by
R . L . S P E C H T ,
Botany Department, University of Adelaide
INTRODUCTION
Most natural plant communities are composed of a heterogeneous collection of species often ranging in habit from tall trees to the smallest of herbs. This complex of geometrical shapes and sizes is apparent even in arid environments, though, of course, the m a x i m u m size and density of the plants are often greatly reduced. To complicate this picture, the specieB are distributed in different patterns and densities. Some m a y occur in dense, even stands; others in sparse but random distributions; but m a n y will be aggregaled in
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clumps, possibly around a parent plant. The ideally smooth contact-surface between the air and the vege-tation—such as is presented by bare ground, a turf, or a uniform, dense pasture, crop or forest—is thus modified into a confusion of micro-reliefs (Fig. 1). The ultimate result of this complex system is to produce (a) consi-derable turbulence in moving air which consequently affects m a n y climatic factors and (b) irregular patterns of incoming solar radiation.
The size and spatial relationships of the individual plants are not the only factors which exert a large effect on the microclimate. The pattern and density of the branches, the size, density and configuration of the leaves, all play a large part in microclimate within and under each specimen. A s well, the depth, nature and distribution of the litter under the plants; the micro-topography of the soil developed under and around each species all influence the microclimate.
This chaotic microclimatic pattern above the ground inevitably influences the microclimates within the soil.
The more arid the community, the more important each factor is likely to become. In mesic environments the vegetation is usually dense and relatively uniform.
Although the factors mentioned above are still opera-tive, the general structure of the community tends to moderate their effects. As the harsh extremes of the arid habitat enable only a comparatively sparse, hetero-geneous stand of plants to survive, great distortion of the macroclimate is likely to result.
The data presented below to illustrate these phenomena have been measured in an environment at the wetter limit of the semi-arid region—rainfall 18 in. (46 cm.) per a n n u m with a summer drought of five months.
The plant community is composed of low, sclerophyllous shrubs and undershrubs, the highest of which is little more than two metres tall. Compared with arid vege-tation, this stand of "heath" is quite dense, but still some 20-30 per cent of bare ground m a y be found between the individual plants. As the specimens are irregularly scattered and each species is of different
Micro-environment (soil) of a natural plant community stature, the surface of the stand is quite irregular. The
leaves of the dominant species vary greatly in size, shape and orientation but all show typical xerophytic characteristics. This vegetation flourishes on a deep siliceous sand which overlies a solonized clay subsoil at least six feet below, a soil which proved ideal for observations on micro-environment. (Specht and Rayson and Specht, unpublished data).
SOIL T E M P E R A T U R E
This community obviously produces a complex pattern of light and shade. Where the soil is bare it is subject to the full force of the sun's rays. As the surface soil is grey in colour m u c h of the heat energy is absorbed. In contrast, on other sites varying amounts of solar energy filter through the bushes. S o m e bushes present a dense mass of broad leaves to the sun; little solar energy reaches the ground. Others present an open framework of needle-like leaves and branches which allow a consi-derable proportion of the energy to penetrate. Thus, varying amounts of the solar radiation strike the soil.
This is amply shown in the following table, recorded when bright sunshine fell on a soil dried below its wilting point (1.0 per cent moisture).
T A B L E 1. Soil temperatures (°C.) at depths of 1, 3, 6 and 12 inches observed under exposed and shaded areas only a few feet apart at noon on 5 March 1951
Depth (inches) The daily and annual fluctuation of atmospheric temperature is paralleled, with some time-lag, b y varia-tions in the soil temperatures. Lateral and vertical conduction and back-radiation of heat are continuous and tend to equalize the differences, but, daily, the system is being upset by further additions of solar energy. The differences are not so great w h e n the soil is moist (it holds up to 6.5 per cent moisture at field capacity) because of the high specific heat of water, but are still apparent. The temperature-dependent processes of all soil micro-organisms and roots are influenced by these daily and seasonal fluctuations. Each species survives only over a limited range of temperatures; if this range is exceeded then a suspension of biological activities, or death, results. However, although the extremes m a y determine the survival of the organism, it is the optimum, middle values which are important for the most efficient biological activities. The organism must find an environment as near to the optimum as possible in order to survive, otherwise its existence, being tenuous, depends entirely on the absence of better-adapted competitors. The plant community modifies its
o w n soil temperature to a certain degree to produce these temperature mosaics. Seedlings germinate and develop in the milder sites but inevitably the resultant vegetation is subjected to the rigours of the macro-climate.
SOIL MOISTURES
The variable nature of the aerial cover not only influences the amount of solar radiation which reaches the ground but also the amount of precipitation. A s m u c h as 70 per cent of the rainfall m a y be intercepted by the leaves and branches of some vegetation. A portion of this intercepted rainfall flows d o w n the stems (stem-flow) or drips off the leaves (foliar-drip), but often as m u c h as 30 per cent of the incident rainfall is evapo-rated directly into the atmosphere from the surface of the plants themselves. Even w h e n it does reach the ground, some of it m a y still be retained by a dense m a t of leaf litter.
S o m e leaves are large and broad, others small, narrow or needle-like; but, in the end, it is the density and orientation of these leaves which determine the total area presented to the incident rainfall. S o m e leaves are so smooth and w a x y that little water adheres to their surfaces; others, furrowed or hairy, m a y provide surfaces easily wetted. A great number of combinations of these characteristics m a y be found in most arid communities.
The penetration of rainfall into heath vegetation varies considerably. The bare areas receive 100 per cent of the rainfall; needle-leafed shrubs with dense litter layers allow only 70 per cent to penetrate; other narrow-leafed shrubs permit up to 90 per cent to reach the ground. Some of the broad-leafed shrubs, however, intercept m u c h more rainfall but redistribute it b y leaf-and stem-flow so that ultimately some 75 per cent reaches the ground. In doing so, marked areas of rain-shadow (50 per cent incident rainfall) are created under the edges of the shrubs while regions of concentration (110 per cent incident rainfall) m a y be observed at their centres. All these striking variations in rainfall inter-ception and redistribution m a y be seen within two to three metres. They finally express themselves by pro-ducing unusual patterns of soil moisture penetration within the deep sand. This is well illustrated in Figs. 2 and 3. The first pattern was observed in late s u m m e r after about 1.00 inch of rainfall had fallen on a soil near its wilting point; the second in the middle of winter. In both figures, values of the soil moisture at or near the wilting point (less than 1 per cent) m a y be seen close to values at the field capacity (greater than 6 per cent).
These moisture patterns are even more important than the variations in soil temperature. The regions of greatest water concentration are always associated with the deep root systems of the dominant species. Even-tually, as these dominants grow larger, more and more
Climatology and microclimatology / Climatologie et microclimatologie
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F I G . 2. Soil moisture pattern under the heath vegetation after 96 points of rain had fallen on a profile near its wilting point (by courtesy Austr. J. Bot.).
rainfall will b e intercepted a n d channelled a w a y from the root systems of the lesser species in the stand.
Records s h o w that, after 5 0 years, few of these species remain.
T h e establishment of a water balance sheet for a natural plant c o m m u n i t y is greatly complicated b y these considerable variations in the soil moisture. A careful survey of the moisture patterns under each major species in the stand will indicate the variation which occurs. T h e placement of soil moisture meters should then be arranged to cover adequately the g a m u t of variation.
EVAPOTRANSPIRATION
Arid plant communities are so open in structure that virtually every leaf, every square inch of ground, is exposed to considerable evaporative conditions. A d m i t -tedly the microclimate will b e less severe in the shaded areas of bushes. Nevertheless, these areas will still b e subjected to considerable evaporation. Unlike the lower levels of a dense crop or forest, the arid c o m m u n i t y c a n rarely develop a stagnant, h u m i d atmosphere even after freak rainstorms. T h u s , each leaf transpires at varying rates which d e p e n d o n the immediate microclimate. Field m e a s u r e m e n t s of the transpiration of individual leaves are crude a n d unsatisfactory. T h e y afford a reasonable comparison
between species under the s a m e environmental condi-tions, but unfortunately the range in microclimates is so wide that the whole variation in transpiration cannot b e investigated so completely as to enable a n accurate estimate of the transpiration of the whole stand. In a n y case the technique ignores direct losses of soil moisture b y evaporation.
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F I G . 3. Soil moisture pattern under the heath vegetation towards the middle of winter (by courtesy Austr. J. Bot.).