physics of water transport through soils and trees to evaporate into the air depends on water potential. Water potential can be thought of as a gradient, similar to a gradient in elevation. A drop of water sitting at rest in a puddle would have a low potential; it would not be possible to obtain work from movement of the water, and its potential could be defined as zero. But if the water could follow the gravitational gradient into the soil, that movement might have an opportunity to do work (though not much!). In this case, a potential for the water at the soil surface would be zero, and the potential of water deeper in the soil would be less than zero (a negative value). Movement along gradients goes from higher potential to lower potential, and zero is higher than negative numbers.
Water at the soil surface might move into the soil along a potential gradient that does not relate to gravity. A key feature of water molecules is an imbalance in electrical charge from one side of the molecule (slightly positive) to the other side (slightly negative). This polar aspect of water makes molecules line up with each other, providing surface tension to water drops. It also causes water to adsorb (stick) onto surfaces such as soil particles. Indeed, the potential for water being adsorbed onto surfaces of soil particles is very low (a large negative value), which means water in a puddle can be “sucked” into dry soil, faster than movement from gravity alone.
The sizes of mineral soil particles are important for influencing water infiltration into soil, movement through the soil, and storage between wetting events (see Chapter 6). The smallest particles are clay‐sized, meaning <2 μm. One gram of clay has more than 1 m2 of surface area to interact with water, so most 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).
Wind Shapes Trees and Forests
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.
Trees experience “low‐to‐average” wind conditions most of the time, but severe wind events can bring forces on tree crowns that are higher than any time in the previous decade or century. When the severe event is not too far beyond the typical range, individual trees topple over, leaving gaps in the forest canopy and to some extent in the soil (Figure 2.20). Trees that blow over and create gaps typically are uprooted, with a large amount of soil raised up and a pit left behind. In other cases, the stem may resist the force of the wind less well than the roots, and the stem snaps off above ground. Severe wind events that greatly exceed typical storms, such as tornadoes, hurricanes and typhoons, can uproot or snap‐off most trees across large areas (Chapter 10).
FIGURE 2.20 Severe winds that are not too extreme may topple individual trees within a forest, leaving most of the canopy intact. High winds toppled many beech trees, and 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 (including small trees). Abrupt edges between harvested and unharvested forests can expose trees on the edge to higher winds, breaking off stems or uprooting trees (bottom left, Vancouver Island, Canada). Severe storms can level entire forests, breaking off stems of trees that are too solidly anchored to uproot, as in this former Scots pine forest in Poland (picture from one year after the storm.
Source: Skłodowski 2020 used by permission.
Events and Interactions Are More Important Than Averages and Single Factors
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.
FIGURE 2.21 What will happen in this forest on a windy day in June? Back in 1928, the forest was a mosaic of trees within a matrix of small grassy meadows
(Source: photo by H. Krauch, US Forest Service photo 16974A).
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