Other causes of forest decline

11.4.1 Effects of plant strategies on forest stability and sustainability

Woodlands and forests, like all ecosystems, are subject to climatic and biotic impacts that threaten their viability and survival. We read almost daily of areas of forest suffering from drought, frost, fires, floods or windstorms on the one hand, or over-grazing, disease, cultivation, pollution or urbanization on the other. There are two key aspects to the response of an individual plant or stand of vegetation to such an extreme event, for which the terms resistance and resilience were coined by Westman (1978). Resistance is the ability to resist displacement from its current state. Resilience is the speed and completeness of the return to the original state (sometimes referred to as elasticity). The two terms are included in the overarching term of stability, defined as the ability of a forest to resist permanent change (the relationship between stability and diversity is discussed in Section 9.5). As Grime (2001) emphasizes, when ecosystems of any sort are exposed to an increasing severity or frequency of extreme events due to such things as climatic change or rising human population density, attention switches from an interest in how the ecosystem reacts to a concern with its sustainability. There may come a point where a forest is so repeatedly disturbed or altered that it cannot return to its original state and may even be permanently changed. A very severe storm or severe fire could, for example, alter the species composition of a forest (despite high resistance) but the forest might return to its original condition within a short time (high resilience). However, frequent storms or fires may give insufficient time for recovery before the next disturbance so the forest never gets beyond an early stage of recovery or the trees may fail altogether and a different habitat develops such as savanna or grassland. Such a change is readily seen with heavy grazing which prevents tree regeneration, eventually leading to a grassland habitat once the existing trees die. Even if the forest does not completely disappear, at what point is the essential character of a woodland or forest ecosystem so impaired that its value is irreparably degraded?

The opportunities for analysis and prediction of ecosystem resistance, resilience and sustainability that arise from recognition of CSR plant strategies by Grime

(2001) - see Section 4.1 for more detail - makes fascinating reading; here there is space only for discussion regarding wooded ecosystems. The four major points made here by Grime apply particularly to these very systems:

(1) Remnants of older landscape such as ancient woodland or old-growth forests, which are usually highly diverse and of lower productivity than their modern counterparts, will continue to diminish in size and become progressively more isolated from similar landscape fragments. The classical island biogeographical theory (MacArthur and Wilson, 1967) then applies. Floras and faunas of such relict systems will lose diversity as species extinctions fail to be compensated by immigrations from remote 'nearest neighbour' populations.

(2) In ecosystems dominated by slow-growing, long-lived plants such as forests, resistance to change is considerable. Once resistance is broken and population reductions and extinctions occur, however, the low reproductive output, limited dispersal range and long juvenile phase of many stress-tolerant species reduce the prospect of successful recovery. Many of the plants and some of the animals that exploit the deteriorating fragments of isolated forests are able to do so because of their life histories and functional characteristics that allow longdistance dispersal and rapid invasion.

(3) Conditions in forests enriched by humans or intensive exploitation are very different from those in fragments of old forests of low productivity. In richer forests there is a more dynamic flux, in space and time, of plant and animal populations with 'weedy' (i.e. R or C) traits. Vegetation processes often proceed rapidly and tend to be constantly returned to an early successional stage by eutrophication (nutrient enrichment) and disturbance in both industrial and agricultural areas.

(4) Species capable of rapid colonization are often already present in a forest at low frequency or in nearby areas and consequently are in a position to cause quick and unpredictable changes in the flora and fauna once a landscape is modernized. Rhododendron Rhododendron ponticum in the UK is a classic example of an alien woodland invader, but the course of invasions by such species remains difficult to predict and there is much to learn regarding the invisibility of plant invaders and the conditions that allow a low-frequency invader to explosively take over.

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