By Bill Mollison. First published in the Permaculture Journal, Issue No. 28, Feb April 1988
INTRODUCTION
What I hope to show is the immense value of trees to the biosphere. We must deplore the rapacity of those who, for an ephemeral profit in dollars, would cut trees for newsprint, packaging, and other temporary uses. When we cut forest, we must pay for the end cost in drought, water loss, nutrient loss, and salted soils. Such costs are not charged by uncaring or corrupted governments, and deforestation has therefore impoverished whole nations. The process continues, with acid rain as a more modern problem, not charged against the cost of electricity or motor vehicles, but with the inevitable account building up that no nation can pay, in the end, for rehabilitation. The “capitalist”, “communist”, and “developing” worlds will all be equally brought down by forest loss. Those barren political or religious ideologies which fail to care for forests carry their own destruction as lethal seeds within their fabric. We should not be deceived by the propaganda that promises: “for every tree cut down, a tree is planted”. The exchange of a 50-gram seedling for a forest giant of 50 1200 tonnes is like the offer of a mouse for an elephant.
No new reafforestation can replace an old forest in energy value, and even this lip service is omitted in the ‘cut and run’ forestry practised in Brazil and the tropics of Oceania.
The planting of trees can assuredly increase local precipitation, and can help reverse the effects of dryland soil salting. There is evidence everywhere, in literature and in the field, that the great body of the forest is in very active energy transaction with the whole environment. To even begin to understand, we must deal with themes within themes, and try to follow a single rainstorm or airstream through its interaction with the forest.
A young forest or tree doesn’t behave like the same entity in older age: it may be more or less frost-hardy, wind-fast, salt-tolerant, drought-resistant or shade-tolerant at different ages and seasons. But let us at least try to see just how the forest works, by taking one theme at a time. While this segmented approach leads to further understanding, we must keeep in mind that everything is connected, and any one factor affects all other parts of the system.
I can never see the forest as an assembly of plant and animal species, but rather as a single body with differing cells, organs, and functions. Can the orchid exist without the tree that supports it, or the wasp that fertilises it? Can the forest extend its borders and occupy grasslands without the pigeon that carries its berries away to germinate elsewhere…? Trees are, for the earth, the ultimate translators and moderators of incoming energy. At the crown of the forest, and within its canopy, the vast energies of sunlight, wind, and precipitation are being modified for the life and growth. Trees not only build but conserve the soils, shielding them from the impact of raindrops and the drying effect of wind and sun.
If we could only understand what a tree does for us, how beneficial it is to life on earth, we would (as many tribes have done) revere all trees as brothers and sisters.
I hope to show that the little we do know has this ultimate meaning: without trees, we cannot inhabit the earth. Without trees we rapidly create deserts and drought, and the evidence for this is before our eyes. Without trees, the atmosphere will alter its composition, and life support systems will fail.
THE BIOMASS OF THE TREE
A tree is, broadly speaking, many biomass zones. These are the stem and crown (the visible tree), the detritus and humus (the tree at the soil surface boundary and the roots and root associates (the underground tree).
Like all living things, a tree has shed its wieght many times over to earth and air, and has built much of the soil it stands in. Not only the crown, but also the roots, die and shed their wastes to the earth. The living tree stands in a zone of decomposition, much of it transferred, reborn, transported, or reincarnated into grasses, bacteria, fungus, insect life, birds and mammals. Many of these tree-lives ‘belong with’ the tree, and still function as a part of it. When a blue jay, currawong, or squirrel buries an acorn (and usually recovers only 80% as a result of divine forgetfulness), it acts as the agent of the oak. When a squirrel or wallaby digs up the columella of the fungal tree root associates, guided to these by a garlic-like smell, they swallow the spores, activate them enzymatically, and deposit them again to invest the roots of another tree or sapling with its energy translator.
The root fungi intercede with water, soil, and atmosphere to manufacure cell nutrients for the tree, while myriad insects carry out summer pruning, decompose the surplus leaves, and activate essential soil bacteria for the tree to use for nutrient flow. The rain of insect faeces may be crucial to forest and prairie health.
What part of this assembly is the tree? Which is the body or entity of the system, and which the part? An Australian Aboriginal person might give them all the same ‘skin name’, so that a certain shrub, the fire that germinates the shrub, and the wallaby that feeds off it are all called Waru, although each part also has its name. The Hawaiians name each part of th Taro plant differently, from its child or shoot, to its nodes and ‘umbilicus’.
It is a clever person indeed who can separate the total body of the tree into mineral, plant, animal, detritus, and life! This separation is for simple minds; the tree can be understood only as its total entity which, like ours, reaches out into all things. Animals are the messengers of the tree, and trees the gardens of animals. Life depends upon life. All forces, all elements, all life forms are the biomass of the tree.
WIND EFFECTS
Vogel (1981) notes that as wind speed increases, the tree’s leaves and branches deform so that the tree steadily reduces its exposed leaf area. Vogel also notes that very heavy and rigid trees spread wide root mats, and may rely more totally on their weight, withstanding considerable wind force with no more attachment than that necessary to prevent slide, while other trees insert gnarled roots deep in rock crevices, and are literally anchored to the ground.
The forest bends and sways, each species with its own amplitude. Special wood cells are created to bear the tension and compression, and the trees on the edge of a copse or forest are thick and sturdy. If we tether a tree halfway up, it stops thickening below the tether, and grows in diameter only above the fixed pint.
Some leaves twist and reverse, showing a white underside to the wind, thus reflecting light energy and replacing it with kinetic or wind energy. In most cases, these strikingly light-coloured leaves are found only in forest-edge species, and are absent or uncommon within the forest.
As streamlines converge over trees or hills, air speed increases. Density and heat may also increase, resulting in fast low-pressure air. To leeward of the obstruction, such streamlines diverge, and an area of slower flow, higher pressure and cooler air may result. If rain has fallen due to the compressions of streamlines, however, the latent heat of evaporation is released in the air, and this drier air can be warmer than the air mas rising over the obstruction. The pressure differentials caused by uplift and descent may affect evaporation as much as wind drying or heat.
Apart from the moisture, the wind may carry heavy loads of ice, dust, or sand. Strand trees (palms, pines and Casuarinas) have tough stems or thick bark to withstand wind particle blast. Even tussock grasses slow the the wind and cause dust loads to settle out. In the edges of forest and behind beaches, tree lines may accumulate a mound of driven particles just within their canopy. The forest removes very fine dusts and industrial aerosols from the airstream within a few hundred meteres.
Forests provide a nutrient net of materials blown by wind,or gathered by birds that forage from its edges. Migrating salmon in rivers die in the headwaters after spawning, and many thousand of tons of fish remains are deposited by birds and other predators in the forests surrounding these rivers. In addition to these nutrient sources, trees actively mine the base rock and soils for minerals.
When we go to any site, we can look at the condition of older trees, which are the best guide to gauge wind effect. Trees indicate the local wind direction and intensity and from these indicators we can place windbreaks to reduce heat loss in homes, to avoid damage in catastrophic winds, and to steer the winds to well-placed wind machines.
TEMPERATURE EFFECTS
Evaporation causes heat loss locally and condensation causes heat gain locally. Both effects may be used to heat or cool air surfaces. The USDA’s Yearbook of Agriculture on Trees (1949) has this to say about the evaporative effects of trees: “An ordinary elm, of medium size, will get rid of 15,000 pounds of water on a clear dry hot day” and “Evapotranspiration (in a 40 inch rainfall) is generally not leass than 15 inches per year.”
Thus, the evaporation by day off trees cools air in hot weather, while the night condensation of atmospheric water warms the surrounding air. Moisture will not condense unless it finds a surface to condense on. Leaves provide this surface, as well as contact cooling. Leaf surfaces are likely to be cooler than other objects at evening due to the evaporation from leaf stomata by day. As air is also rising over trees, some vertical lift cooling occurs, the two combining to condense water, thus having twice the specific heat of soil, remaining cooler than the soil by day and warmer at night. Plants generally may be 15° or so warmer than the surrounding air temperature.
Small open water storages or tree clumps upwind of a house have a pleasant moderating effect. Air passing over open water is cooled in summer. It is warmed and has moisture added even in winter. Only water captured by trees, however, has a dehumidifying effect in hot and humid tropical areas, as trees are capable of reducing hujmidity by direct abasoption except in the most extreme conditions.
Reddish-coloured leaves, such as developed in some vines and shrubs, reflect chiefly red light rays. Sharp decreases in temperature may result by interposing reddish foliage between a thermometer and the sun, up to 20° C (36° F) lower than with green pigmented plants (Daubenmire, 1974). Whitish plants such as wormwood and birch may reflect 85% of incoming light, whereas the dark leaves of shade plants may reflect as little as 2%. It follows that white or red-coloured roof vines over tiles may effectively lower summer temperatures within buildings or in trellis systems. Additional cooling is effected by fitting fine water sprays and damp mulch systems under a trellis, thus creating a cool area of dense air by evaporation. This effect is of great use in moderating summer heat in buildings, and for proviidng cool air sources to draw from by induced cross-ventifilation.
TREES AND PRECIPITATION
Trees have helped to create both our soils and atmosphere. The first by mechanical (root pressure) and chemical (humic acid) breakdown of rock, adding life processes as humus and myriad decomposers. The second by gaseous exchange, establishing and maintaining an oxygenated atmosphere and an active water-vapour cycle essential to life.
The composition of the atmosphere is the result of reactive processes, and forests may be doing about 80% of the work, with the rest due to oceanic or aquatic exchange. Many cities, and most deforested areas such as Greece, no longer produce the oxygen they use.
The basic effects of trees on water vapour and windstreams are:
Compressions of streamlines, and induced turbulence in air flows.
Condensation phenomena, especially at night.
Rehumidification by the cycling of water to air.
Snow and meltwater effects.
provision of nucleii for rain.
We can deal with each of these in turn (realising that they also interact).
Compression and Turbulence Effects
Windstreams flow across a forest. The streamlines that impinge on the forest edge are partly deflected over the forest (almost 60% of the air) and partly absorbed into the trees (about 40% of the air). Within 1000 metres, the air entering the forest, with its tonnages of water and dust, is brought to a standstill. The forest has swallowed these great energies, and the result is an almost imperceptible warming of the air within the forest, a generally increased humidity in the trees (averaging 15 18% higher than the ambient air), and air in which no dust is detectable.
Under the forest canopy, negative ions produced by life processes cause dust particles (++) to clump or adhere each to the other, and a fall-out of dispersed dust results. At the forest edge, thick-stemmed and specially wind-adapted trees buffer the frontline attack of the wind. If we cut a windward forest edge, and remove these defences, windburn by salt, dust abrasion, or just plain windforces may well kill or throw down the inner forest of weaker stems and less resistant species. This is a commonly observed phenomenon, which I have called “edge break”. Converesely, we can set up a forest by planting tough, resistant trees as windbreak, and so protect subsequent downwind plantings. Forest edges are therefore to be regarded as essential and permanent protection and should never be cut or removed.
If dry hot air enters the forest, it is shaded, cooled, and humidified, If cold humid air enters the forest, it is warmed, dehumidified, and slowly released via the crown of the trees. We may see this warm humid air as misty spirals ascending from the forest. The trees modify extremes of heat and humidity to a life-enhancing and tolerable level.
The winds deflected over the forest cause compression in the streamlines of the wind, an effect extending to twenty times the tree height, so that a 12 metres high line of trees compresses the air to 244 metres above, thus creating more water-vapour per unit volume, and also cooling the ascending air stream. Both conditions are conducive to rain.
As these effects occur at the forest EDGE, a single hedgerow of 40% permeability will cause similar compression. In flat country, and especially in the path of onshore winds, fine grid placements of rain gauges in such countries as Holland and Sweden reveal that 40% of the rainfall measure downwind of trees and mounds 12 metres or more in height is caused by this compression phenomena. If wind speeds are higher (32 km/h or more), the streamlines may be preserved and rain falls perpendicular to the windbreak. However, at lower wind speeds (the normal winds), turbulence and overturn occurs.
Wind streaming over the hedgerow or forest edge describes a spiral section repeated 58 times downwind, so that a series of compression fronts, this time parallel to the windbreak, are created in the atmosphere. This phenomena was first described by Eckman for the compression fronts created over waves at sea.
Eckman Spirals
The Eckman spirals over trees or bluffs may result in a ranged series of clouds, often very regular in their rows. They are not perfectly in line ahead, but are deflected by drag and the Coriolus forces to change the wind direction, so that the wind after the hedgerow may blow 5 15° off the previous course. (One can imagine that ranks of hedgerows placed to take advantage of this effect would eventually bring the wind around in a great round spiral).
Winds at sea do in fact form great circuses, and bring cyclonic rains to the westerly oceanic coasts of all continents. These cyclones themselves create warm and cold fronts which ridge up air masses to create rain. In total, hedgerows across wind systems have a profound effect on the airstreams passing over them, and a subsequent effect on local climate and rainfall.
Condensation Phenomena
On the sea-facing coasts of islands and continents, the relatively warmer land surface creates quiet inshore airflows towards evening, and in many areas cooler water-laden air flows inland. Where this humid air flows over the rapidly cooling sufaces of glass, metal, rocks, or the thin laminae of leaves, condensation occurs, and droplets of water form.
On leaves, this may be greatly aided by the colonies of bacteria (Pseudomonas) which also serve as nucleii for frost crystals to settle on leaves.
These saturated airsteams produce seaward-facing mosses and lichens on the rocks of fresh basalt flows, but more importantly condense in trees to create a copious soft condensation which, in such conditions, may far exceed the precipitation caused by rainfall. Condensation drip can be as high as 80 86% of total precipitation of the upland slopes of islands or sea coasts, and eventually produces the dense rainforests of Tasmania, Chile, Hawaii, Washington-Oregon, and Scandinavia. It produced the redwood forests of California and the giant laurel forests of pre-conquest Canary Islands (now an arid area due to the almost complete deforestation by the Spanish).
Percentages of Moisture
A single tree such as a giant Til (Ootea foetens) may present 40 acres of laminate leaf surface to the sea air, and there can be 40 or so such trees per surface acre, so that trees enormously magnify the available condensation surface. The taller the trees, as for example the giant Redwoods and White Pines, the larger the volume of moist air intercepted, and the greater the precipitation that follows.
All types of trees act as condensers; examples are Canary Island Pines, Laurels, Holm Oaks, Redwoods, Eucalypts, and Oregon Pines. Evergreens work all year, but even deciduous trees catch moisture in winter. Who has not stood under a great tree which ‘rains’ softly and continuously at night, on a clear and cloudless evening? Some gardens, created in these conditions, quietly catch their own water while neighbours suffer drought.
The effects of condensation of trees can be quickly destroyed. Felling of the forest causes rivers to dry out, and drought to grip the land. All this can occur in the lifetime of a person.
Re-humidification of Airstreams
If it rains again, and again, the clouds that move inland carry water mostly evaporated from forests, and less and less water evaporated from the sea. Forests are cloud-makers both from water vapour evaporated from the leaves by day, and water transpired as part of life process. On high islands, standing clouds cap the forested peaks, but disappear if the forests are cut. The great bridging cloud that reached from the forest of Maui to the island of Kohoolawe, has disappeared as cutting and cattle destroyed the upper forests on Maui. With the cloud forest gone, and the rivers dry, Kohoolawe is a true desert island, now used as a bombing range for the U.S. Air Force.
A large evergreen tree such as Eucalyptus Globulus may pump out 800 1000 gallons of water a day which is how Mussolini pumped dry the Pontine marshes of Italy. With sixty or so of these trees to the hectare, many thousand of gallons of water are returned to the air to become clouds.
A forest can return (unlike the sea) 75% of its water to air, “in large enough amounts to form new rain clouds”. (Bayard Webster, “Forests’ Role in Weather Documented in Amazon”, NY Times, July 5, ’83). forested areas return ten times as much moisture as bare ground, and twice as much as grasslands. In fact, as far as the atmosphere itself is concerned, “the release of water from the trees and other plants accounts for half, or even more of all moisture returned to air”. (Webster) This is a critical finding that adds even more data to the relationship of desertification by deforestation. Clouds form above forests, and such clouds are now mixtures of oceanic and forest water vapour, clearly distinguishable by careful isotopes analysis. The water vapour from forests contain more organic nucleii and plant nutrients than does the ‘pure’ oceanic water. Oxygen isotopes are measured to determine the forests’ contribution, which can be done for any cloud system.
Of the 75% percent of water returned by trees to the air, 25% is evaporated from leaf surfaces, and 50% transpired. The remaining 25% of rainfall infiltrates the soil and eventually reaches the streams. the Amazon discharges 44% of all rain falling, thus the remainder is either locked into the forest tissue or returns to air. Moreover, over the forests, twice as much rain falls than is available from the incoming air, so that the forest is continually recycling water to air and rain, producing 50% of its own rain (Webster, ibid). These findings forever put an end to the fallacy that trees and weather are unrelated.
It is a wonder to me that we have any water available after we cut the forests, or any soil.There are dozens of case histories in modern and ancient times of such desiccation as we find on the Canary Islands following deforestation, where rivers once ran and springs flowed. Design strategies are obvious and urgent save all forest that remains, and plant trees for increased condensation on the hills that face the sea.
Effects on Snow and Meltwater
Although trees intercept some snow, the effect of shrubs and trees is to entrap snow at the edges of clumps, and hold 75 95% of snowfall in shade. Melting is delayed for 2 10 days compared with bare ground, so that release of snowmelt is a more gradual process. Of the trapped snow within trees, most is melted, while on open ground snow may sublime directly to air. Thus, the beneficial effects of trees on high slopes is not confined to humid coasts. On high cold uplands such as we find in the continental interiors of the USA or Turkey near Mt. Ararat, the thin skeins of winter snow either blow off the bald uplands, to disappear in warmer air, or else they sublime directly to water vapour in the bright sun of winter. In neither case does the snow melt to groundwater, but is gone without productive effect, and no streams result on the lower slopes.
Even a thin belt of trees entraps large quantities of driven snow in drifts. The result is a protracted release of meltwater to river sources in the higlands, and stream-flow at lower altitudes. When the forests were cleared for mine timber in 1846 at Pyramid Lake, Nevada, the streams ceased to flow, and the lake levels fell. Add to this effect that of river diversion and irrigation, and the whole series of lakes rich with fish and waterfowl have become dustbowls, as has Lake Winnemucca. The Cuiuidika’s Indians (Paiute) who live there lost their fish, waterfowl, and fresh water in less than 100 years. The ‘cowboys’ have won the day, but ruined the future to do so.
Provision of Nucleii for Rain
The upward spirals of humid air coming up from the forest carry insects, pollen, and bacteria aloft. this is best seen as flights of gulls, swifts and ibis spiral up with the warm air and actively catch insects lifted from the forest; their gastric pellets consist of insect remains. It is these organic aerial particles (pollen, leaf dust, and bacteria mainly) that create the nucleii for rain.
The violent hailstorms that plague Kenya tea plantings may well be caused by tea dust stirred up by the local winds and the feet of pickers, and “once above the ground the particles are easily drawn up into thunderheads to help form the hailstorms that bombard the tea-growing areas in astounding numbers”. (New Scientist, 22 March ’79). thus, the materials given up by vegetation may be a critical factor in the rainfall inland from forests.
All of these factors are clear enough for any person to understand. To doubt the connection between forests and the water cycle is to doubt that milk flows from the breast of the mother, which is just the analogy given to water by tribal peoples. Trees were “the hair of the earth” which caught the mists and made the rivers flow. Such metaphors are clear allegorical guides to sensible conduct, and caused the Hawaiians (who had themselves brought about earlier environmental catastrophes) to ‘Tabu’ forest cutting or even to make tracks on high slopes, and to place mountain trees in a secred or protected category. Now that we begin to understand the reasons for these beliefs, we could ourselves look at trees as our essential companions, giving us all the needs of life, and deserving of our care and respect.
It is our strategies on-site that make water a scarce or plentiful resource. To start with, we must examine ways to increase local precipitation. Unless there is absolutely no free water in the air and earth about us (and there always is some), we can usually increase it on-site. Here are some basic strategies of water-caputure from air:
We can cool the air by shade or by providing cold surfaces for it to flow over, using trees and shrubs, metals, and glass.
We can cool the air by shade or by forcing it to higher altituudes, by providing windbreaks, or providing updraughts from heated or bare surfaces (large concreted areas), or by mechanical means (big industrial fans).
We can provide condensation fucleii for rainfrops to form on; from pollen, bacteria, and organic particles.
We can compress air to make water more plentiful per unit volume of air, by forcing streamlines to converge over trees and objects, or forcing turbulent flow in airstreams (Eckman spirals).
If by any strategy we can cool air, and provide suitable condensation surfaces or nucleii, we can increase precipitation locally. Trees, especially crosswind belts of tall trees, meet all of these criteria in one integrated system. They also store water for local climatic modification. Thus we can clearly see trees as a strategy for creating more water for local use.
In summary, we do not need to accept ‘rainfall’ as having everything to do with total local precipitation, especially if we live within 30 100 kilometres of coasts (as much of the world does), and we do not need to accept that total precipitation cannot be changed (in either direction) by our action and designs on-site.
HOW A TREE INTERACTS WITH RAIN
Rain falls, and many tons of rain may impact on earth in an hour or so. On bare soils and thinly spaced or cultivated crops, the impact of droplets carries away soil, and many typically remove 30 tons per acre (… tonnes per hectare), or up to 400 tons (… tonnes) in extreme downpours. When we bare the soil, we lose the earth.
Water runoff and pan evaporation, estimated as 80 90% of all rain falling on Australia, carries off nutrients and silt to the sea or to inland basins. As we clear the land, runoff increases and for a while this pleases people, who see their dams fill quickly. But the dams will silt up and the river eventually ceases to flow, and the clearing of the forests will result in flood and drought, not a long-regulated and steady supply of clean water.
When rain falls on a forest, a complex process begins. Firstly, the tree canopy shelters and nullifies the impact effect of raindrops, reducing the rain to a thin mist below the canopy, even in the most torrential showers. Thre is slight measurable silt loss from mature forests, exceeded by the creation of soils by forests. If the rain is light, little of it penetrates beyond the canopy, but a film of water spreads across the leaves and stems, and is trapped there by surface tension. The cells of the tree absorb what is needed, and the remainder evaporates to air. Where no rain penetrates through the canopy, this effect is termed ‘total interception’.
INTERCEPTION is the amount of rainfall caught in the crown. It is the most important primary effect of trees or forests on rain. The degree of interception is most influenced by these factors:
Crown thickness
Crown density
Season
Intensity of rain
Evaporation after rain.
Broadly speaking, interception commonly falls between 10 15% of total rainfall. Least interception occurs in thinned and deciduous forests, winter rain, heavy showers, and cloudy weather conditions, when it is as little as 10% of rain. Most interception occurs with dense, evergreen trees, light summer rain, and sunny conditions, when it may reach 100% of the total.
However, if more rain falls, or heavy rains impact on the trees, water commences to drift as mists or droplets to earth. This water is called ‘throughfall’. THROUGHFALL depends on the intensity of rain, and there is little interception effect in heavy downpurs. As an average figure, the throughfall is 85% of rain in humid climates. At this point, throughfall contains many plant cells and nutrients, and is in fact a much richer brew than rainfall. Dissolved salts, organic content, dust, and plant exudates are included in the water of throughfall.
Nor can throughfall be measured in rain gauges, for the trees often provide special receptors, conduits, and storages for such water. The random fall of rain is converted into well-directed patterns of flow that serve the needs and growth in the forest. In the stem bases of palms, plantains, and many epiphytes, or the flanged roots of figs, water is held as aerial ponds, often rich in algae and mosquitoes. Stem mosses and epiphytes absorb many times their bulk of water, and the tree itself directs water via insloping branches and fissured bark to its tap roots, with spiders catching their share on webs, and fungi soaking up what they need. Some trees trail weeping branches to direct throughfall to their fibrous peripheral roots.
With the aerial reservoirs filled, the throughfall now enters the humus layer of the forest, which can itself (like a great blotter) absorb 1 cm of rain for every 3 cm of depth. In old beech forests, this humus blanket is at least 40 cm deep, and earth below is a mass of fungal hyphae. In undisturbed rainforest, deep mosses may carpet the forest floor. So for 40 60 cms depth, the throughfall is absorbed by the decomposers and living systems of the humus layer. Again, the composition of the water changes, picking up humic exudates, and water from the deep forests and bogs may then take on a clear golden colour, rather like tea.
pH can reach as low as 3.5 or 4.0 from natural humus layers, and rivers run like clear coffee to the sea. Below the humus lies the tree roots, each clothed in fungal hyphae and the gels secreted by bacterial colonies. 30 40% of the bulk of the tree itself lies in the soil; most of this extends over many acres, with thousands of kilometres of root hairs lying mat-like in the upper 60 cm of soil (only 10 12% of the root mass lies below this depth but the remaining roots penetrate as much as 40 metres into the rocks below). The root mat actively absorbs the solution that water has become, transporting it up the tree again to transpire to air. Some dryland plant roots build up a damp soil surround, and may be storing surplus water in the earth for daytime use; this water is held in the root associates as gels. Centrosema and Gleditzia both are dryland woody legumes which have ‘wet root zones’, and other plants are also reported to do the same in desert soils (Prosopis spp).
The soil particles around the tree are now wetted with a surface film of water, as were the leaves and root hairs. This bound water forms a film available to roots, which can remove the water down to 15 atmospheres of pressure, when the soil retains the last thin film. Once soil is fully charged (at ‘field capacity’), free water at last percolates through the interstitial spaces of the soil and commences a slow progression to the streams and thence to the sea.
At any time, trees may intercept and draw on these underground reserves for growth, and pump the water again to air. If we imagine the visible (above-ground) forest as water (and all but about 5 10% of this mass is water), and then imagine the water contained in soil humus, and root material, the forests represent great lakes of actively-managed and actively recycled water. No other storage system is so beneficial, or results in so much useful growth, although fairly shallow ponds are also valuable in a productive landscape.
At the crown, forceful raindrops are broken up and scattered, often to mist or coalesced into small bark-fissured streams, and so descend to earth robbed of the kinetic energy that destroys the soil mantle outside forests. Further impedance takes place on the forest floor, where roots, litter, logs, and leaves redirect, slow down, and pool the water.
Thus in the forest, the soil mantle has every opportunity to act as a major storage. As even poor soils store water, the soil itself is an immense potential water storage.
INFILTRATION to this storage along roots and through litter is maximised in forests. The soil has several storages:
RETENTION STORAGE as a film of water bound to the soil particles, held by surface tension.
INTERSTITIAL STORAGE as water-filled cavities between soil particles.
HUMUS STORAGE as swollen mycorrhizal and spongy detritus in the humic content of soils.
A lesser storage is as chemically-bound water in combination with minerals in the soil. As a generalisation, 2.5 7cm (1 3 inches) of rain is stored per 30 cm (12 inches) of depth of soil mantle in retention storage.
Thus the soil becomes an impediment to water movement, and the free (interstitial) water can take as long as 1 to 40 years to percolate through to streams. Greatly alleviating droughts, it also recharges the retention storages on the way. Thus, it almost seems as though the purpose of the forest is to give soil time and means to hold fresh water on land. this is, of course, good for the forests themselves, and enables them to draw on water reserves between periods of rain. (Odum, 1974)
SUMMARY
Let us now be clear about how trees affect total precipitation. The case taken is where winds blow inland from an ocean or large lake:
1. The water in the air is that evaporated from the surface of the sea or lake. In contains a few salt particles but is ‘clean’. A small proportion may fall as rain (15 20%), but most of this water is CONDENSED out of clear night air or fogs by the cool surfaces of leaves (80 85%). Of this condensate, 15% evaporates by day and 50% is transpired. The rest enters the groundwater. Thus trees are responsible for more water in streams than the rainfall alone provides.
2. Of the rain that falls, 25% again re-evaporates from crown leaves, and 50% is transpired. This moisture is added to clouds, which are now at least 50% ‘tree water’. These clouds travel on inland to rain again. Thus trees may double or multiply rainfall itself by this process, which can be repeated many times over extensive forested plains or foothills.
3. As the air rises inland, the precipitation and condensation increases, and moss forests plus standing cluods may form in mountains, adding considerably to total precipitation and infiltration to the slower slopes and streams.
4. Whenever winds pass over tree lines or forest edges of 12 metres (40 feet) or more in height, Eckman spirals develop, adding 40% or so to rainfall in bands which roughly parallel the tree lines.
5. Within the forest, 40% of the incident air mass may enter and either lose water or be rehumidified.
6. And in every case, rain is more likely to fall as a result of organic particles forming nucleii for condensation, whereas industrial aerosols are too small to cause rain and instead produce dry, cloudy conditions.
Thus, if we clear the forest, what is left but dust?
REFERENCES
Chang, Jen-hu, Climate and Agriculture, Aldine Publishing, Chicago, 1968
Daubenmire, Rexford F., Plants and Environment: a textbook of plant autoeology, Wiley & Sons, New York, 1974
Geiger, Rudolf, The Climate Near the Ground, Harvard University Press, 1974
Odum, Eugene, Fundamentals of Ecology, W. B. Saunders, London, 1974
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