Multiples Of Shelter Belt Height

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Figure 12.13 The influence of shelter belts on wind-velocity distributions (expressed as percentages of the velocity in the open). (A) The effects of one shelter belt of three different densities, and of two back-coupled medium-dense shelter belts. (B) The detailed effects of one half-solid shelter belt.

Sources: (A) After Nageli, and Geiger (1965). (B) After Bates and Stoeckeler, and Kittredge (1948).

c Modification of the humidity environment

The humidity conditions within forest stands contrast strikingly with those in the open. Evaporation from the forest floor is usually much less because of the decreased direct sunlight, lower wind velocity, lower maximum temperature, and generally higher forest air humidity. Evaporation from the bare floor of pine forests is 70 per cent of that in the open for Arizona in summer and only 42 per cent for the Mediterranean region.

Unlike many cultivated crops, forest trees exhibit a wide range of physiological resistance to transpiration processes and, hence, the proportions of forest energy flows involved in evapotranspiration (LE) and sensible heat exchange (H) vary. In the Amazonian tropical broad-leaved forest, estimates suggest that after rain up to 80 per cent of the net solar radiation (Rn) is involved in evapotranspiration (LE) (Figure 12.14). Figure 12.15

compares diurnal energy flows during July for a pine forest in eastern England and a fir forest in British Columbia. In the former case, only 0.33 Rn is used for LE due to the high resistance of the pines to transpiration, whereas 0.66 Rn is similarly employed in the British Columbia fir forest, especially during the afternoon. Like short green crops, only a very small proportion of Rn is ultimately used for tree growth, an average figure being about 1.3 W m-2, some 60 per cent of which produces wood tissue and 40 per cent forest litter.

During daylight, leaves transpire water through open pores, or stomata. This loss is controlled by the length of day, the leaf temperature (modified by evaporational cooling), surface area, the tree species and its age, as well as by the meteorological factors of available radiant energy, atmospheric vapour pressure and wind speed. Total evaporation figures are therefore extremely varied. The evaporation of water intercepted by the

Figure 12.14 A computer simulation of energy flows involved in the diurnal energy balance of a primary tropical broad-leaved forest in the Amazon during a high-sun period on the second dry day following a 22-mm daily rainfall.

Source: After a Biosphere Atmosphere Transfer Scheme (BATS) model from Dickinson and Henderson-Sellers (1988), by permission of the Royal Meteorological Society, redrawn.

vegetation surfaces also enters into the totals, in addition to direct transpiration. Calculations made for a catchment covered with Norway spruce (Picea abies) in the Harz Mountains of Germany showed an annual evapotranspiration of 34 cm and additional interception losses of 24 cm.

The humidity of forest stands is linked closely to the amount of evapotranspiration and increases with the density of vegetation present. The increase in forest relative humidity over that outside averages 3 to 10 per cent and is especially marked in summer. Vapour pressures were higher within an oak stand in Tennessee than outside for every month except December. Tropical forests exhibit almost complete night saturation irrespective of elevation in the trunk space, whereas by day humidity is related inversely to elevation. Measurements in Amazonia show that in dry conditions daytime specific humidity in the lower trunk space (1.5 m) is near 20 g kg-1, compared with 18 g kg-1 at the top of the canopy (36 m).

Recent research in boreal forests shows that they have low photosynthetic and carbon draw-down rates, and consequently low transpiration rates. Over the year, the uptake of CO2 by photosynthesis is balanced by its loss through respiration. During the growing season, the evapotranspiration rate of boreal (mainly spruce) forests is surprisingly low (less than 2 mm per day). The low albedo, coupled with low energy use for evapotranspiration, leads to high available energy, high sensible heat fluxes and the development of a deep convective planetary boundary layer. This is particularly marked during spring and early summer due to intense mechanical and convective turbulence. In autumn, in contrast, soil freezing increases its heat capacity, leading to a lag in the climate system. There is less available energy and the boundary layer is shallow.

The influence of forests on precipitation is still unresolved. This is due partly to the difficulties of comparing rain-gauge catches in the open with those in forests, within clearings or beneath trees. In small clearings, low wind speeds cause little turbulence around the opening of the gauge and catches are generally greater than outside the forest. In larger clearings, downdrafts are more prevalent and consequently the precipitation catch increases. In a 25-m high pine and beech forest in Germany, catches in 12-m diameter clearings were 87 per cent of those upwind of the forest, but the catch rose to 105 per cent in clearings of 38 m. An analysis of precipitation records for Letzlinger Heath (Germany) before and after afforestation suggested a mean annual increase of 6 per cent, with the greatest excesses occurring during drier years. It seems that forests have little effect on cyclonic rain, but they may slightly increase orographic precipitation, by lifting and turbulence, of the order of 1 to 3 per cent in temperate regions.

A more important influence of forests on precipitation is through the direct interception of rainfall by the canopy. This varies with crown coverage, season and rainfall intensity. Measurements in German beech forests indicate that, on average, they intercept 43 per cent of precipitation in summer and 23 per cent in winter. Pine forests may intercept up to 94 per cent of low-intensity precipitation but as little as 15 per cent of high intensities, the average for temperate pines being about 30 per cent. In tropical rainforest, about 13 per cent of annual rainfall is intercepted. The intercepted precipitation either evaporates on the canopy, runs down the trunk, or drips to the ground. Assessment of the total precipitation reaching the ground (the through-fall) requires careful measurements of the stem flow and the contribution of drips from the canopy. Canopy interception contributes 15 to 25 per cent of total evaporation in tropical rainforests. It is not a total loss of moisture from the forest, since the solar energy used in the evaporating process is not available to remove soil moisture or transpiration water. However, the vegetation does not derive the benefit of water

Figure 12.15 Energy components on a July day in two forest stands. (A) Scots and Corsican pine at Thetford, England (52°N), on 7 July 1971. Cloud cover was present during the period 00:00 to 05:00 hours. (B) Douglas fir stand at Haney, British Columbia (49°N), on 10 July 1970. Cloud cover was present during the period 11:00 to 20:00 hours.

Figure 12.16 Seasonal regimes of mean daily maximum and minimum temperatures inside and outside a birch-beech-maple forest in Michigan.

Figure 12.15 Energy components on a July day in two forest stands. (A) Scots and Corsican pine at Thetford, England (52°N), on 7 July 1971. Cloud cover was present during the period 00:00 to 05:00 hours. (B) Douglas fir stand at Haney, British Columbia (49°N), on 10 July 1970. Cloud cover was present during the period 11:00 to 20:00 hours.

Sources: (A) Data from Gay and Stewart (1974), after Oke (1978). (B) Data from McNaughton and Black (1973), after Oke (1978).

cycling through it via the soil. Canopy evaporation depends on net radiation receipts, and the type of species. Some Mediterranean oak forests intercept 35 per cent of rainfall and almost all evaporates from the canopy. Water balance studies indicate that evergreen forests allow 10 to 50 per cent more evapotranspiration than grass in the same climatic conditions. Grass normally reflects 10 to 15 per cent more solar radiation than coniferous tree species and hence less energy is available for evaporation. In addition, trees have a greater surface roughness, which increases turbulent air motion and, therefore, the evaporation efficiency. Evergreens allow transpiration to occur year-round. Nevertheless, research to verify these results and test various hypotheses is needed.

Figure 12.16 Seasonal regimes of mean daily maximum and minimum temperatures inside and outside a birch-beech-maple forest in Michigan.

Source: After US Department of Agriculture Yearbook (1941).

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Renewable Energy 101

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  • igor
    2 years ago
  • aatifa
    What are the effects of shelter belt height?
    2 years ago

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