Disturbance patch dynamics and scales of change

Natural disturbance and patch dynamics (Pickett and White, 1985) are involved in the ecology of many woodlands and forests. The various possible forms of disturbance, from a burrowing animal to a forest fire, cause very different degrees and scales of change.

9.3.1 Earthquakes and avalanches

A notable example involves very large (> 0.25 ha) patches in the forests of the Lake District of southern Chile. Many undisturbed forests here have a profusion of lichens, mosses, ferns and vines, yet seedlings and saplings of the huge southern beeches (Nothofagus) which can reach over 45 m, have potential lifespans of 400-500 years and tower over the rest of the forest, are virtually absent. Tree species of the lower canopy, in contrast, normally possess individuals of all size classes. Are the southern beeches on their way out? The answer to this riddle appears to lie in the unstable nature of the region, which is amongst the most seismically active in the world.

In 1960, a series of catastrophic earthquakes in this area caused the northern coastline to rise by almost 2 m, with a corresponding lowering in the south. There were thousands of extensive debris avalanches, landslides and mud-flows. At the same time volcanoes erupted in the Andes, showering ash throughout the district. When surfaces exposed by the 1960 slides were examined 15 years later, numerous tree saplings with average heights exceeding 3 m were present. In the lower elevation forests Hualle (Nothofagus obliqua), Eucryphia cordifolia (a eucryphia native to Chile) and Weinmannia trichospermia were the most abundant colonizing species, with Coigue (N. dombeyi) being the commonest species above 500 m. Nearly all reproductively mature trees of Reuli (N. alpina), which is by far the most valuable timber tree in the region, had been felled, but where it was present in more remote areas its saplings were also represented on the exposed slides. The shade-tolerant species common in the adjacent forests, in which their seedlings flourish, were absent or extremely rare on surfaces exposed in 1960. Furthermore, inspection of old-growth forests showed that many had developed on previous landslide scars or debris deposits.

The evidence assembled by Veblen (1985) thus strongly supports the view that the abundance of southern beeches in much of the mid- and low-elevation Andean forests is largely a consequence of the repeated periodic seismic disturbances known to have occurred. Indeed, of the 47 notable earthquakes which occurred in this region of southern Chile between 1520 and 1960, seven were roughly equivalent in magnitude to the main shock of 1960.

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