Permaculture Designers Manual
CHAPTER 6 –
TREES AND THEIR ENERGY TRANSACTIONS IN PERMACULTURE
Section 6.6 –
How a Tree Interacts with Rain in Permaculture
Rain falls, and many tons of rain may impact on earth in an hour or so.
On bare soils and thinly spaced or cultivated crop, the impact of droplets carries away soil, and may typically remove 80 t/ha, or up to 1,000 tons in extreme downpours.
“When we bare the soil, we lose the earth.”
Water run-off 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, run-off increases and for a while this pleases people, who see their dams fill quickly.
But, the dams will silt up and the river will eventually cease to flow and the clearing of forests will result in flood and drought, not a long-regulated and steady supply of dean 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.
There 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:
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 the earth.
This water is called THROUGHFALL. Throughfall depends on the intensity of rain, and there is little interception effect in heavy downpours.
As an average figure, the throughfall is 85% of rain in humid climates.
At this point, throughfall is no longer just rainwater, any more than your bathwater is rainwater; throughfall contains many plant cells and nutrients and is in fact a much richer brew than rainwater.
Dissolved salts, organic content, dust, and plant exudates are included in the water of throughfall (Table 6.2).
“The results show that rain washes large amounts of potassium and smaller amounts of nitrogen, phosphorus, calcium, and magnesium from the canopies to the surface soil. Litter adds organic matter and is a rich source of calcium and nitrogen and a moderately rich source of magnesium and potassium.” (Murray, J. S. and Mitchell, A.; Red Gum and the Nutrient Balance: Soil Conservation Authority, Victoria, Australia, undated).(Figure 6.6)
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 Terminalia trees and 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 in sloping 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 I cm of rain for every 3 cm of depth.
In old beech forests, this humus blanket is at least 40 cm deep, and the earth below is a mass of fungal hyphae.
In undisturbed rainforest, deep mosses may carpet the forest floor. So, for 40-60 cm 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 deep forests and bogs may then take on a clear golden color, rather like tea.
pH can reach as low as 3.5 or 4.0 from natural humic layers, and rivers run like clear coffee to 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 kilometers 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 m deep in 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 Gleditsia both are dry land 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 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 a valuable 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 facility.
INFILTRATION to this storage along roots and through litter is maximized 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 myrorrhizal 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 generalization, 2.5-7 cm (1-3 inches) of rain is stored per 30 cm (12 inches) depth of soil mantle in retention storage, although soils of fine texture and high organic content may store 10-30 cm (4-12 Inches) of rain per 30 cm depth.
In addition, 0-5 cm (0-2 inches) may be stored as interstitial storage.
Thus the soil becomes an impediment to water movement and the free (interstitial) water can take as long as 1-40 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 forests is to give soil time and means to hold fresh water on land. That is, of course, good for the forests themselves, and enables them to draw on water reserves between periods of rain. (Odum, 1974)