Permaculture Designers Manual
CHAPTER 5 – CLIMATIC FACTORS IN PERMACULTURE
Section 5.7 –
Landscape Effects in Permaculture
Heat is transported on a world scale by two great circulations:
That of the air masses, and those of oceanic currents.
Of these, air masses are more wide-spread in their effect, and are least limited by land masses.
Oceanic currents, or indeed proximity to any large body of water, have their greatest moderating effect on down-wind shorelines.
Such effects may have little inland influence.
The concept of continental climates \ was evolved to describe those extreme and widely fluctuating inland climatic zones that are not buffered by the effects of sea currents, and which demonstrate periods of extreme heat and cold, all the more marked \ on high mountains.
Thus, the third complication on the simple temperature-rainfall classifications is CONTINENTALITY.
After this, only one special factor remains and it is the effect of hills or ranges of mountains on local climate; these effects are very like the latitudinal effects on a global scale.
LATITUDE AND ALTITUDE
An average measure of temperature fall with altitude is: 9.8°C/km (5.4°F/1000 feet) in rainless or dry air; or 4-9°C/km (2.2-5°F/ 1000 feet) in humid and saturated conditions.
As a rough approximation, every 100m (330 feet) of altitude is equivalent to 1° of latitude, so that at 1000 m (3300 feet) on the equator, the temperatures are about equivalent to a climate 10° off the equator with the same humidity.
At 10° latitude off the equator, a plateau at 1850m (6000 feet) has a climate more like that at 30′ latitude, with a probability of wind chill to below freezing.
For high islands or ranges of mountains, this altitudinal factor is crucial to design strategies for homes and gardens.
Altitude effect alone enables us to grow a wide range of plant species on a high island, using the area from ocean to mountain top.
High Altitude Effects
Mountains are not in fact strictly “latitude equivalents“, as the air is more rarefied, air pressure less, and radiation therefore higher.
On very high mountains of 4000m (13,000 feet) and more, people may experience oxygen deficiency (mountain sickness), snow or radiation blindness and suffer from the extremes of day-night temperature fluctuations.
The mountain sickness of oxygen stress is not felt by locals, but can cause extreme fatigue, insomnia, and labored breathing in visitors.
In the Peruvian Andes, day temperatures remain at 16-19°C (61-66°F) all year, but night temperatures fall rapidly to -10°C (14°F).
Shade temperatures are lower than at sea level, due to the less effective heat transfer and insulation effects of the rarefied air.
Water boils at lower temperatures due to the low atmospheric pressure, and snow may sublime directly to water vapor rather than melting.
High mountains reduce the range of foods available. (See Figure 5.24)
Snow cover may serve as an insulating blanket and prevent early spring thawing, or even autumn freezing if it covers unfrozen ground.
Snow cover also causes intense reflection, and raises air temperatures just above the snow by day.
At night, radiation from snow causes an extremely cold ground air layer, so that any plants protruding from snow suffer these extremes of diurnal temperature.
As great as the effect of altitude is, the effect of slope is even more pronounced.
Daubenmire (1974) records that slopes of 5° towards the poles “reduce soil temperatures as much as 168km distance” towards the poles, so that even a gentle slope away from the sun creates very much cooler conditions locally.
The effect of cold ravines in near-permanent shadow is extreme indeed and one may stand in hot sunlight in the Himalayas and gaze into icy depths where only the hardiest life forms exist, and where ice may permanently cover rocks and spray zones caused by waterfalls or rapids.
We have referred to the chill of narrow, shaded, high altitude gorges, but an opposite effect occurs in sun facing wider valleys, sheltered from winds.
Here, hot air builds up rapidly, soils are drier, and strong winds may be generated (upslope and up valley by day, down slope and down valley at night).
Figure 5.25 demonstrates this effect in moderate mountain areas of 3,000-4,000m (9,850-13,000 feet).
In large valleys, and especially on cool moist climates, the upslope wind may result in the generation of a chain of cumulus clouds at the valley head, trailing off as a succession of clouds from mid-morning to evening.
In more tropical humid climates, the cloud may be continuously held on the mountain tops; this forms part of the standing cloud of high islands.
Such cloud (and rainfall) effects are accentuated by forest on the valley sides and ridges, as trees actively humidify the air streams by transpiration in hot weather.
Valleys in tundra and desert support tree populations absent from the plain or peneplain areas surrounding them, but the reasons may differ in that tundra valleys are likely to be protected by (driven) deep snow cover.
This preserves warm or sub-lethal soil temperatures in winter (as well as providing excess summer melt moisture).
Valleys in deserts remain moist due to the deep detritus which fills their floors; the shaded soils lose less moisture to evaporation.
Lethal soil temperatures are also avoided by partial shading. Both ice blast and sand blast are modified or absent in valley floors, so that unprotected seedlings can survive high winds in the shelter of valleys.
Thus, valleys (or wadis) are preferred growing sites in deserts, and provide tree products in otherwise treeless tundra’s, although the latter sites are rarely occupied by human settlement.
In the field, we often notice a sudden coldness just before dawn in valley areas; this is the time of the greatest depth of cold air and hence the greatest intensity of cold.
Air flowing down from the mountains has pooled all night and just before the sun rises, we (and many animals) are at our greatest exposure and lowest ebb.
It is at this time that winds off glaciers flowing down cold valleys reach their maximum speed.
Without wind or air flow, radiation frost can form, as it does in sheltered hollows and tree clearings. In these areas, opening up the clearings or draining them of cold air may help reduce frost, if that is the aim (Figure 5.13).
When, some years ago, I grew such crops as tomatoes and cucurbits inside open tree canopies, I did prevent frost, but lost crop due to low light levels and a lack of wind or insect pollination.
In such cases, a shade side screen of reflector plants facing the sun would help to keep light levels up, and plants to attract bees need to be placed around the clearing.
Arboreal or ground browsers within forests are also worrisome in gardens (possum and porcupine for example).
Green-leaf vegetables’s, however, are not usually eaten, and can be successfully grown in small forest clearings or in open forest in frosty areas.