Combinations of factors

Damage to forests often results from the combined impact of several factors as in north-west Colorado, where over 10 000 ha of subalpine forest was blown down in 1997 (Kulakowski and Veblen, 2002). The study area, shown in Fig. 9.15, ranges from 2400 to 3400 m (8000-11000 ft) in altitude, and its forests are dominated by lodgepole pine Pinus contorta, quaking aspen Populus tremuloides, Engelmann spruce Picea engelmannii and subalpine fir Abies lasiocarpa. It was concluded that both topographic position and fire history contributed to susceptibility to wind damage. Damage was least in the younger stands, at lower elevations and in areas away from ridges where wind speeds were lower. Stands

Forest Fire Factors

2400 2600 2800 3000 3200 3400 3600 0 200 400 600 800 1000 1200

Elevation (m) Distance to closest ridge (m)

Figure 9.15 (a) Location and (b) fire history classes of a 4400 ha study area of subalpine forest in Colorado, USA. Dates correspond to year of the last stand-replacing fire. (c) Expected (open bars) and observed (filled bars) blowdown of trees at different elevations within the study area. (d) Expected (open bars) and observed (filled bars) blowdown of trees in areas of different distances from the closest ridge. (From Kulakowski and Veblen, 2002. Journal of Ecology 90, Blackwell Publishing.)

2400 2600 2800 3000 3200 3400 3600 0 200 400 600 800 1000 1200

Elevation (m) Distance to closest ridge (m)

Figure 9.15 (a) Location and (b) fire history classes of a 4400 ha study area of subalpine forest in Colorado, USA. Dates correspond to year of the last stand-replacing fire. (c) Expected (open bars) and observed (filled bars) blowdown of trees at different elevations within the study area. (d) Expected (open bars) and observed (filled bars) blowdown of trees in areas of different distances from the closest ridge. (From Kulakowski and Veblen, 2002. Journal of Ecology 90, Blackwell Publishing.)

resulting from recent fires were even less affected because the early pioneer, aspen, is more windfirm than the later pine, and much more so than the eventual spruce-fir forest. When natural disturbances interact, as did fire and blowdown here, forest dynamics are influenced by synergistic (i.e. combined) effects, even when disturbance is infrequent.

Cases such as those discussed above, together with the physical and climatic changes experienced to varying degrees by all regions of the world, mean that the existence of vegetational end-points stable in the long term is most unlikely.

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Renewable Energy 101

Renewable Energy 101

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