Under the canopy

In the cooler forest understory, out of the direct sun, overheating is not a problem and leaves can grow bigger than at the top of the canopy. Many of the types of plants that grow down near the floor of the forest have large plate-like leaves 30 cm or more across; undivided leaves this size are hardly ever seen up in the forest canopy.

On the forest floor, the overwhelming impression is of stillness and quiet. The calls of birds up in the canopy are muffled by the leaves. There may be barely any breeze even as branches of the trees far above wave about in the wind. Friction with the leaves and branches of the tree crowns slows down the wind, so only the uppermost parts of the canopy get the full force of it. The wind speed tends to be at its least in the lower part of the canopy where the high density of leaves blocks movement of air. Down below on the more open forest floor, a light breeze may sometimes blow through between the trunks of the trees.

While overheating is not a problem on the forest floor, and dehydration is much reduced, the plants that grow there have their own problems to cope with. In a really dense forest—such as primary tropical rainforest or under a dark boreal conifer forest—more than 97% of the daylight may be filtered out by the canopy. The light levels are so low that it can be difficult to get a good photo without using a flash. In this twilight, photosynthesis can only be carried out slowly, and there is just a sparse layer of plants on the forest floor, many of them barely making a living.

The spectrum as well as the amount of the light is very different at the forest floor compared with the canopy. There is almost no UV, and blue and red light have been filtered out by chlorophyll so what is left is mostly green. The ratio of red to far red light is also shifted by the sunlight passing through leaves above, with chlorophyll in the photosynthetic cells absorbing most of the red.

Since there is not much photosynthesis going on under the canopy, CO2 is not used up quickly. Yet there is plenty of decay of fallen leaves and branches, pushing CO2 into the air. So, CO2 levels near the forest floor will often be several times higher than they are in the earth's atmosphere in general. In contrast, up in the rapidly photosynthesizing canopy, CO2 levels can be much lower than the "average" of the broader atmosphere. Effectively, within the forest there is a "carbon pump'', taking CO2 by photosynthesis and pulling it down (as dead leaves and other material) to decay on the forest floor.

Out of the drying influence of the sun, under a dense canopy of leaves the relative humidity can be much higher than above the canopy. Rain that has fallen and dampened the ground also adds greatly to the humidity. Where a stream passes through the forest, the evaporation of water from it tends to give even higher humidity and cooler temperatures to the nearby areas of forest floor.

Perhaps 50 meters above, the intense heating of the upper canopy by the sun tends to form a stable layer of air—less dense because it is warmer—floating within the canopy during the day. This stable cap of warmer air helps to seal off the forest floor from the world outside. CO2 gas released from respiration tends to build up during the day, until the inversion layer disperses in the evening.

Figure 4.6.

During the day, heating of the canopy leaves by the sun gives an

"inversion layer'' of warm air floating NIGHT

After the sun *

goes down, the canopy loses heat to the night sky, and the air floating just

WARM AIR HEATED BY SUN ON CANOPY

Inversion layer

RELATIVELY WARM AIR

Inversion layer

CANOPY SURFACE COOLS BY RADIATION

CANOPY SURFACE COOLS BY RADIATION

above it is left relatively warm, as it fails to cool down as fast.

However, the forest's interior is not totally insulated from the air above. To some extent, turbulence created by wind blowing over the upper surface of the canopy drags moist air up from within the forest, and spins dry hot air down inside. Convection rising from the hot leaves of the canopy also has a similar effect by sucking air up from below. This amount of air exchange with the surface tends to limit how much the forest can "make" its own interior climate by shade and by evaporation of water.

After sunset, air movement above the canopy tends to settle down. As the surface of the canopy cools off—radiating to the night sky—another "inversion layer'' may now form above it as the daytime air stays relatively warm (Figure 4.6). The evening chorus of monkeys and other creatures in tropical rainforests seems to take advantage of the boundary of this inversion layer to bounce sound sideways across the canopy, allowing them to send their signals much farther than they would be able to during the day. It is called an "inversion layer'' because it turns upside-down the normal pattern of temperature with altitude, which is for things to get cooler higher up.

If part of the forest has been cut, air blowing to the forest interior from the open ground at its edge is likely to have a very different temperature and humidity. The air entering the forest understory from recently cleared land has been heated by the full force of the sun, and there is not the dense mass of leaves to evaporate water and keep the air cool and moist. This "edge effect'' of dry hot air blowing in can alter the ecology of the forest floor and the lower parts of the canopy, with its influence extending some tens of meters into the forest. The presence of edges in both tropical and mid-latitude forests has been found to have noticeable effects on the types of understory plants that will grow there. Close to the forest edge, the plants that require low light levels and high humidity (see below) are replaced by tougher species that can cope with intense sunlight and dehydration.

Even with the edge effect diluting the influence of the forest, the contrast in temperature and humidity is immediately apparent to a casual observer stepping from underneath trees to an open area in direct sunlight. Studies of the microclimates of small grassy clearings around 10 or 20 metres wide have shown that they are around 2-4°C hotter during the day than the understory of undisturbed tropical forest, even without the direct heating effect of sun on the leaf surfaces which adds much more to the heat loading on plants. Extensive clearings of several hectares or more can get warmer still; generally speaking, the bigger the clearing the hotter it gets. The increased air temperature is due to the sparser leaf cover of the clearing: fewer leaves mean less evaporation of water to cool the air. But at night the situation is reversed. Temperatures stay slightly warmer under the closed forest than out in the clearing, because the dense canopy of leaves above blocks the loss of heat to the night sky. In the clearing there is no such blanket of leaves overhead, so infra-red is radiated out to the sky more easily.

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