Forest Ecology. Dan Binkley. Читать онлайн. Newlib. NEWLIB.NET

Автор: Dan Binkley
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: Биология
Год издания: 0
isbn: 9781119704416
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water molecules are close enough to clay surfaces to slow their mobility. Water molecules that interact with surfaces have a lower (more negative) water potential than free water. The story of potentials also explains how water moves up trees, ascending tens of meters (or even one hundred meters) upward against the pull of gravity. Dry air has a tremendously negative potential compared to the insides of leaves, so water is sucked from leaves into the air, driving a potential gradient that goes against the gradient provided by gravity (Chapter 4).

      Forests have a complex relationship with wind. For example, wind moving past a leaf cools the leaf in two interacting ways. Leaves have a “boundary layer” of air that restricts the exchange of energy, water and CO2 with the atmosphere at large. Winds strip away the outer portion of the boundary layer, making it thinner and facilitating more transfer of energy and matter. How much does this matter? A leaf exposed to sunshine with no wind might have a temperature about 3–5 °C cooler in a breeze than in still air (as noted earlier in this chapter). If the tree is well supplied with water and the air's vapor pressure deficit is not a problem, the leaf would be another 3 or 4 °C cooler, for a total temperature difference of about 7 °C (Knoerr and Gay 1965). This difference in temperature in relation to wind (with interacting effects on transpiration) would be large enough to change photosynthesis and respiration in a leaf by 10–50% (Figure 2.4).

      Moving from the scale of leaves and minutes up to trees and years, wind affects the way trees form stemwood. Trees may experience high wind if they grow in a windy location, or if they're at the edge of a forest with few sheltering neighbors. Tree crowns act like sails on ships, catching the wind and enduring very large forces that bend stems. Trees that are chronically exposed to strong winds develop a strong taper, broad at the base and thin at the top. Trees that are less exposed to wind have less taper, with tall slender stems that decline less in diameter going up the tree. Some of the windiest environments occur at high elevation, where crowns appear to be pushed to the downwind side of trees (flagging), or even pressed down close to the ground (krummholz, from German, “twisted wood”). The upper‐elevation limit of tree occurrence may relate as much to severe winds as to short growing seasons. High winds contribute great stresses in these extreme locations for trees, including wind‐blown ice crystals that damage leaves.

Photos depict a few spruce trees, in this mixed forest in central Germany. Treefalls create gaps in the canopy that allows more light, water, and nutrients to be available for neighboring trees and understory plants.

      Source: Skłodowski 2020 used by permission.

      For learning purposes, it may be reasonable to talk about patterns and processes of energy, temperature, water and wind separately. Forests are influenced and respond to all of these all at once, with complex interactions that have legacies that can last for centuries. We can enter a forest and look around, employing knowledge and measurements to understand what's going on currently in the forest, including temperatures, water use by trees, and connections with growth. The current composition and structure of the forest resulted from a legacy of environmental factors in the past, especially the big events of storms, fires, and insect outbreaks. These historical events often leave traces that can be unearthed with careful study. The future of any forest will depend on the current operations of environmental factors, on the historical legacies of past major events, and on big events that may or may not happen soon. Any forest can be examined with these three questions (what's up with this forest, how did it get that way, and what's next?), but the answers always require an inconvenient amount of local detail. And when it comes to “what's next?”, only broad, hazy insights are possible because big events just can't be predicted with much clarity.

Photos depict the forest that was a mosaic of trees within a matrix of small grassy meadows

      (Source: photo by H. Krauch, US Forest Service photo 16974A).

      A partially logged forest would require a windstorm of over 200 km hr−1 for a fire to spread from crown to crown across the forest (which might be strong enough to topple the trees). With heavy cattle grazing and fire suppression for eight decades, the grass meadow matrix was replaced by a high density of closely packed trees (white arrow points to the same rock;

      Source: photo by Andrew Sanchez‐Meador),

      and a wind of only 50 km hr−1 could spread a crown fire. For a related, spatially explicit example, see Figure 11.14.

      (Source: D.W. Huffman, J.D. Bakker, D.M. Bell, and M.M. Moore, unpublished).