Читать книгу Forest Ecology - Dan Binkley - Страница 21

Over the course of an hour, the tree leaves would intercept about 140 MJ of sunlight, and about half of the light arrives at wavelengths that can be used in photosynthesis. Perhaps 10–15% of the light reaching leaves reflects back into the environment, with no effect on the leaves. About one‐third to one‐half is converted to heat, warming the leaves, which then lose heat to the surrounding air (especially if the wind is blowing). Most of the rest of the intercepted energy is consumed as water evaporates from moist leaf interiors into the dry air, also cooling the leaves.

FIGURE 1.1 The Tree. This tulip poplar is a typical tree for temperate forests. The tree may live for a few centuries, integrating daily, seasonal, and yearly fluctuations in environmental conditions to turn carbon dioxide and water into wood (and thousands of types of chemicals).

A few percent of the radiant energy hitting leaves is harnessed to drive photosynthesis, producing about 30 g of sugar in this tulip poplar in an hour. The carbon contained in the newly formed sugar enters the leaves as carbon dioxide (CO₂) during the same hour as the light interception. Small, adjustable openings (stomata) in the underside of the leaves allow CO₂ to diffuse into the interior of the leaves as photosynthesis depletes the concentration of CO₂ inside leaf cells. The rate of diffusion from the air into the leaf depends on the difference in concentration between CO₂ in the atmosphere and inside the leaf. The air has about six times the concentration of CO₂ that would be found inside photosynthesizing leaves, providing a steep gradient for the movement into the leaves. The remarkable biochemical processes in the leaves depend on the presence of more than a dozen elements in the tree, including 500 g of nitrogen (N) and 50 g of phosphorus (P). The bulk of these nutrients were taken up from the soil earlier in the season, but a sizable portion came from reserves that were recycled from last year's leaves and stored over winter in the wood.

The 30 g of sugar produced during an hour would be associated with a release of about 30 g of oxygen (O₂), as oxygen is released when water is split as part of photosynthesis. It may seem that this oxygen could be an important source of oxygen for the atmosphere, but it isn't. As with all accounting in ecology, half a picture might lead to the wrong conclusion. The sugar produced by the tree may be “respired” fairly soon to support the growth of new cells or to maintain old cells, and oxygen is consumed (reforming water) in this reaction. Some of the sugar ends up in longer‐lived cells, but even these tend to be oxidized back to CO₂ over years or centuries. Unless the carbon content of a forest increases across generations of trees, the generation of oxygen in photosynthesis is matched by consumption during respiration and decomposition, leaving no extra oxygen in the atmosphere.

Some of the sugar produced by photosynthesis is consumed within the leaf to produce and support the metabolic needs of cells in the leaf. More than three‐quarters of the sugar is loaded into the phloem and sent to flowers, twigs, branches, stems, roots and symbiotic root fungi (mycorrhizae).

Exposing the moist interiors of leaves to the dry air allows for uptake of CO₂, but also allows water to be pulled into the dry air. The production of one molecule of sugar entails an unavoidable loss of hundreds of molecules of water. The production of 30 g of sugar in an hour would be accompanied by a far greater loss of water, perhaps 10 liters (10 kg) of water. The water transpired by the leaves during an hour of photosynthesis would have been found lurking in the soil a day earlier, and may have been in the atmosphere a day or a week before.

The tree has tremendous surface area developed within the soil to facilitate uptake of water and nutrients. The surface area of fine roots may be in the order of 100 times the surface area of leaves in the crown, and the surface area of mycorrhizal fungi that colonize roots contribute more than 10 times the surface area of roots. This vast surface area of absorbing roots and fungal mycelia collects water (and nutrients) that move up through the sapwood of the tree. The sapwood is comprised of xylem vessels, each measuring about 0.1 mm in diameter by 1 mm in length. The water passes through more than 1000 vessels for every meter of tree height, taking half a day or a day to move from all the way from roots to leaves.

Lifting water from the soil to the crown requires energy to overcome gravity, about 300 J for 10 liters. This is a tiny amount of energy compared to energy consumed as liquid water in leaves becomes water vapor in the atmosphere (about 2400 kJ for each liter, or 24 MJ for 10 liters). All the energy consumption and dissipation by the tree crown result in a deep shade beneath the tree. The air temperature in the shade may be a degree or two cooler than the air above the crown, but the shade will feel much cooler to a person sitting under the tree because of the greatly reduced energy load from the incoming sunlight.