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wonder, efficiency and diversity of a guild, or waru
design landscape sites to achieve at least 40 per cent tree cover at maturity.
If we don’t have design aims for forestry and windbreaks:
fresh water is polluted
rivers and lakes silt up and flood soil water is depleted
animal and crop productivity decreases energy costs are higher
water leaves land fast, and soil erodes easily
windborne diseases and weed seeds are not filtered by vegetation.
Figure 7.1 The waru: a complementary relationship between organisms. The red gum provides a safe habitat for the animals; in return the animals carry out the tree’s need for pollination, seed dispersal and nutrients.
The functions of a forest
You can think of forests as living organisms carrying out special and unique processes for life to continue on Earth. The primary purpose of forests is to give soils the time and the
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means to hold and cleanse water on land before it moves to rivers, lakes and aquifers. It requires an entire forest to do this, with each individual tree carrying out its co-operative function to achieve the purpose of the forest.
The forest as a co-operative
Like all organisms, a forest has several parts. Trees, shrubs, herbs, grasses and
groundcovers are the fixed species of a forest and they carry out special functions. To do this they need pollinating, seed dispersal, fertilising and cultivating. Other species are mobile, like birds, insects, mammals and spiders. The mobile species work for the fixed species and the fixed species provide them with food, safety, medicine and nesting materials. Together the tree and its associates can be imagined as a guild, or waru (see Figure 7.1).
A large planting area of monospecific trees such as pines is only a plantation and not a forest because of its limited functions, habitats and components.
The cells of the forest
When you visualise an old forest as a single organism, then each tree can be seen as cells of the forest, each one shedding its own weight many times in its lifetime. Once we understand the way they work, we can see that trees are miracles of engineering.
Trees are far from uniform and each tree’s many parts are like mini-ecosystems. The bark, the roots, the flowers, leaves and growing points comprise quite different zones of one tree, and perform quite different functions. To understand the forest we need to
understand how trees work with wind, sun and water—the components of climate that you read about in Chapter 5. Trees are superb regulators of air quality, temperature, humidity and wind.
Forests and wind
Wind pruning and anchorage: Trees are ‘pruned’ or deformed by prevailing winds and from this you can fairly accurately predict local wind direction, intensity and places for windbreaks. Heavy trees with large canopies such as oaks rely mainly on their weight to withstand severe winds. Other trees with lighter canopies insert roots deep into the ground and anchor themselves. It is important to use anchoring trees in cyclone areas.
Trees on the edge of forests: Trees with light-coloured bark and leaves grow on the prevailing wind side and this light colour deflects both wind and light to some extent. In deciduous forest this role is performed by the birch, and in Australia,
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light-barked eucalypts.
The edge as a wind filter: Wind, like water, carries a ‘load’. It carries ice particles, sand, dust, bacteria, viruses and seed. Some trees, especially on the windward side, have a thickened bark to withstand particle blast. Because trees with small fine leaves can ‘capture’ the load and deposit it as nutrients, the edge of a forest can be seen as a nutrient trap, and the edge facing the prevailing wind will have richer soils and hardier plants than the edge on the leeward side of a forest. Figure 7.2 shows how the edge must be kept permanently intact because if it is destroyed then windburn, abrasion, disease, pests and weeds enter the forest and destroy its integrity.
Figure 7.2 The edge of a forest must be kept intact to prevent the intrusion of windburn, abrasion, disease, pests and weeds.
Deflecting winds: Typically, about 60 per cent of the wind stream is deflected up and over the forest. This is because of the specialised forest edge which is essential to the lift of the wind. Edge species are usually pruned to a 45-degree or 60-degree angle, and these species are dense, small-leafed and tough with thick stems. The 40 per cent of wind that penetrates the ‘edge’, or forest closure, is cleaned of most of its load and its energy is absorbed. The lifted and deflected wind is then
compressed in a belt up to 20 times the height of the tree canopy. If this is humid air then it is compressed again, cools, condenses and it rains.
Modifying wind temperature and humidity: If the wind is cold, it is warmed by condensation as it passes over the leaves of trees and shrubs. If the wind is hot, it is cooled by evaporation. Within 500 metres of the edge the wind comes to stillness.
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At this point in the forest the air is clean, warm, still and slightly humidified. This is a perfect growing place. It is what we want to design for intensive, productive, protected growing and, like this place, we need to farm in forest clearings.
Trees and sun
Photosynthesis: Trees absorb the sun’s light energy and turn it into chemical energy by the process called photosynthesis. Where leaves are dark green or reddish, as often found in the tropics, more light is absorbed and local temperatures are
reduced. Photosynthesis requires water from the soil. Evaporation and transpiration (sweating) from the underside of leaves works as a pump to pull up the water.
Evaporation and transpiration: Trees evaporate and transpire water into the
atmosphere as humidity. This evaporation is accompanied by cooling so that by day it is cooler in and near a forest than it is in unvegetated areas. At night in humid conditions, moisture condenses on the leaves and warms the surrounding air.
Because leaves are 86 per cent water, they are cooler by day and warmer by night than bare ground (the microclimate effect of a water body). Figure 7.3 shows evaporation and transpiration from plants. Together they are called
evapotranspiration. In very dry areas, the evapotranspiration from trees humidifies dry air, and in very damp areas water captured by leaves dries the air.
Figure 7.3 Photosynthesis and water movement in plants.
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Working in this way, forests can be seen as natural airconditioners that cleanse the air and regulate extremes of humidity and temperature. And, as you know from Chapter 6, tree roots pull up water and keep the water table low, so preventing the salts, which cause soil salinity, from concentrating in the top few centimetres of soil. By absorbing sunlight trees also reduce glare.
Trees and precipitation
Condensation drip forests: Where the airstream is very humid (for example, on sea-facing coasts and islands), the air flows rapidly and condenses on leaf surfaces.
Dense rainforests grow from the condensation harvested from leaf surfaces, which can be 80–86 per cent of the total precipitation. When you consider that a single tree can present 16 hectares of leaf surface, a forest has huge potential to capture water through condensation even if it doesn’t rain.
Trees as evaporative pumps: Trees pump moisture into the air as they transpire and return 75 per cent of received precipitation this way. The Tasmanian blue gum, Eucalyptus globulus, pumps 4000 litres into the atmosphere per day and averages about 60 trees to a hectare in a mixed forest. This is a huge return of water to an airstream, which can condense and rain elsewhere. It has been calculated that as much as 60 per cent of inland water comes from forest transpiration. Therefore, forest removal in one area can relate directly to drought in another.
Trees protect soils: The forest canopy protects soils from water and wind erosion.
Bare earth can lose 80 tonnes per hectare of soil in one heavy deluge. Topsoil and organic matter is removed first. This often ends up in the sea and is irretrievable. In addition, the topsoil and subsoil dry out and become hard like a claypan. And with unimpeded run-off, rivers flood and dams silt up.
Rain over forests
The interception layer: When it rains over a forest the rain is spread, as a film of water bound by surface tension, over all the leaves of the trees and is caught in stems, bark, cobwebs, flowers, and insect and bird nests. Some evaporates from these places. The amount caught in the canopy is influenced by the crown’s thickness, density of tree canopy, branches and trunk. For 100 per cent of rain
falling, 10–25 per cent is caught in the canopy, called the interception layer, without ever reaching the ground. More is caught in evergreen trees than deciduous (see Figure 7.4).
The through fall: The rest of the rain drifts through the canopy as mist and droplets.
It contains organic salts, dust, plant exudates, insect droppings and sheddings. It is a
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nutrient-rich soup and is directed towards the outer leaf canopy, also known as the dripline, below which are the feeding roots.
Mulch blotter: Before the water can reach the roots, however, bark, taproots, fungi and the humus layer of the soil act like a great water blotter, soaking up
centimetre of rain for every 3 centimetres of depth and holding it for release or use when the soil begins to dry out again.
Filtering through soil: Through the next 40–60 centimetres of soil the through fall is absorbed into water and air channels, nests and burrows; absorbed by more soil fungi and bacteria; filtered by humus and mineral particles; and, of course, taken up by the tree roots. This water is first bound by particles of clay and humus and then the excess percolates slowly through the soil. At any time some of this water is available to soil organisms. Some water is bound and held firmly, some is stored in cavities in the soil and in humus.
Filtered water: Once all this has been accomplished, excess water starts very slowly to move to rivers, springs and the sea. When it does, it is clean.
Windbreaks
You draw on your knowledge of how forests function when you design windbreaks. Well-designed windbreaks protect land and increase crop yields by carrying out similar
functions to the forest edge. They also provide additional yields such as bee fodder, firewood and building timbers. A line of pine trees is not an efficient windbreak because once the lower branches fall off, the wind velocity under the trees is increased and the long black tree shadows reduce productivity.
Wind is fluid and, like water, it can be deflected sideways and lifted. It forms layers naturally because hot air rises and cooler air flows underneath it. Every site has a
predictable wind pattern. Sometimes you can learn about it from weather records. In other cases you will need to observe how tree shapes are deformed (wind-pruned) and the
amount of wear on buildings.
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Figure 7.4 How a forest metabolises rain (after Bill Mollison et al., Permaculture Design Course Handbook).
Each windbreak design is site-specific. You decide where windbreaks are needed from your sector analysis and from your microclimate analysis. Your task in this chapter is to identify where the harshest winds—hot or cold or strong—come from on your land, how long they last and in what season they arrive.
Design windbreaks as part of the 35 per cent of permanent tree cover needed for each site.
Use the natural characteristics of winds to create desired microclimates.
When you don’t have windbreaks
Cold winds remove heat from the surface of plants, buildings, water and living bodies.
With the increase in wind speed and the evaporation of fluids, a chill factor is created. The result is that on windy sites the climate will actually be cooler than the temperature figures show. Plant growth is retarded and both height and yield decrease, while solar devices and insulation work less efficiently on buildings.
The ecological functions of windbreaks are to:
serve as suntraps
increase wind velocity or cooling
decrease evaporation from water or land control erosion
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provide shelterbelts for stock act as dust filters
form nutrient traps for wind and water.
Figure 7.5 Advantages of windbreaks. Climate modification, improved plant and animal production, and energy conservation are some of the benefits provided by windbreaks.
TABLE 7.1: SPECIFIC ADVANTAGES OF WINDBREAKS
Advantages Examples and design techniques
Protected animals
Windbreaks protect animals from harsh winds during hot and cold weather. In very cold weather animals eat 16 per cent less and in summer heat also have a reduced intake. Ideally, no grazing animal should be more than 400 metres from good shade or the losses in heat and moisture are greater than gains in meat and wool production.
(Animals require shelterbelts which are more like open woodlands than dense forest and have a canopy closure of about 70 per cent.)
Protected soils
These retain more moisture and frosts are reduced. Soil on slopes is held better and not as susceptible to wind and water erosion. Soil temperatures are lower in summer and warmer in winter and soils lose less moisture.
Reduced plant damage
With windbreaks, damage in citrus orchards is reduced by 50 per cent. In all orchards there is increased blossom and fruit set, increased pollination and less breaking of branches and uprooting. The overall increase in production is about 25 per cent. The most sensitive plants to wind damage are citrus, avocado, kiwifruit, deciduous fruit, corn, sugar cane and bananas.
Reduced energy loss from buildings
Houses can lose up to 60 per cent of their warmth in winter. Savings of 30 per cent in heating fuels are usual with windbreaks, even in moderate climates. In hot climates large evergreen shady trees designed in an avenue can bring cooled air into the home.
Shady trees over roofs and on western walls are also effective.
Cooling very hot sites Specially designed windbreaks can speed, channel and cool air that is uncomfortably hot for plants and animals.
Protect human settlements
Windbreaks prevent snowdrift in cold climates. They protect roads from high-velocity winds and have an impact on human health. In dry climates they filter the dust that causes ear, eye, nose and lung sicknesses in people. There is some evidence that human and animal epidemics, known to be carried by winds, spread faster when there are no encircling forests or windbreaks around human settlements.
Windbreaks for energy
generation Designed windbreaks can increase the wind speed by directing it to the generator.
Height, density and shape of windbreaks
Figure 7.6 shows how wind moves when it is fully blocked and how it can be directed up and away from soft areas or plantings. A windbreak is effective to a distance along the ground equivalent to about 20 times its maximum height. However, its effectiveness decreases the further away you move from the actual windbreak. There must be some movement of air through a windbreak or the wind eddies, often quite destructively, on the other side of the barrier. The principle is to create the equivalent of the forest ‘edge’, which will lift the wind. The windbreak can be shaped so it tapers at the ends, reducing wind velocity.
The most effective shape for a windbreak is a boomerang or parabola. This directs the wind off to the sides and also functions as a suntrap. A site can have several of these. The wind, once lifted on the prevailing-wind side, is kept high. This is particularly effective over orchards.
How to design your windbreak
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Start with successional planting, as discussed in Chapter 3. Plant small tough species, including local nurse and pioneer species, which prepare the soil for the climax species.
Plants with the following characteristics should be included in your windbreak design:
hardy with deep anchoring root systems
fire- and wind-resistant with fibrous stems and fleshy or small hairy leaves plants with fast early growth—pioneers will fulfil this role
nitrogen-fixing plants with good leaf fall, which are self-mulching and have additional yields.
Assess the risks to your new plants and, if you require it, provide extra protection with tree guards or fence the whole area.
Windbreaks for different ecosystems and climates
Coastal areas, desert areas, inland regions of heavy frost, subtropical regions in danger of cyclones, and cool climates lend themselves to special windbreak designs (see Figure 7.7).
From your analysis you know which way the winds blow in different seasons—which are mild and warm; which are cold and harsh. Perhaps you need dust-filtering windbreaks on the sunny aspect and they may need to be deciduous to receive winter sun.
Small windbreaks
Although you may not need to design and plant windbreaks for large areas, windbreaks are necessary in most gardens, although they need not be permanent. For example, Jerusalem artichokes make an excellent summer windbreak or suntrap in cool climates.
They can collect and direct sun to ripen tomatoes.
Windbreaks can be small hedges of herbs only knee-high in some cases. For example, lavender could be used for this purpose in a cool climate. Near the coast where wind is abrasive, desiccating and saline, small windbreaks can be sufficient to raise a summer or winter crop inside them.
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Figure 7.6 Penetrability of windbreaks. A solid barrier increases air turbulence on the leeward side, whereas a porous windbreak, consisting of mixed plantings, reduces wind speed and velocity.
Try these:
Twice a day, in the early morning and in the late afternoon at sunset, walk around your garden or land and feel with your face or hands where the local air currents are. Sketch these on your plan.
Design a windbreak for a site you are very familiar with.
Think about whether you need permanent windbreaks or just at certain seasons. Make up a short list of possible plant species.
If you were the local town planner, where would you want wind breaks:
for shopping in comfort to rest from shopping
to eat a picnic with children
to walk to church, the post office or other institution?
What is the negative effect that you wish to moderate—hot dry winds, cold harsh winds, or dusty winds? What species and plant characteristics would you need?
5. What do you think is happening in the city when people talk about the ‘canyon’ effects of winds?
Figure 7.7 Examples of windbreaks suited to different landscapes.