Forests

The vertical structure of a forest, which depends on the species composition, the ecological associations and the age of the stand, largely determines the forest microclimate. The climatic influence of a forest may be explained in terms of the geometry of the forest, including morphological characteristics, size, cover, age and stratification. Morphological characteristics include amount of branching (bifurcation), the periodicity of growth (i.e. evergreen or deciduous), together with the size, density and texture of the leaves. Tree size is obviously important. In temperate forests the sizes may be closely similar, whereas in tropical forests there may be great local variety. Crown coverage determines the physical obstruction presented by the canopy to radiation exchange and airflow.

Different vertical structures in tropical rainforests and temperate forests can have important microclimatic effects. In tropical forests the average height of the taller trees is around 46 to 55 m, with individual trees rising to over 60 m. The dominant height of temperate forest trees is generally up to 30 m. Tropical forests possess a great variety of species, seldom less than forty per hectare (100 hectares = 1 km2) and sometimes over 100, compared with less than twenty-five (occasionally only one) tree species with a trunk diameter greater than 10 cm in Europe and North America. Some British woodlands have almost continuous canopy stratification, from low shrubs to the tops of 36-m beeches, whereas tropical forests are strongly stratified with dense undergrowth, simple trunks, and commonly two upper strata of foliage. This stratification results in more complex microclimates in tropical forests than temperate ones.

It is convenient to describe the climatic effects of forest stands in terms of their modification of energy transfers, airflow, humidity environment and thermal environment.

a Modification of energy transfers

Forest canopies change the pattern of incoming and outgoing radiation significantly. The short-wave reflectivity of forests depends partly on the characteristics of the trees and their density. Coniferous forests have albedos of about 8 to 14 per cent, while values for deciduous woods range between 12 and 18 per cent, increasing as the canopy becomes more open. Values for semi-arid savanna and scrub woodland are much higher.

Besides reflecting energy, the forest canopy traps energy. Measurements made in summer in a thirty-year old oak stand in the Voronezh district of Russia, indicate that 5.5 per cent of the net radiation at the top of the canopy is stored in the soil and the trees. Dense red beeches (Fagus sylvatica) intercept 80 per cent of the incoming radiation at the treetops and less than 5 per cent reaches the forest floor. The greatest trapping occurs in sunny conditions, because when the sky is overcast the diffuse incoming radiation has greater possibility of penetration laterally to the trunk space (Figure 12.11A). Visible light, however, does not give an altogether accurate picture of total energy penetration, because more ultraviolet than infra-red radiation is absorbed into the crowns. As far as light penetration is concerned, there are great variations depending on type of tree, tree spacing, time of year, age, crown density and height. About 50 to 75 per cent of the outside light intensity may penetrate to the floor of a birch-beech forest, 20 to 40 per cent for pine and 10 to 25 per cent for spruce and fir. However, for tropical forests in the Congo the figure may be as low as 0.1 per cent, and 0.01 per cent has been recorded for a dense elm stand in Germany. One of the most important effects of this is to reduce the length of daylight. For deciduous trees, more than 70 per cent of the light may

Figure 12.11 The amount of light beneath the forest canopy as a function of cloud cover and crown height. (A) For a thick stand of 120 to 150-year-old red beeches (Fagus sylvatica) at an elevation of 1000 m on a 20° southeast-facing slope near Lunz, Austria. (B) For a Thuringian spruce forest in Germany over more than 100 years of growth, during which the crown height increased to almost 30 m.

Source: After Geiger (1965).

Figure 12.11 The amount of light beneath the forest canopy as a function of cloud cover and crown height. (A) For a thick stand of 120 to 150-year-old red beeches (Fagus sylvatica) at an elevation of 1000 m on a 20° southeast-facing slope near Lunz, Austria. (B) For a Thuringian spruce forest in Germany over more than 100 years of growth, during which the crown height increased to almost 30 m.

Source: After Geiger (1965).

penetrate when they are leafless. Tree age is also important in that this controls both crown cover and height. Figure 12.11B shows this rather complicated effect for spruce in the Thuringian Forest, Germany.

b Modification of airflow

Forests impede both the lateral and the vertical movement of air (Figure 12.12A). Air movement within forests is slight compared with that in the open, and quite large variations of outside wind velocity have little effect inside woods. Measurements in European forests show that 30 m of penetration reduces wind velocities to 60 to 80 per cent, 60 m to 50 per cent and 120 m to only 7 per cent. A wind of 2.2 m s-1 outside a Brazilian evergreen forest was reduced to 0.5 m s-1 at 100 m within it, and was negligible at 1000 m. In the same location, external storm winds of 28 m s-1 were reduced to 2 m s-1 some 11 km deep in the forest. Where there is a complex vertical structuring of the forest, wind velocities become more complex. Thus in the crowns (23 m) of a Panama rainforest the wind velocity was 75 per cent of that outside, while it was only 20 per cent in the undergrowth (2 m). Other influences include the density of the stand and the season. The effect of season on wind velocities in deciduous forests is shown in Figure 12.12B. In a Tennessee mixed-oak forest, forest wind velocities were 12 per cent of those in the open in January, but only 2 per cent in August.

Knowledge of the effect of forest barriers on winds has been used in the construction of windbreaks to protect crops and soil. Cypress breaks of the southern Rhône valley and Lombardy poplars (Populus nigra) of the Netherlands form distinctive features of the landscape. It has been found that the denser the obstruction the greater the shelter immediately behind it, although the downwind extent of its effect is reduced by lee turbulence set up by the barrier. A windbreak of about 40 per cent penetrability (Figure 12.13) gives the maximum protection. An obstruction begins to have an effect about eighteen times its own height upwind, and the downwind effect can be increased by the back coupling of more than one belt (see Figure 12.13).

There are some less obvious microclimatic effects of forest barriers. One of the most important is that the reduction of wind speed in forest clearings increases the frost risk on winter nights. Another is the removal of dust and fog droplets from the air by the filtering action of forests. Measurements 1.5 km upwind on the lee side and 1.5 km downwind of a kilometre-wide German forest gave dust counts (particles per litre) of 9000, less than 2000 and more than 4000, respectively. Fog droplets can be filtered from laterally moving air resulting in a higher precipitation catch within a forest than outside. The winter rainfall catch outside a eucalyptus forest near Melbourne, Australia, was 50 cm, whereas inside the forest it was 60 cm.

Wind Profiles And Forests

Figure 12.12 Influence on wind-velocity profiles exercised by: (A) a dense stand of 20-m high ponderosa pines (Pinus ponderosa) in the Shasta Experimental Forest, California. The dashed lines indicate the corresponding wind profiles over open country for general wind speeds of about 2.3, 4.6 and 7.0 m s-1, respectively. (B) A grove of 25-m high oak trees, both bare and in leaf.

Figure 12.12 Influence on wind-velocity profiles exercised by: (A) a dense stand of 20-m high ponderosa pines (Pinus ponderosa) in the Shasta Experimental Forest, California. The dashed lines indicate the corresponding wind profiles over open country for general wind speeds of about 2.3, 4.6 and 7.0 m s-1, respectively. (B) A grove of 25-m high oak trees, both bare and in leaf.

Sources: (A) After Fons, and Kittredge (1948). (B) After Geiger and Amann, and Geiger (1965).

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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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