Permaculture Designers Manual
CHAPTER 6 –
TREES AND THEIR ENERGY TRANSACTIONS IN PERMACULTURE
Section 6.3 –
Wind Effects in Permaculture
Vogel (1981) notes that as wind speed increases, the trees leaves and branches deform so that the tree steadily reduces its exposed leaf area.
At times of very high winds (in excess of 32 m/sec) the interception of light, efficient water use and convective heat dissipation by the tree becomes secondary to its survival.
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 about the fixed point.
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-colored 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 compression of streamlines, however, the latent heat of evaporation is released in the air and this drier air can be warmer than the air mass rising over the obstruction.
The pressure differentials caused by uplift and descent may affect evaporation as much as wind drying or heat. (Figure 6.2)
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 meters.
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.
The effect of the wind on trees is assessed as the Griggs and Putnam index (Table 6.1), and the accompanying deformations in both crown shapes and growth (as revealed in stems) is given a value which is matched to wind speeds with an average 17% accuracy.
Such scales and field indicators are of great use in design.
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 windbreak to reduce heat loss in homes, to avoid damage in catastrophic winds and to steer the winds to well-placed wind machines.