Forest vegetation has an important effect on micro-scale temperature conditions. Shelter from the sun, blanketing at night, heat loss by evapotranspiration, reduction of wind speed and the impeding of vertical air movement all influence the temperature environment. The most obvious effect of canopy cover is that, inside the forest, daily maximum temperatures are lower and minima are higher (Figure 12.16). This is particularly apparent during periods of high summer evapotranspiration, which depress daily maximum temperatures and cause mean monthly temperatures in tropical and temperate forests to fall well below that outside. In temperate forests at sea-level, the mean annual temperature may be about 0.6°C lower than that in surrounding open country, the mean monthly differences may reach 2.2°C in summer but not exceed 0.1°C in winter. On hot summer days the difference can be more than 2.8°C. Mean monthly temperatures and diurnal ranges for temperate beech, spruce and pine forests are given in Figure 12.17. This also shows that when trees transpire little in the summer (e.g. the forteto oak maquis of the Mediterranean), the high daytime temperatures reached in the sheltered woods may cause the pattern of mean monthly values to be the reverse of temperate forests. Even within individual climatic regions it is difficult to generalize, however. At elevations of 1000 m the lowering of temperate forest mean temperatures below those in the open may be double that at sea-level.
Source: Food and Agriculture Organization of the United Nations (1962).
The vertical structure of forest stands gives rise to a complex temperature structure, even in relatively simple stands (Figure 12.18). For example, in a ponderosa pine forest (Pinusponderosa) in Arizona the recorded mean June to July maximum was increased by 0.8°C simply by raising the thermometer from 1.5 to 2.4m above the forest floor. In stratified tropical forests the thermal picture is more complex. The dense canopy heats up considerably during the day and quickly loses its heat at night, showing a much greater diurnal temperature range than the undergrowth (Figure 12.18A). Whereas daily maximum temperatures of the second storey are intermediate between those of the tree-tops and the undergrowth, the nocturnal minima are higher than either tree-tops or undergrowth because the second storey is insulated by trapped air both above and below (Figure 12.18B). During dry conditions in the Amazonian rainforest, there is a similar decoupling of the air in the lower storey from the upper two-thirds of the canopy, as reflected by the reduced amplitude of diurnal temperature range. At night, the pattern is reversed: temperatures respond to radiative cooling in the lowest two-thirds of the vegetation canopy. Temperature variations within a layer up to 25-m height are now decoupled from those in the tree-tops and above.
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