Wind cold and wave regeneration

Wave regeneration is a particularly regular form of cyclic change in which mature trees continually die off at the front edge of the wave, which lies behind an opening in the forest canopy and is thus exposed to the prevailing wind. In balsam fir (Abies balsamea) forests at over 1000 m in north-eastern USA, young trees spring up after the wave has passed on at a speed of 1-3 m y_1 (Fig. 9.8). Though the dead trees often remain standing for some time, they no longer exert sufficient shade to inhibit growth of the young seedlings and saplings growing beneath them. Balsam fir is moderately short-lived and becomes increasingly susceptible to stresses, especially those caused by pathogens, by the age of 50-60 years. Trees bearing the brunt of the prevailing wind

-Prevailing wind

-Prevailing wind

Figure 9.8 Diagrammatic vertical section of a regeneration wave in a balsam fir Abies balsamea forest, Whiteface Mountain, New York State, USA. The initially erect dead trees shown are crescentic in plan view; in many cases fusion results in a long multi-crescent wave which progresses up the mountainside. Beyond the ridge top, the waves pass out of the fetch of the prevailing wind, cease to move and eventually disappear. (From Sprugel, 1976. Journal of Ecology 64, Blackwell Publishing.)

Figure 9.8 Diagrammatic vertical section of a regeneration wave in a balsam fir Abies balsamea forest, Whiteface Mountain, New York State, USA. The initially erect dead trees shown are crescentic in plan view; in many cases fusion results in a long multi-crescent wave which progresses up the mountainside. Beyond the ridge top, the waves pass out of the fetch of the prevailing wind, cease to move and eventually disappear. (From Sprugel, 1976. Journal of Ecology 64, Blackwell Publishing.)

are very commonly partially senescent, but are finally killed by environmental stresses including damage caused by winter accumulations of rime-ice, winter desiccation and decreased primary productivity which results from summer cooling. Forests with abundant balsam fir in New York State, New Hampshire and Maine all show regeneration waves of a similar type. Balsam fir has a relatively short life-span (80-100 years) compared with the red spruce Picea rubens that commonly grows with it which lives up to 300 years and is favoured by long disturbance-free periods. Areas of mixed forest have a high proportion of fir for the first century after cutting or major disturbance, while spruce becomes increasingly dominant as the stand ages and the mature firs die out. It is not surprising, therefore, that firs tend to dominate those areas of the forest where regeneration waves occur. Despite continual change, the overall composition of areas of forest possessing these regeneration waves remains relatively constant and in a steady state, provided interventions such as major changes in climate or disease patterns do not occur. Forests of Veitch and Maries' firs (Abies veitchii and A. mariesii) at 2000-2700m in Japan show very similar regeneration waves, although these are further apart than those on Whiteface Mountain. This is only to be expected as these species are slower growing and longer lived (to about 100 years).

Crescent-shaped regeneration waves which move straight downwind appear to result from the death of a single tree or small group of trees, thus exposing a downwind arc of living trees to much higher wind speeds. Indeed, wind speeds in the canopy at the exposed edge of the old forest stand may be over 50% higher than those in the rest of the canopy. While a single crescent might

Wind Speed And Rain Angle

Figure 9.9 An illustration of how helical roll vortices form wave forests. The angle of the major axis of the helical roll vortices in relation to the geostrophic wind direction (generated by atmospheric pressure differences and the Coriolis force) implies the classic Eckman Spiral where surface currents in water flow at an angle to the surface wind. (From Robertson, 1987. Canadian Journal of Forest Research, 17.)

Figure 9.9 An illustration of how helical roll vortices form wave forests. The angle of the major axis of the helical roll vortices in relation to the geostrophic wind direction (generated by atmospheric pressure differences and the Coriolis force) implies the classic Eckman Spiral where surface currents in water flow at an angle to the surface wind. (From Robertson, 1987. Canadian Journal of Forest Research, 17.)

possibly move gradually all the way up to the top of the mountain alone, adjacent crescents appear to coalesce in long multi-crescent waves of the type common near the ridgetop. The more complex aerodynamics of the balsam fir forest of Spirity Cove, Newfoundland, the largest wave forest known, were investigated by Robertson (1987), who suggests that the regeneration waves found here are mainly caused by a series of helical roll vortices (Goertler vortices). These result in large numbers of sinusoidal dead tree strips, 100-150 m apart and orientated at an acute angle to the direction of the prevailing wind, which move in 55-year cycles (Fig. 9.9). This type of repeated change, literally driven by the wind, is in some respects similar to the cyclical turnover of vegetation which Vera (2000) considers to have occurred in the earlier European Holocene forest in response to the activities of large forest animals (see Section 5.9).

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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