bog and goes through the sequence with which species take over the open water in the pool. The final species in this series then forms a hummock, which is then also inhabited by a series of species. This can be proven by the layers of plant material in the peat. 

He states that each of these phases of the “regeneration complex” were once considered a community in itself, but Watt believes that each cycle is brought about by the last one and the spatial relation between patches and their change in level. 

Dwarf Callunetum

This is an analysis of the dwarf Callunetum on an exposed slope of the Cairngorms (a national park in Scotland). The two main species he talks about are Calluna and Arcostaphylos. These two have the same habitat, but Arcostaphylos is almost always found leeward of Calluna. Arcostaphylos first grows over the eroded soil, and then Calluna (the dominant plant) grows over the older parts of Arcostaphylos. This, he says, is proof of a dynamic relationship between the two species and the process of succession. It is a three-phase system with the phases arranged in linear series. 

Eroded Rhacomitrietum

This is a similar phenomena to the Calluna in that wind determines the direction of growth. This community consists of a network of patches of bare soil and of vegetation. These are characterized by a series of bryophytes ending with Rhacomitrium. This plant is then eroded by the wind and and the exposed accumulated humus is dispersed and the mineral soil bare again. 

Bracken

This study is about the Pteridietum (ferns) that occurs in a hinterland where the fronds are patchily distributed and the axes of the plants form a loose network (Figure 4). There are patches with fronds and without fronds. The patches with fronds vary features of frond and rhizome, and exist in four different stages - pioneer, building, mature, and degenerate. These exist in what Watt calls the marginal zone and the hinterland, and he says the processes are the same in these two - when the vegetation cycle changes, there is also a change in the factors of habitat. 

Grassland A (Breckland)

This is a patchy grassland that has hollows and hummocks with the habitat resembling the regenerative complex. Here, the four stages Watt classifies are hollow, building, mature, and degenerate. The life of the community centers around Festuca, the dominant plant, and in the spaces around this, lichens grow. Every part of this community can be assigned to one of the four stages, and the cover percentage of fescue to bare soil are inversely related. Holophytic bryophytes are excluded from the building and mature phases, and reach their peak in the hollow where competition from fescue is the least.

Grass-heath on acid sands (Breckland)

This community consists of rings of vegetation, and Watt says it epitomizes the spatial and temporal changes within a community. The two main species being studied are Agrostis and Cladonia, the first of which gives way to the second. Drought has a large effect on this system - it bypasses the first stage completely and goes straight to the second one.

Beechwood

In this study, Watt wants to investigate if there is a correspondence between structure and current meteorological factors within long-lived dominants. There is a cyclic relation between the tree species and the causes having long-continued effects in structure. Watt also remarks that if a whole gap phase regenerates at the same time, there an age class of abnormal area is initiated.

Supplementary Evidence

In this section, Watt mainly gives evidence of where else these dynamic principles could apply; not just to temperate regions, but to extreme regions like the high arctic. The tropical forest is not expected to fit into a simple dynamic interpretation. He states that Aubreville has somewhat disproven this, that although the tropical forest is diverse and has much larger number of species, there is a change in the dominant species over time. This leads him to conclude that the community itself largely determines the “distribution, density, and gregariousness of its component parts.”

Comparison and Synthesis

Watt introduces the idea of an upgrade of a community and a downgrade. The upgrade results in a building up of plant material for an ultimate positive balance, and a downgrade is an ultimate negative balance. He also looks at a quantitative graphical expression of the change in an upgrade series and says that a logistic curve of growth expresses the general course of change (a similar negative curve for downgrade fits also). Figure 11 gives a nice approximation of this. Most species are limited in space, but also in time based on when the gap phase is receptive. This may lead us to assume that the plant community in a constant environment will show a definite proportion between constituent phases. 

Some Implications

He takes some time to point out that Clements and Shelford do not incorporate the effects of animals and micro-organisms into their study. He thinks the degree of intimacy varies, but their key positions in the system should be considered a part of it. Fungi also play a role in the maintenance of the ecosystem. Watt then moves on to “shatter” the unified system that Tansley calls the ecosystem, and points out that the living plants and animals (biome) must be separated from the non-living habitat. He also points out Gleason’s “randomicity” of the species in an association, and says that although two species may be randomly distributed, they may show a high degree of association. Gleason minimizes the significance of the relations between the components of a community. 

Watt ends with saying that drawing a line between the plant community and the ecosystem is difficult, but we must have even an idealistic objective of melding all of the fragments into one original unity. So many problems in nature are problems of the ecosystem and not just of soil, animals, or plants, and because of this, we must know how all are connected.