TO 30 20 10 0 10 20 30 Days Before After Leaf Emergence

Figure 6.3. As the leaves come out, the progressive warming into spring halts for a few days because of the latent heat taken up by evaporation from the leaves. After Bonan.

surrounding air cooler than it would otherwise be (Chapter 4). The moisture from leaves also affects broader-scale aspects of regional climate. It increases the humidity—giving, for example, the sticky summer climate of the southeastern USA when plenty of heat and plenty of rainfall combine in a predominantly forested landscape. This humidity itself keeps the night warm: as the air cools in the evening, some of this water vapor condenses out yielding heat that helps prevent the air from cooling further. And the water vapor itself acts as a "greenhouse gas", trapping heat radiated by the forest canopy during the night and sending it straight back down to earth.

Leafing out in spring in the temperate forest zone has an immediate effect on temperature due to the onset of transpiration. The progressive increase in temperature into spring is halted for a few days by the transpiration from these newly formed tree leaves (Figure 6.3). These patterns seem to be paralleled more extensively in the tropics, where models and observations suggest that transpiration from forest keeps the climate cooler and rainier (see below), and less variable between night and day.

So, in at least some cases such as these, Chekov's characters were right after all: the forest does seem to moderate the climate.

Another way that forests modify atmospheric processes is through "roughness". The crowns of forest trees often look like heads of broccoli packed all against one another, with bumps on the surface and valleys between them. This uneven surface lets the wind blow down between their crowns but then find its way blocked by others in front, and it tends to set the air rolling, a type of motion known as "turbulence" (Chapter 4). The big trunks and branches also act as barriers for the wind, slowing the wind down and making it more turbulent. All this turbulence set off by the forest canopy tends to carry heat and water vapor upwards more effectively. So, the roughness of forest surfaces makes them feed water to the atmosphere more rapidly, compared with smoother surfaces such as grassland, crops or bare desert.

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