Читать книгу Forest Ecology - Dan Binkley - Страница 34

A leaf increases in thermal energy when exposed to radiant energy from the sun, and when exposed to hot air. Absorption of solar radiation raises leaf thermal energy, raising leaf temperature. The temperature of the leaf continues to rise until the gain of energy is offset by energy losses. Leaves can lose energy by exposure to cooler air, by evaporating water, and by emitting (shining) radiant energy.

FIGURE 2.5 On an afternoon when air temperature was 25 °C, the temperatures of leaves of beech and spruce were sustained at higher levels as a result of heating by sunlight. The temperatures of leaves covered a range for each species, because of varying angles of leaf exposure to the sun's rays, and the variable levels of shade in the crowns. Beech leaves were hotter (most were about 30 °C) than spruce leaves (most were about 27 °C), indicating beech leaves had higher energy gain (absorbing more light), or lower energy loss (lower transpiration, or poorer coupling with the cooler air).

Source: Data from Leuzinger and Körner (2007).

Objects such as tree leaves and bird feathers, lose energy to the air by conduction and convection if the air is cooler than the object (Figure 2.5). The rate of energy transfer into the air depends on the gradient in temperatures, and how well the object is “coupled” to the air. Large surfaces, such as beech leaves, have stable layers of air molecules comprising a thick boundary layer, slowing energy transfer from the leaf into the air at large. Small surfaces have thinner boundary layers allowing faster transfer of thermal energy; the boundary layer for the spruce needles was only one‐fourth that of the beech leaves, explaining a large portion of the difference in temperatures between species. The coupling of surfaces to the atmosphere is improved by winds, as the thickness of the boundary layer declines with increasing wind. A wind of a few meters per second (a few kilometers per hour) can lower leaf temperatures by a few degrees, and increase the delivery of CO₂ to stomata along with loss of water to the air.

The evaporation of water requires large amounts of energy, about 2.4 MJ/l evaporated. This energy flow is referred to as “latent heat,” because the temperature of the water molecules remains unchanged as the phase changes from liquid to gas. The amount of energy removed from an object such as a leaf depends of course on the rate of evaporation, and rates of evaporation depend on the water status of the plant, the dryness of the air, and the presence of wind to reduce the boundary layer. Evaporation from a leaf exposed to dry air without any wind may lower leaf temperature by about a degree; the addition of a light wind can increase evaporation enough to cool leaves by 3–5 °C.