Microclimates are caused by local differences in the amount of heat or water received or trapped near the surface. A microclimate may differ from its surroundings by receiving more energy, so it is a little warmer than its surroundings. On the other hand, if it is shaded it may be cooler on average, because it does not get the direct heating of the sun. Its humidity may differ; water may have accumulated there making things damper, or there may be less water so that it is drier. Also the wind speed may be different, affecting the temperature and humidity because wind tends to remove heat and water vapor. All these influences go into "making" the microclimate.
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4.1.1 At the soil surface and below
Soil exposed to the sun heats up during the day and cools during the night. Within a few centimeters of the surface, the temperatures during the day can be extreme: 50 °C or more in a dry desert climate when there is no water to evaporate and cool the soil. Even high on mountains, exposed dark soil surfaces heated directly by the sun can reach 80 °C—hot enough to kill almost any lifeform.
At night a bare soil surface cools off rapidly and by morning it may end up more than 20 °C cooler than during the day. Yet, only 15 cm down the fluctuation between night and day is only about 5°C, because the day"s heat is slow to travel through soil. Thus, the soil at depth has its own quite separate climate: a microclimate distinct from that at the surface. Down at 30 cm there is essentially no difference between temperature of night and day because the soil is so well insulated from the surface; it stays at about the average temperature of all the days and nights combined over the last few weeks. At about 1 meter depth, there is no difference between temperatures in winter and summer—the soil remains right at the yearly average without fluctuation.
These differences are all-important to plant roots and the small animals and microbes that live within the soil. At depth, the extremes of heat or cold are much less and survival is often easier. But in high latitudes where the average annual temperature is too low, below —3°C, the soil at depth always remains frozen, for it is never reached by the heat of the summer. Water that once trickled down into the soil forms a deep layer of ice, known as permafrost, that may stay in place for many thousands of years. Where there is permafrost, roots cannot penetrate and plants must make do with rooting into the surface layer above which at least thaws during the summer. In the far north, patches of trees in the tundra seem to promote the formation of permafrost in the soil underneath themselves. The freezing of the soil eventually kills the roots, causing the trees to die and give way to tundra again. Permafrost forms under these tree patches because, in the shade cast by the leaves and branches, there is no direct heating of the ground by sunshine in the peak of summer. The frozen sub-surface of the soil never thaws out, and that equals permafrost. This is despite the fact that the covering of trees absorbs sunlight and heats up the air above the ground in the warmer months, and warms the local and regional climate overall (see Chapter 5). This extra warming does not reach into the ground, however; at least not strongly enough to compensate for the lack of the intense direct heating of the sun that would be found on open tundra soil in summer.
4.1.2 Above the surface: the boundary layer and wind speed
If we now go upwards from the soil surface into the air above, there is another succession of microclimates. When wind blows across bare soil or vegetation, there is always some friction with the surface that slows the wind down. This slowing down causes the air just above the soil to form a relatively still layer known as the boundary layer. Within a few millimeters of the soil surface, the friction is severe enough that the air is almost static (Figure 4.1). Air molecules are jammed against the surface, and the molecules above them are jammed against the air molecules below, and so on. Moving up a few centimeters or tens of centimeters above the surface, the dragging influence of friction progressively lessens as the "traffic jam" of air molecules gets less severe, and there is a noticeable increase in average wind speed because of this. In fact, what with the decreasing friction from plants, trees, buildings, etc. the average wind speed keeps on increasing with higher altitudes, until it really tears past a mountain top. It is no coincidence that the strongest wind gust ever recorded was at the top of a mountain (372km/hr at the summit of Mount Washington, USA). Even so, mountains are not always windy. Some days in the mountains will have hardly any breeze, when the weather favors calm conditions.
In a sense there is a succession of boundary layers, each on top of one another and with the air higher up moving faster. "Boundary layer" is really a relative term: it is a layer of slower moving air caused by being closer to a rough surface, below a faster moving one above that is less affected by the surface. The term "boundary layer" is used at many different scales in the field of climatology, and can be very confusing because different sub-disciplines each use the term in their own way at the scale they are most interested in.
The boundary layer fundamentally affects the heat balance at the surface and in the air above, up to the height of a few centimeters or a few meters. If sunlight is hitting the surface, being absorbed and heating the surface up, heat is being
(Wind speed increases with height from surface)
Figure 4.1. The boundary layer over a surface. Source: Author.
conducted gradually to the air above it. The relatively static air in the boundary layer will be able to heat up as it is close to the surface, and because it stays still and accumulates heat it will be quite a bit warmer than the mixed air in the wind above. As this boundary layer air is not being continually whisked away, the surface will not lose heat as fast either. In effect, the warmed boundary layer air acts like a blanket over the surface. The thicker the blanket, the warmer the surface can become. If the surface below the boundary layer air consists not of soil but of living leaves (as it does above a forest canopy, for instance), this extra warmth can be very important for their growth and survival. In a cold climate, there may be selection on the plants to maximize the thickness and the stillness of the boundary layer. In a hot climate, on the other hand, the plants may be selected to disperse the boundary layer, to prevent the leaves from overheating.
So, in a layer of still air the temperature can be several degrees higher than the mixed-in air just above it. This can make a lot of difference to the suitability of the local environment for particular plants and animals. For instance, in a tundra or high mountain environment, at the very edge of existence for plants, this small amount of shelter can determine whether plants can survive or not. On the upper parts of mountains, with strong winds and short grassy vegetation, a local boundary layer can make a big difference to the temperature the plants experience. If a spot is sheltered—for instance, between rocks or in a little hollow—the wind speed is also lower; there is a small space of static air with almost no wind movement. On a mountain slope in the mid or low latitudes, the intense sunlight can deliver a lot of energy directly to the surface. If the shelter of a hollow prevents this heat from escaping to the cold air above, it can become much warmer and types of plants that require more warmth are able to survive.
By making their own boundary layer climate, plants can turn it to their advantage. The upper limit to where trees can grow on a mountain—the tree-line—occurs below a critical temperature where the advantage shifts from trees towards shrubs or grasses. Trees themselves standing packed together create a layer of relatively still air amongst them that can trap heat, but there comes a limit up on a high mountain slope at which this heat-trapping effect is no longer quite enough for trees to form a dense canopy. In a looser canopy, much of the heat-trapping effect collapses and suddenly beyond this point the trees are left out in the cold. This effect helps to produce the sudden transition in vegetation that is often seen at a certain altitude up on many mountains.
Often, right above the treeline on a mountain, dense woody shrubs take over. It is thought that shrubs can thrive at mountain temperatures too cold for trees because they can create a strong boundary layer against the wind among their tightly packed branches. Wind cannot blow between the branches, so the sun"s direct heat is not carried away as fast, and their leaves can thrive in the warmer temperatures of the trapped air (Figure 4.2). Trees, by contrast, have a much looser growth form; so, if they are standing out on their own the wind can blow straight through their branches and carry away the sun"s heat. Shrubs—with their heat-trapping growth form—can keep their leaves as much as 19°C warmer than the trees, making all the difference between success and failure in the high mountains.
Figure 4.2. Shrubs trap more heat amongst their branches than trees do, because the wind cannot blow between the tightly packed branches of a shrub. Source: Author.
Higher even than shrubs can grow on a mountain is the "alpine" zone of cushion plants (Figure 4.3*). These exquisite little plants, from many different plant families in mountains around the world, form a little dense tussock of short stems and tiny leaves. Many of them look at first sight like cushions of moss, but they are flowering plants—often producing a flush of pretty flowers on their surface in the summer. The cushion plant growth form seems to be adapted to a version of the same trick that mountain shrubs use. A cushion plant, which needs all the heat it can get, creates a miniature zone of static air in the small gaps down between its tightly packed leaves. Leaves within the tussock are heated directly by the sun, and because the wind cannot blow between them everything within the tussock stays warmer. The plant is able to photosynthesize, grow and reproduce in an extreme environment by creating its own miniature boundary layer and microclimate amongst the leaves. Measurements show that on sunny days in the mountains, the leaf temperature of these cushion plants is often 10 to 20°C higher than the air immediately above. One reason why such alpine cushion plants are difficult to grow in sunny, warm lowland climates is that they are so good at trapping heat. They essentially fry themselves when ambient temperatures are already warm, raising their own leaf temperatures to levels that would also kill any lowland plant.
* See also color section.
Many cushion plants use an additional trick to trap heat: above the dense cushion of leaves is a layer of hairs—transparent, and matted. These act like a little greenhouse, letting in sunlight and trapping warmed air underneath because it is not carried away by convection or by the breeze. This miniature greenhouse significantly increases the temperature of the leaves underneath, presumably resulting in more photosynthesis and better growth.
4.1.3 Roughness and turbulence
Although an uneven surface creates a boundary layer by slowing the air down, it can actually help set the air just above the boundary layer in motion by breaking up the smooth flow of the wind. The surface of a forest canopy, with lumpy tree crowns and gaps between them (Figure 4.4*), can send rolling eddies high up into the air above. This turbulent zone created by the canopy often reaches up to several times the height of the trees themselves. A more miniature turbulent layer will also be created above scrub vegetation when the wind blows across open ground between the bushes and then jams against their leaves and branches. Generally, whatever the height of the biggest plants in the ecosystem, the rolling turbulence that they create will extend for at least twice their own height into the atmosphere above.
The turbulent microclimate created by air blowing over uneven vegetation surfaces also helps to propel heat and moisture higher up into the atmosphere, altering the temperature on the ground and feeding broader scale climate processes. In Chapters 5 and 6 we will see various case studies where changes in vegetation roughness seem to affect climate quite noticeably.
4.1.4 Microclimates of a forest canopy
The canopy and understory of a forest are like two different worlds, one hot and illuminated by blinding sunlight, the other dark, moist and cool. Parts of a large forest tree can extend all the way between these two worlds, and trees will often spend their early years in the deep shade before pushing up into the light above. Both the canopy and the understory microclimates present their own distinct challenges, and the plants need adaptations to meet these.
It is remarkable how hot the surface of a temperate or tropical forest canopy can become on a sunny summer"s day, with leaf temperatures exceeding 45°C. In tropical rainforests, although it is cloudy and humid much of the time, a few sunny hours are enough to dry out the air at the top of the canopy and really bake the leaves.
It is critical that a leaf exposed to strong sunlight keeps itself cool enough to avoid being killed by heat. A leaf can lose heat very effectively by evaporating water brought up by the tree from its roots; the heat is taken up into the latent heat of evaporation, vanishing into water vapor in the surrounding air—it is the same principle by which sweating cools the human body. Evaporation from the leaves occurs mostly through tiny pores known as stomata, which are also used to let CO2 into the leaf for photosynthesis (see Chapter 8). When the evaporation occurs through these stomata, ecologists call it "transpiration". As we shall see in the later chapters of this book, both the heat uptake and the supply of water to the atmosphere by transpiration are also important in shaping the regional and global climate.
Slowing down heat loss by transpiration presents a dilemma for the plant. On one hand, if its stomata are open and it is transpiring, a leaf can keep cool. However, keeping cool in this way gets through a lot of water. If the leaves "spend" too much water, there is a risk that eventually the whole tree will die of drought because its roots cannot keep up with the rate of loss. Even if there is plenty of water around the tree"s roots, the afternoon sun can evaporate it from leaves faster than the tree can supply it through its network of vessels. If water is indeed limiting, the leaves will shut their stomata to conserve it. Tropical forest leaves in sun-lit microclimates also have a thick waxy layer, to help cut down on evaporation when water is in short supply.
If leaves close their stomatal pores and swelter, they risk being damaged by heat. It is thought that certain chemicals which are naturally present in leaves, such as isoprene, may help to protect their cells against heat damage in situations where they cannot evaporate enough water to keep cool. A breeze over the forest canopy will always help the leaves to lose heat even without any transpiration going on, and the faster the wind blows the better the leaves will be able to cool. The size and shape of leaves can also be important in avoiding heat damage. A big leaf is at all the more risk of overheating than a small leaf, because it creates a wider, thicker boundary layer that resists the cooling effect of the breeze. These sorts of problems are thought to limit the size that leaves of canopy trees can reach without suffering too much water loss or heat damage. The only exceptions are big-leaved tropical "weed trees"" such as Macaranga, that can have leaves 50 cm across. They seem to keep themselves cool by sucking up and transpiring water at a high rate. Perhaps because of the risks of overheating, in temperate trees the "sun leaves"" (see below) exposed at the top of the canopy tend to be smaller than the "shade leaves"" hidden down below, even on the same tree.
The most intense aridity in the forest is likely to be felt by smaller plants that grow perched on the branches of the big trees: the epiphytes. In tropical and temperate forests where there is high rainfall and high humidity year-round, these plants are able to establish themselves and grow even without any soil to provide a regular water supply. But, because they are isolated from the ground below, and only rooting into a small pocket of debris accumulated on the branches, epiphytes are at the mercy of minor interruptions in the supply of water from above. When it has not rained for a while, epiphytes up in the canopy can only sit tight, either tolerating dehydration of their leaves or holding in water by preventing evaporation from their
waxy leaves. Some epiphytes live rather like cacti within the rainforest, having thick fleshy leaves that store water for times of drought. One very important group of epiphytes in the American tropics, the bromeliads, tends to accumulate a pool of rainwater in the center of a rosette of leaves. They are thought to be able to draw upon this water reserve to keep themselves alive when it has not rained for a while. Other bromeliads are able to tolerate drying out and then revive and photosynthesize each time it rains. One well-known example is Spanish moss (Tillandsia) which festoons trees in the Deep South of the USA.
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Sometimes trees can in effect water themselves. High up on many tropical mountains, around 2,000 m above sea level, are "cloud forests" which thrive in the layer where clouds tend to hit the mountain slopes (Figure 4.5*). The cloud droplets condense on leaves in the forest canopy and drip to the ground. Walking under the trees when clouds shroud the mountain, cold water condensed from the fog continuously drips onto the back of one"s neck. Often this contributes 30% or more of the water that reaches the trees" roots. Similarly, in northern California where coastal fogs constantly roll in off the sea, the water captured from fog droplets plays an important part in the survival of the giant redwoods (Sequoia sempervirens).