Environmental Engineering Reference
In-Depth Information
increases and the active zone extends from the higher canopy down to the soil surface.
The extreme example of this effect is seen in mature forest. So much has been written
about the microclimate within a forest that the term
forest climate
is frequently used to
indicate the wide variety of conditions that can be experienced.
On a hot summer's day it is noticeable that temperatures in a forest are much lower
than outside, providing a respite from the strong glare and baking heat of the sun. Air
movement is weak. It feels humid, and the impression is quickly gained of an entirely
different climate. This affects plant and animal life as well as people. Quite different
ecosystems develop because of the climatic environment produced by the forest. Because
of the differences in scale, the microclimates within a forest are more distinct than those
in grassland or low crops.
RADIATION EXCHANGES IN WOODLAND
It is apparent on entering a forest that the forest canopy cuts out much of the incoming
radiation. Most of the energy is absorbed by the tree canopy. A significant proportion is
reflected - about 5-15 per cent, on average, although in some cases reflection may reach
over 30 per cent (Table 8.2). Only a very small proportion reaches the ground directly,
normally in the form of small patches of light called sunflecks. The remainder penetrates
the vegetation indirectly; it is scattered by the atmosphere and arrives as diffuse radiation.
Table 8.2 Tree albedos (%)
Aleppo pine
17
Monterey pine
10
Loblolly pine
11
Lodgepole pine
9
Scots pine
9
Oak
Summer
15
Spring
12
Eucalyptus
19
Sitka spruce
12
Norway spruce
12
Birch and aspen
Late winter
25
Orange trees
32
Tropical rain forest
13
Cocoa
16
