Overstory #92 - Trees and Their Energy Transactions
This article deals with the complex interactions between trees and the incoming energies of radiation, precipitation, and the winds or gaseous envelope of earth. The energy transactions between trees and their physical environment defy precise measurement as they vary from hour to hour, and according to the composition and age of forests, but we can study the broad effects.
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 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 keep 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 life and growth. Trees not only build but conserve the soils, shielding them from the impact of raindrops and the desiccation 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.
In this article, 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 weight 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 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 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 the 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 manufacture 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 Aborigine 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 the 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.
Apart from 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 wind and cause dust loads to settle out. In the edges of forests 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 metres.
Forests provide a nutrient net for 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 thousands 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.
Evaporation causes heat loss locally and condensation causes heat gain locally. Both effects may be used to heat or cool air or 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 less 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 moisture on the forest. We find that leaves are 86% 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°C 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 humidity by direct absorption except in the most extreme conditions.
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:
- Compression 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; and
- Provison of nucleii for rain.
We can deal with each of these in turn (realizing 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 m (3,300 feet) 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 front-line attack of the wind. If we cut a windward forest edge, and remove these defences, windburn by salt, dust abrasion, or just plain windforce 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." Conversely, 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.
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 surfaces 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 airstreams 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 almost complete deforestation by the Spanish).
The effects of condensation of trees can be quickly destroyed. Felling of the forests causes rivers to dry up, swamps to evaporate, shallow water to dry out, and drought to grip the land. All this can occur in the lifetime of a person.
Rehumidification 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 processes. On high islands, standing clouds cap the forested peaks, but disappear if the forests are cut. The great bridging cloud that reached from the forests of Maui to the island of Kohoolawe, remembered by the fathers of the present Hawaiian settlers, has disappeared as cutting and cattle destroyed the upper forests on Maui and so lifted the cloud cap from Kohoolawe, leaving this lower island naked to the sun.
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 210 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 U.S.A. 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. 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 spiralling up with the warm air and actively catching 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.
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 on earlier environmental catastrophes) to "tabu" forest cutting or even to make tracks on high slopes, and to place mountain trees in a sacred or protected category. Now that we begin to understand the reasons for these beliefs, we could ourselves look on 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 capture from air:
- We can cool the air by shade or by providing cold surfaces for it to flow over, using trees and shrubs, or metals, including glass.
- We can cool air by forcing it to higher altitudes, 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 nucleii for raindrops 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 (Ekman 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 km 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.
Chang, Jenhu, Climate and Agriculture, Aldine Pub. Co., Chicago, 1968.
Daubenmire, Rexford F., Plants and Environment: a textbook of plant autecology, Wiley & Sons, N. Y., 1974.
Geiger, Rudolf, The Climate Near the Ground, Harvard University Press, 1975.
Odum, Eugene, Fundamentals of Ecology, W. B. Saunders, London, 1974.
Plate, E. J., The Aerodynamics of Shelterbelts, Agricultural Meteorology 8, 1970.
U. S. Department of Agriculture, Trees, USDA Yearbook of Agriculture, 1949.
Vogel, Stephen, Life in Moving Fluids, Willard Grant Press, Boston, 1981
This edition of The Overstory is excerpted with the kind permission of the author and publisher from:
Mollison, B. 1988. PERMACULTURE: A Designer's Manual. Tagari Publications, Tasmania, Australia. ©Bill Mollison
TAGARI PUBLICATIONS Tagari Publications - Permaculture Institute 31 Rulla Road Sisters Creek Tasmania 7325 Australia Ph: 61 (0)3 6445 0945 Fax: 61 (0)3 6445 0944 email: email@example.com
About the author
Bill Mollison, founder and director of the Permaculture Institute, Bill is the most experienced Permaculture teacher and designer today. He has taught and developed projects from the Arctic through Sub-tropics and Equatorial regions of the planet. The Peoples of the Pacific, South East Asia, South Africa and seven Amazonian language groups have been inspired by and acted on his teachings, embracing Permaculture as a dynamic tool. He has also given Courses in the drylands and developed projects with Native Americans, Indigenous Australians, tribal women of the Deccan, Kalahari, San groups and Pima people of the Sonora. In the USA, Europe and Scandinavia, Bill has lectured and helped to develop ecological designs for urban and rural properties, including many city-farms and CSA's (Community Supported Agriculture). Bill Mollison's many roles include: scientist, naturalist and University professor. Later he became a vigorous campaigner against environmental exploitation which lead him to develop Permaculture as a positive solution. Since then, Bill has devoted his energies towards designing sustainable systems, writing books and articles on Permaculture, and most importantly, teaching. Bill's publications include Introduction to Permaculture, PERMACULTURE: A Designer's Manual, and The Permaculture Book of Ferment and Human Nutrition.