The existence of microclimates has long been recognised in its practical application in the importance of aspect and contour in determining the climate of local areas. It has been recognised in the difference between sunny and shady slopes in hill country; in the occurrence of cold air drainage and frost in the siting of crops and orchards; in the tolerance of forest trees to shading and its relation to natural regeneration; and even in the choice of position for the different plants in the home garden.

Microclimate has been defined as the climatic environment of a very local area, such as the north- or the south-facing slope of a hill, or an even smaller area. It refers strictly to local combinations of atmospheric factors, which differ from the macroclimate because of uneven topography or differences in plant cover. Within the area of one macroclimate there may exist a whole series of microclimates some of which may differ sufficiently to be of ecological importance. Biologists have frequently pointed out that the climate in which plants and animals actually live is very different from that measured by the meteorologist in a Stephenson screen at 4 ft. 6 in. off the ground; but in the past many studies of plant and animal habitats have relied almost entirely on measurements of the physical factors of the environment obtained from the meteorologist.

Apart from the practical applications in forestry and agriculture, research on microclimate has been confined to the habitats of a few plants and animals. The main source of inspiration for studies of microclimate has been the work of the meteorologist R. Geiger (1927, 1942) who showed that the climate near the ground differed from the macroclimate because of the effect of the ground and the presence of a plant cover. Microclimate differs from micro meteorology since it is concerned with the local atmospheric conditions as they affect the living conditions of plants and animals, and not with the mechanics of the weather itself. The factors measured are the same as in ordinary meteorological studies, but the type of ground, the presence of vegetation and the type of plant are important in determining the microclimate. A few of the ways in which vegetation affects the climatic factors are considered below in a little more detail.

Temperature

Geiger was the first to demonstrate the steep temperature gradients which exist above the ground or vegetation surfaces, often amounting to 15° C. in 18 inches. The greatest extremes of temperature (that is the highest day-time and the lowest night-time temperature) occur at what is page 53 known as the ‘outer active surface’. This is the surface at which the sun's energy is absorbed and from which it is radiated; and is either the surface of the ground or the region of maximum leaf development of the plant cover; here the temperature fluctuates more widely in 24 hours than does the air temperature.

Where there is free air movement there is no difference in air temperatures in the sun and the shade, but in the absence of wind the hot air formed over bare areas rises vertically during the day and has little influence on the air temperature under the adjacent shade, but during the night cold air formed over the open ground spreads out under the adjacent cover.

Even a thin cover of vegetation reduces the heating of the soil by the sun, and in the shade the soil surface temperatures remain less than the air temperatures, even in the hottest part of the day. At night the rate of loss of heat is retarded by a plant cover, and the temperature of the soil does not drop as low as on adjacent open areas. Thus because of the opposite effects of vegetation during the day and during the night there is less fluctuation in temperature under a plant cover.

Daily maximum temperatures may be higher above a plant cover than at an equal distance above bare ground if the plants are too widely spaced to shade the ground but are effective in preventing wind from blowing away the heated air. Similarly night minimum temperatures may be lower if the wind moves the air over bare areas, but does not move the cold air which settles among the plants.

The presence of plants reduces the steepness of the temperature gradients above bare earth, and the type of plant changes the height at which the outer active surface occurs. Geiger measured the temperature gradients in different types of vegetation and found that in a bed of Antirrhinums the daily maxima occurred at the upper leaf surface, but in rye was distributed down the stems; at night temperature inversion occurred in the Antirrhinums but not in the rye because the close stems impeded air movements. The latent heat of evaporation of the water produced by transpiration from the plants has a damping effect, similar to the formation of dew, which results in a considerable reduction in the amount of cooling which occurs at the soil surface.

In valleys where temperature inversion on a large scale occurs at night, a layer of warm air is formed between two cold layers. Where this belt of warm air meets the valley sides the ‘thermal belt’ is formed. This is a well-known phenomenon in mountainous areas, especially in Europe where orchards and vineyards are located in the thermal belt, to escape the late spring and early autumn frosts which occur on the valley floor below.

The nature of the soil surface has a great influence on the amount of temperature change, a sandy soil being warmer and undergoing a greater range of temperature fluctuation than a forest or bog soil. The wetter the soil the slower the temperature change, so patches of impeded drainage in an otherwise well-drained soil will be much colder. On areas of bare soil page 54 when the temperature drops slowly below freezing point ice is formed as a layer of vertically orientated crystals which grow upwards; but if freezing is rapid the entire surface soil is frozen. Additional layers accumulate from below, building up a considerable frozen mass. Little soil is lifted or heaved on the ice, but plants frozen in the surface ice are lifted out of the ground by alternate freezing and thawing. Frost heaving is possible only when the soil has a very high water content and is most pronounced when the vegetative cover is sparse. Soil is frozen to a greater depth in the open than under a plant cover and least under forest. A frozen soil is impervious to rain and the soil just below the surface may be quite dry despite heavy rain.

Precipitation

In areas of uneven topography snow is blown from the windward side of hills, and deposited in hollows and on the lee slopes. Snow can be a protective cover, but heavy accumulations may break and kill plants. The boundaries between areas of scanty and excessive accumulation are often very sharp, and some plants show a strong preference for a heavy snow cover in winter and a very short growing season after the thaw. These form the characteristic snow patch vegetation usually dominated by sedges. Woody plants tend to be excluded from areas where deep drifts accumulate and persist.

Variation in rainfall in small areas is often very difficult to assess. Condensation from a local fog belt may effectively add to the annual rainfall, and local variations in topography and the presence of vegetation of different heights can cause a rain shadow effect similar to that caused by mountain ranges in macroclimates. The main differences in microclimate are caused by variations in the effectiveness of the precipitation; local temperature and humidity conditions influence the rates of evaporation and transpiration, and a plant cover shades the soil. The total quantity of each rainfall which is intercepted before reaching the soil seems to vary with the density, rather than type, of vegetation, and the amount and rate of precipitation.

Humidity

In the study of macroclimate, the relative humidity of the air is measured, but in ecological studies the saturation deficit or evaporative power of the air with its influence on the rate of evaporation and transpiration is probably more important. Saturation deficit increases rapidly with height above the ground, so that plants of different height are subject to quite different evaporative powers, and there is a sharp increase just above plant level. An economic application of studies of humidity relationships is the research on the microclimates of pests of stored products, and the temperature and humidity conditions under which these pests cannot live.

Light

In microclimates illumination fluctuates constantly. Apart from the changing angle of the sun, the differences in the time of day, and of season, and the effect of weather, under a canopy of vegetation the movement of leaves by the wind results in rapid and wide variations in the amount of light energy received at a given point.

The reduction in the amount of light caused by a plant cover is important ecologically, for it determines the presence or absence of a ground cover or plants beneath the canopy. But as wind velocity, humidity, soil moisture and temperature vary concomitantly with reduction in light intensity, it is difficult to evaluate the light factor alone.

Leaves transmit less than 2% of the light impinging on them; the transmitted light also differs in quality, depending on the type of vegetative cover. For example, in forests the light transmitted through a broadleaf canopy differs from that through a canopy of conifers, but as far as is known the difference in quality has little influence in determining the composition of the undergrowth.

The variation in the intensity and daily duration of insolation caused by the direction and slope of the land is well known. In general, however, the differences in vegetation are probably determined by the increased temperature and decreased relative humidity of the air and thus on the temperature and water relations of the soil on the slopes exposed to the sun.

Air Movement

Even a cover of low herbaceous plants reduces the velocity of wind along the ground, and a forest causes a considerable reduction. The effect of a natural grove of trees or of a planted windbreak may extend up to 70 times the height of the trees; there is also a small area of comparatively still air on the windward side due to deflection upwards of the air currents. But in the planting of windbreaks to create these conditions of decreased velocity the creation of a distinct microclimate must also be considered, including the different temperature and humidity conditions on the lee side, and an increase in the duration of frosts.

Differences in Shrub Community Caused by Microclimate

The small streams which flow out from the Western Hutt hills to the Hutt River have cut deep gullies more or less at right angles to the steep front of the Wellington fault scarp. When a study was made of the natural regeneration of lowland tawa forest on these hills it was found that the slopes with a northerly aspect were covered in Leptospermum and Cassinia, whereas on the south-facing slopes mixed broadleaf scrub and tree ferns were dominant.

One of these gullies was examined in detail. It is about 120 feet across at the widest part, and about 80 feet from ridge to the streambed. On the south-facing slope Coprosma robusta and Melicytus ramiflorus (mahoe) page 56 dominate mixed broadleaf scrub 14-16 ft. high. Associated shrubs are Brachyglottis repanda (Rangiora), Nothopanax arboreum (fivefinger) and Macropiper excelsum. On the north-facing slope there is a mixed shrub community on the lower part, and on the upper part a pure stand of Leptospermum scoparium (manuka) with a ground cover of grasses and weeds, such as Dactylis glomerata (cocksfoot), Holcus lanatus and Hypochaeris radicata (cat's ear). This difference in plant community on the two slopes was first thought to be due to differences in the dates of fires and the number of fires on each area, but the same pattern of distribution was found repeated in other valleys along these hills.

Fortnightly readings of the climatic factors showed consistent differences large enough to be recorded with ordinary meteorological instruments. The temperature on the north-facing slope is 5-10° F. higher during the winter and 15-20° during the summer. Rain gauges collected and retained ½ in. to 2 ½ in. more rain on the south slope, which is sheltered from the prevailing north-west winds but not from the rain-bearing southerly winds. The north-facing slope is partially sheltered from the north winds by a belt of pines, as well as being sheltered from the southerly winds. The relative humidity on the north-facing slope was 15-20% lower than on the opposite slope, and on sunny days 5-10% lower than in the open. Isolated readings of the rate of evaporation on still days gave values 8-10 times that for the south-facing slope.

The soil on these steep slopes is a skeletal soil developed from greywacke, and small surface slips are a common feature especially on the south-facing slope where the soil is damp. Small pockets of soil and litter are accumulated between large rock fragments. On the north -facing slope the soil is compacted into a hard dry surface on which little litter is accumulated and run-off is very rapid.

The most noticeable difference between these two slopes was the incidence of sunlight in the winter months. In summer the north-facing slope is in the sun for most of the day; in the winter, however, the area covered in Leptospermum scoparium is the only part in the sun. In June, July and part of August the sun's rays are almost at right angles to the slope and the ground receives the maximum heat from the sun. It is suggested that it is this incidence of winter sunlight which controls the dominance of manuka, a plant which tolerates rather dry conditions. It is not the light, but the heat of the sun which causes a higher temperature, lower relative humidity and thus an increase in the rates of evaporation and transpiration, and therefore a lower amount of available soil moisture. Shelter from the rain-bearing winds increases the dryness of this north-facing slope.