Microclimates and vegetation

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Climate on the broad scale, across hundreds of kilometers, brings about the broad-scalc distribution of vegetation types (Chapters 1 and 2). However, even looking at the world much more locally, wc see that there arc also very substantial differences in the average climatc. For example, a south-facing slope has a different climate from a north-facing one. The year-round temperature and rainfall conditions under a tree will be different from those just a few meters away in the open. The temperature right at the soil surface is different from the temperature a few centimeters under the surface.

Such local differences make up what are known as "microclimates". These arc little climates that exist to some extent everywhere and vary on a scale of a few tens of meters, a few centimeters or even a few millimeters. Such differences are all-important to plants, and also the animals that live amongst them.

Microclimates help to explain part of the patchiness in vegetation that occurs on smaller scales; they determine which plants can grow where. They are also important in understanding how so many different species of plants manage to coexist, without them all being out-competed by one strong species. And microclimates can explain certain features of growth form, leaf shape and physiology of plants.

Furthermore, microclimates are the building blocks of climate. The broad-scale climate is in part the product of these countless little climates, added up and averaged out. If we really want to understand how climate on the global scale is made, including how plants themselves help to form it (Chapters 3 and 4), we have to understand microclimates.


Microclimates arc 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.

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, the bare soil surface cools off rapidly and by morning it may end up more than 20°C cooler than during the day. Yet, only 10cm 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 arc much less and survival is often easier. But in high latitudes where the average annual temperature is too low, below 0 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 that at least thaws during the 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 arc 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

(Wind speed increases with height from surface)

(Wind speed increases with height from surface)

Boundary layer (nearly static air)

Figure 4.1. The boundary layer over a surface. Source: Author.

Figure 4.1. The boundary layer over a surface. Source: Author.

fact, what with the decreasing friction from plants, trees, buildings, etc. the 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).

The boundary layer fundamentally a fleets 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 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 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, below 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 surfacc. 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 own advantage. The upper limit to where trees can grow on a mountain the treeline occurs below a critical temperature where the advantage shifts from trees towards shrubs or grasses. Frees themselves standing packed together create a layer of relatively still air amongst them that can trap heat, but there conies a critical point 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 cflcct 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 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.

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, bui they are flowering plants—often producing a flush of pretty flowers on their surfacc 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 arc difficult to grow in sunny, warm lowland climates is that they arc so good at trapping heat. They essentially fry themselves when ambient temperatures

* Sec also color section.

Lotus Flower With Hands Clipart
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.

are already warm, raising their own leaf temperatures to levels that would also kill any lowland plant.

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 becausc 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, 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

Alpine Cushion Plants
Figure 4.3. An alpine cushion plant. S He tie exscapa. The growth form of cushion plants maximizes trapping of heat in the cold high mountain environment. Source: Christian Koerner.

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 they also use to let CO: 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 arc 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 wrhen 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. 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 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.

'I'he most intense aridity in the forest is likely to be felt by smaller plants that grow pcrchcd 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 arc able to establish themselves and grow even without any soil to provide a regular water supply. But, because they arc 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 bromcliads, tends to accumulate a pool of rainwater in the ccntcr of a rosette of leaves. They arc thought to be able to draw upon this water reserve to keep themselves alive when it has not rained for a while. Other bromcliads 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 IJSA.

4.1.5 Under the canopy

In the cooler forest undcrstory, out of the direct sun. overheating is not a problem and leaves can grow bigger than at the top of the canopy. Many of the types of plants that grow down near the floor of the forest have large plate-like leaves 30 cm or more across; undivided leaves this size arc hardly ever seen up in the forest canopy.

On the forest floor, the overwhelming impression is of stillness and quiet. The calls of birds up in the canopy arc muffled by the leaves. There may be barely any breeze even as branches of the trees far above wave about in the wind. Friction with the leaves and branches of the tree crowns slows down the wind, so only the uppermost parts of the canopy get the full forcc of it. The wind speed tends to be at its least in the lower part of the canopy where the high density of leaves blocks movement of air. Down below on the more open forest floor, a light breeze may sometimes blow through between the trunks of the trees.

While overheating is not a problem on the forest floor, and dehydration is much reduced, the plants that grow there have their own problems to cope with. In a really dense forest—such as primary tropical rainforest or under a dark boreal conifer forest more than 97% of the daylight may be filtered out by the canopy. The light levels are so low that it can be difficult to get a good photo without using a flash. In this twilight, photosynthesis can only be carried out slowly, and there is just a sparse layer of plants on the forest floor, many of them barely making a living.

The spectrum as well as the amount of the light is very different at the forest floor compared with the canopy. There is almost no U V, and blue and red light have been filtered out by chlorophyll so what is left is mostly green. The ratio of red to far red light is also shifted by the sunlight passing through leaves above, with chlorophyll in the photosynthetic cells absorbing most of the red.

Since there is not much photosynthesis going on under the canopy, CO: is not used up quickly. Yet there is plenty of decay of fallen leaves and branches, pushing C02 into the air. So, C02 levels near the forest floor will often be several times higher than they are in the earth's atmosphere in general. In contrast, up in the rapidly photosynthesizing canopy, C02 levels can be much lower than the "average" of the broader atmosphere. Effectively, within the forest there is a "carbon pump'1, taking C02 by photosynthesis and pulling it down (as dead leaves and other material) to decay on the forest floor.

Out of the drying influence of the sun, under a dense canopy of leaves the relative humidity can be much higher than above the canopy. Rain that has fallen and dampened the ground also adds greatly to the humidity. Where a stream passes through the forest, the evaporation of water from it tends to give even higher humidity and cooler temperatures to the nearby areas of forest floor.

Perhaps 50 meters above, the intense heating of the upper canopy by the sun tends to form a stable layer of air—less dense bccausc it is warmer—floating within the canopy during the day. This stable cap of warmer air helps to seal off the forest floor from the world outside. CO: gas released from respiration tends to build up during the day, until the inversion layer disperses in the evening.

However, the forest's interior is not totally insulated from the world above. To some extent, turbulence created by wind blowing over the upper surface of the canopy drags moist air up from within the forest, and spins dry hot air down inside. Convection rising from the hot leaves of the canopy also has a similar cflcct by sucking air up from below. This amount of air exchange with the surfacc tends to limit how much the forest can "make" its own interior climate by shade and by evaporation of water.

After sunset, air movement above the canopy tends to settle down. As the surface of the canopy cools off radiating to the night sky another "inversion layer" may now form above it as the daytime air stays relatively warm. The evening chorus of monkeys and other creatures in tropical rainforests seems to take advantage of the boundary of this inversion layer to bounce sound sideways across the canopy, allowing them to send their signals much farther than they would be able to during the day.

If part of the forest has been cut, air blowing to the forest interior from the open ground at its edge is likely to have a very different temperature and humidity. The air entering the forest understory from recently cleared land has been heated by the full force of the sun, and there is not the dense mass of leaves to evaporate water and keep the air cool and moist. This "edge effect" of dry hot air blowing in can alter the ecology of the forest floor and the lower parts of the canopy, with its influence extending some tens of meters into the forest. The presence of edges in both tropical and mid-latitude forests has been found to have noticeable effects on the types ol understory plants that will grow there. Close to the forest edge, the plants that require low light levels and high humidity (see below) are replaced by tougher species that can cope with intense sunlight and dehydration.

Even with the edge effect diluting the influence of the forest, the contrast in temperature and humidity is immediately apparent to a casual observer stepping from underneath trees to an open area in direct sunlight. Studies of the microclimates of small grassy clearings around 10 or 20 metres wide have shown that they arc around 2-4 C hotter during the day than the understory of undisturbed tropical forest, even without the direct heating effect of sun on the leaf surfaces which adds much more to the heat loading on plants. Extensive clearings of several hectares or more can get warmer still; generally speaking, the bigger the clearing the hotter it gets. The increased air temperature is due to the sparser leaf cover of the clearing: fewer leaves mean less evaporation of water to cool the air. But at night the situation is reversed. Temperatures stay slightly warmer under the closed forest than out in the clearing, because the dense canopy of leaves blocks the loss of heat to the night sky. In the clearing there is no such blanket of leaves overhead, so infra-red is radiated out to the sky more easily.

4.1.6 Big plants "make" the microclimates of smaller plants

The plants that live on the forest floor—at low light levels, milder temperatures and higher humidity—are specialized to a microclimate made for them by the canopy trees that absorb most of the sunlight. Their photosynthetic chemistry is specialized to low light levels and they cannot cope with direct sunlight. These forest floor plants tend to have soft leaves, because leaves underneath the canopy have no need to be "toughM—they arc not blown about by the wind, nor arc they dehydrated in direct sunlight. An example of one of these forest floor plants is the African violet (Saintpaulia), a common house plant which requires shade. As many houseplant owners know all too well, it dies quickly when exposed to direct sunshine.

Some forest floor plants have peculiar adaptations to help them gather as much as possible of the light that falls upon them. Certain herbaceous plants such as the southeast Asian vine spike moss (Selaginella wilhlenowii) and some species of Begonia (Figure 4.4*) have a bluish sheen (known as iridescence) to their leaves. This is caused by little silica beads within the epidermis of the leaf. Experiments have suggested that these beads help the leaf to focus in light from a range of directions, sending it straight into the photosynthetic cells below. In Selaginella each cell underneath a silica bead has a single large chloroplast which seems to be precisely located to receive this focused beam of light.

The leaves at the top of a tree also make the microclimate for the leaves below them. Even on the same tree, leaves that arc out in full sunlight develop slightly differently from those in the shaded branches down below. The "sun leaves" are thicker with more layers of photosynthetic cells packed in, to take advantage of the abundant light. The lacquer-like cuticle on the upper surface of a sun leaf also tends to be thicker, to help reduce unnecessary evaporation. On a sun leaf there are more stomata the pores which open to let C02 in so that the leaf can take advantage of high light levels to bring in more C02 for photosynthesis when it has enough water. As soon as evaporation through the stomata becomes too intense and the leaf is in danger of dehydrating, the stomata are clamped shut and the leaf relies on its cuticle to prevent further water loss.

The chemistrv and color of sun leaves also tends to be different from shade leaves. Shade leaves tend to be a darker green because they are richer in a particular dark green form of chlorophyll (chlorophyll b) that is good at harvesting light at low intensities and at the wavelengths filtered by leaves above. Sun leaves have more of the chlorophyll a form which exploits high light intensities more effectively. The upper epidermis of sun leaves is also packed with natural sunscreen compounds such as flavenoids which absorb most UV light and prevent it from damaging the sensitive photosynthetic cells below. Just putting a shade-grown tree seedling out into direct sunlight shows how important this protection is: in a few days the shade-grown leaves are bleached and useless.

Tropical Microclimate

Figure 4.4. This species of Begonia lives in the understory of mountain rainforests in southeast Asia. The bluish metallic "sheen" of many species of rainforest understory plants is thought to come from the refractive effect of silica beads which help to gather in light for the leaves. Source: Author.

Figure 4.4. This species of Begonia lives in the understory of mountain rainforests in southeast Asia. The bluish metallic "sheen" of many species of rainforest understory plants is thought to come from the refractive effect of silica beads which help to gather in light for the leaves. Source: Author.

Tree seedlings often survive for years in the shaded forest floor environment. Depending on how much light they are getting, they may either stay more or less the same size, or slowly grow up into the canopy. These young trees from the forest floor can go through different phases in their life, with physiological adaptations to different light levels. For example, many of Australia's Eucalyptus trees have an "early" phase with an entirely different leaf form, suited to growing at low light levels within the darker forest interior. Typically, the juvenile leaves of such cucalypts form a disk with the stem in the center, while the adult leaves are long and strap-shaped. It is thought that the disk-like leaves—arranged along the stem like a kebab are good for harvesting light coming down from a small gap in the canopy above; it helps to keep the photosynthetic area "all lined up" within a shaft of sunlight as the seedling grows its stem up to follow the light.

Often a young tree on the forest floor will only really be able to start growing fast when a bigger tree or a large branch—falls from the canopy to give a patch of sunlight that illuminates its leaves. This is the turning point that gives the young tree enough energy to fix enough carbon to lay down wood and grow tall, rather than merely surviving.

The subtle range of opportunities provided by microclimates is thought to help maintain the species diversity of forests and other plant communities. A tropical rainforest can have more than two hundred species of trees packcd into a hectare, and in ecological terms it is difficult to explain how they can all manage to exist sidc-bv-sidc. Simple ecological theory suggests that eventually just one species that can compete more effectively should increase its numbers and push the rest out, so that it dominates the forest. Yet, obviously this does not happen. Ecologists suspect that part of the reason such exclusion of species does not happen is that small differences in light level, as well as soil texture and nutrient levels, determine which tree species gets established in any particular spot on the forest floor. It is thought that for trees in particular a critical stage which determines whether a species grows in any one spot is its early growth as a seedling and small sapling. Each spccies might be adapted when it is a seedling to a narrow range of light intensities, or light of particular wavelengths or angles. If it finds itself in its forte, it will out-compete seedlings of other species. Once that critical seedling stage is passed and the tree has established itself, it is essentially guaranteed a place in the forest. Although this is quite a compelling theory, there is still only limited evidence that this sort of specialization on the forest floor is important in the competition and survival of forest trees.

4.1.7 The importance of sun angle

Just as sun angle makes the difference overall between temperatures at different latitudes of the earth, it makes a significant difference on a local scale too. If a slope is angled towards the sun when the sun is low in the sky, it gets more of a full beam and so the surface temperature of soil or leaves (and the air just above) will be warmer. On a slope that is in the "wrong" direction relative to the sun. much of the day is spent in shadow or being sunlit at an angle, so it will be colder than if it had been on the flat.

On the equator, the sun travels a path right overhead and does not shine more on either a southern or a northern slope: in fact, it shines slightly more on east and west-facing slopes which catch additional energy from the sun around sunrise and sunset. At higher latitudes in the southern hemisphere, the sun tends to be in the northern half of the sky, so a north-facing slope will be warmer. In the northern hemisphere, south-facing slopes are warmest because the sun stays mostly in the southern half of the sky. For example, one study during a summer's day on a hill in Massachusetts found that the maximum temperature reached during the day was 3.5 C warmer on the south-facing slope than on ihe north-facing slope (Figure 4.5). In fact, in the mid-latitudes the sun does wander slightly into the "other" half of the sky during the early and late parts of the day during the summer; but always more energy is received from the south in the northern hemisphere, and from the north in the southern hemisphere.

Such local slope angle effects can make a difference to the ecology. A study on flowering times in a wooded valley in Indiana found that several species of wild-flowers bloomed about a week earlier on a south-facing slope than a north-facing one. This is because plants often need to be exposed to a certain amount of heat

Diff Parts

Figure 4.5. Distribution of temperatures on a sunny summer's day on a hill in Massachusetts. The more southerly-facing slope has warmer temperatures than the opposite slope facing northwards. Elevation from 340m to 540m by 40m. Temperature from 19.0 C to 22.5'C bv 0.5 C. After Bonan.

Figure 4.5. Distribution of temperatures on a sunny summer's day on a hill in Massachusetts. The more southerly-facing slope has warmer temperatures than the opposite slope facing northwards. Elevation from 340m to 540m by 40m. Temperature from 19.0 C to 22.5'C bv 0.5 C. After Bonan.

av during the season before they will flower; on the warmer sunlit slope this required "heat sum" was reached sooner.

The differences with aspect tend to be most striking for types of plants which are right at the edge of their ranges, and barely able to survive in the local climate. Sometimes, they are warmer-climate plants that are at the poleward edge of their range. For example, on sand dunes on the coast of eastern England there grows a type of wild lettuce known as prickly lettuce (Lactuca virosa) which is at the northern edge of its distribution range in Europe. In England it will grow only on the south-facing slopes of dunes, gathering just enough energy for itself to grow and set seed. On coastlines farther south in Europe (e.g., most of France) prickly lettucc grows on both the north and south sides of dunes bccausc the microclimate is warm enough even on the north sides, given the generally warmer air temperatures. Similarly, the stemlcss thistle (Onopordum acaulon) only grows on the south side of hills at the northern edge of its range in Yorkshire, northern England. In southern England, there is enough warmth for it to grow on both the northern and southern sides of hills.

As well as temperature, the severity of aridity differs between north and south-facing slopes. The stronger the beam of sunlight, the droughtier the conditions as more water is evaporated. In semi-arid areas of southern Europe, many "north European" plant species requiring cool damp climates only survive on north-facing slopes. 1 remember once walking on the steep northward-facing slope of a hill in Provence in the south of France. It was covered in beech (Fagtts sylvaiica) forest and in the shade and dampness of the understory 1 could for all the world have been in my rainy native land of England a strangely comforting form of dejci vu. Yet, when I topped the brow of the hill to the southern side, in the space of a few meters I was back into hot, dry air, surrounded by typical open Mediterranean scrub. The influence of sun angle had made all the difference between survival of deciduous forest, and its replacement by oily brush that burns every few years.

The difference in moisture availability with aspect can even be noticeable on a more miniature scale on tree trunks; the northern side of a tree trunk in northwestern Europe tends to have a lot more mosses growing on it than the drier, hotter south-facing side.

4.1.8 Bumps and hollows in the landscape have their own microclimate

As I mentioned above, a group of rocks that provides shelter can allow a pocket of still air to form on an exposed mountain slope. Small bowl-shaped hollows in the landscape, a few meters or even just a few centimeters across, can also act as solar energy collectors (like a parabolic satellite dish which concentrates the signal into the middle), gathering heat into the center to give a warmer microclimate. In tundra the grassy or shrubby vegetation which exists in very cold Arctic and alpine environments (Chapter 2) the extra heat concentrated in small hollows in the landscape is crucial to the growth of certain plants, and the survival of certain species of insects. This sort of heat-concentrating effect is also very common among the bowl-shaped hollows in coastal sand dunes at lower latitudes too; temperatures can be many degrees higher on a sunny day in the hollow between several dunes than on the tops of the dunes. In an interesting variant of this heat-gathering ellect, the white flowers of the "Arctic rose" Dryas (a widespread plant of the Arctic) also act as parabolic heat collectors concentrating light into the center of the flower. This warms up the center of the flower increasing the chances that the pollen will grow and fertilize the seeds. It also apparently wanns up the bees that visit the flowers, speeding up their activity and helping them to carry pollen between the plants more efficiently.

Where there is a hollow in the landscape (caused by a small valley, or geological features such as a kettle hole or sink hole), it may have warmer sunny days because of the concentration of the sun's heat into the center, sheltered from the breeze. However, it can also have more severe winters. 1 lived for a while in the Appalachian fold country of cast Tennessee, where small, cosy farmed valleys known as "hollers" make up some of the most beautiful countryside 1 have seen anywhere. My house stood perched on the somewhat cooler sloping side of a holler, and I remember how the short walk down the track to the center of the valley on a summer's day could seem like entering a furnace.

The frosts at the bottom of the holler were also more extreme; the old lady whose house was on the valley floor bemoaned the fact that her tender spring vegetables and flowers would often be hit by late frosts, while those in other people's gardens a few yards higher on the slopes survived intact. In a study of the landscape of part of the Appalachian fold country in Virginia, the date of the last frost in spring was almost a month later in small valleys that formed "frost hollows", compared with areas on the flat. Frost hollows occur because the cold air that forms at the ground surface on the valley slopes and ridge tops during a cold night (as the ground loses infra-red radiation to space) is heavy and drains downslope as a fluid. As it enters a valley bottom, the cold air also tends to pool up to a certain level, producing a sharp transition between frosty air below and the warmer air above (Figure 4.6). One can sometimes sec a "burn level" in small vallcvs on leafina-out deciduous trees or ferns where frostv r W ✓

air accumulated in a hollow, like water filling a pond; I have seen the transition from no damage at all to every leaf killed in less than 50 cm vertically, all around the edge of a small valley.

Drainage of cold air also leads to more transient patterns in microclimate. On bare Mediterranean hillsides after a hot sunny day, the air near the top of a hill begins to cool after the sun goes down. Being denser it drains downslope, often forming invisible "rivers" that flow down dry stream valleys and along goat paths. Walking through the garrigue scrub in the evening one often passes through these cool rivers of air and then steps back into warm air within the space of few meters. Local people

Dan Kay 316 Rust 800

Relative Elevation (meters)

Figure 4.6. Temperature profile against height on a cold spring morning in a Pennsylvania valley that acts as a frost hollow. Sub-freezing temperatures are only present in the lowermost parts of the valley. Source: Bonan.

Relative Elevation (meters)

Figure 4.6. Temperature profile against height on a cold spring morning in a Pennsylvania valley that acts as a frost hollow. Sub-freezing temperatures are only present in the lowermost parts of the valley. Source: Bonan.

sometimes explain these patches of air as being the spirits of the dead that wander the hills, chilling or warming the people they encounter during their journeys.

Such microclimatc-scalc differences brought about by cold air drainage arc a miniature version of the same process that can occur in large valleys, producing the mid-elevation warm belt mentioned in Chapter 1. Often, the only distinction between microclimates and mesoclimates is a matter of scale, not the fundamental processes involved.

4.1.9 Life within rocks: endolithic lichens and algae

Even in the coldest places on earth there is life, and favorable microclimates make it possible. For example, in the Antarctic mountains, where the air never gets above freezing, a north-facing rock surface on a sunny day can get much warmer. Although temperatures right on the rock surface can rise above freezing in the direct sunlight, the very dry air and rapid fluctuations in temperature prevent any form of life from growing there. Over just a few minutes, the temperature can rise above freezing and dip back down below, and this sort of instability seems to be too much for even the hardiest organisms to cope with. However, conditions are warmer and more stable a few millimeters down between the grains of a sandstone rock. The grains are transparent quartz, which allows sunlight in but provides insulation from the chilling air outside. Temperatures in the tiny gaps between these quartz grains can get much warmer than the surrounding air: to around IO C', which is some 20 C warmer than the air outside ever gets. It is within these miniature greenhouses that specialized "endolithic" (meaning "within-rock") lichens live, photosynthesizing and growing during the few weeks each year that are warm enough, and perhaps living for centuries in a mainly dormant form.

In another of the earth's most extreme environments, lichens can also survive within rocks due to the right microclimates. Death Valley in California holds the record as the hottest place on earth. Sandstone rock faces baked by the sun and parched by lack of rain seem an unlikely place to find life, and indeed the rock surface itself has nothing growing on it. Yet, a few millimeters deep inside the rock temperature extremes are lessened and there is moisture that trickled in between the rock grains when it last rained. Endolithic lichens survive here, harvesting sunlight that reaches through the quartz grains of the rock.

4.1.10 Hants creating their own microclimate

We have considered already how forest canopies create a special environment underneath themselves, how the Arctic rose keeps its flowers warm and how cushion plants trap extra heat for themselves. There are other instances too of plants making their own microclimate.

4.1.11 Dark colors

Many algae and lichens growing on rocks in cold climatcs are dark-colored, even black. This helps them absorb the visible wavelengths that contain most energy from the sun (i.e., they have low albedo). It has been suggested that this dark color is a spccial feature evolved to cope with cold climates: the extra heating that results from this might allow better metabolism and growth. Thus, the plant modifies its own microclimate to make itself warmer. It is reasonable to suppose that in the cold, being dark might benefit the plant. However, it is not clear that dark colors have been specifically selected for in cold climates even in the tropics algae and lichens living on rock surfaces are often dark-colored, perhaps accumulating the pigment as a defense against damaging UV light.

4.1.12 Protection against freezing

Fleshy succulent plants known as tree groundsels (Senecio) and giant lobelias (Lobelia) living on the tops of high mountains in the east African tropics have to cope with frost at night, even though the days are above-freezing. These plants seem to protect their soft, sensitive growing tips by accumulating a little "basin" of water in a rosette of leaves; this covers the growing tip with water that has heated up during the sunny days, protecting it from frost each night.

4.1.13 Internal heating

There is a widespread family of plants known as the arums—containing many thousands of species—which create heat in the flowering structures by burning up sugars, to vaporize certain chemicals that attract flies to come and pollinate the flowers. The most proficient of these self-heating plants seems to be the skunk cabbage (Symplocarpus foetidus), which grows in swamps in North America, creating quite a stink with the chemicals that it evaporates.The amount of heat released by a skunk cabbage can raise the temperature inside its flowering head to 35 C (almost as warm as the human body temperature), even when the air temperature is below-freezing. Early in spring, when snow is still on the ground, the skunk cabbage flower heads are able to melt the snow around them, poke up and flower before any other species.

4.1.14 Volatiles from leaves

Volatile chcmicals arc abundant in desert scrub and Mediterranean vegetation. The scentcd oils evaporating from the leaves of evergreen Mediterranean scrub (such as garriguc, Chapter 2) can make a hillside smell like one big pot pourri, and the sagebrush of the American southwest can also smell rather strong. It is generally thought that these compounds have a protective role in making the plants distasteful or indigestible to grazers. However, some ccologists have suggested that the plants use them for an additional purpose: being kept warmer and frost-free bccausc of the

"greenhouse" heat-trapping properties of these chemicals (which strongly absorb infra-red light). However, atmospheric physicists calculate that the "heat-trapping" effect of these chemicals is probably not strong enough to make any significant difference.

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  • Adelmio Monaldo
    How do forests provide microclimate?
    3 years ago
  • susan
    How does a forest floor help create a microclimate?
    2 years ago
  • alfred
    What is a rainforest microclimate?
    2 years ago
  • martina
    How vegetation create microclimate?
    1 year ago
  • Gorhendad
    How far below ground surface do pedogenic processes pentrate in tropical rainforests?
    6 months ago
  • jessica
    How does aforest effect a microclimate?
    1 month ago
  • caterina
    How does Rainforest vegetation modifies the climate on the forest floor?
    7 days ago

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