by
R . M . M O O H E , Division of Plant Industry,
CSIRO, Canberra
INTRODUCTION
In arid regions and indeed in any region with extremes of climate the m a x i m a , minima, and ranges of the various factors of the environment are more important determinants of plant distribution than means. A single occurrence of a m a x i m u m or a m i n i m u m of a climatic factor above or below the tolerance range of a cultivated crop m a y be sufficient to kill it, but in the case of native species it is probably the frequency of such extremes which determines their occurrence and distribution within a particular region. T h e change in microclimate associated with variations in soil, slopes, aspects, etc., m a y have a controlling effect on the distribution of different species, particularly in arid and alpine regions where the modifying effects of such natural phenomena are likely to be quite marked.
The critical period of a plant's existence is from germination to establishment or the development of a root system and in dry regions this means that for any particular plant species to germinate there must be a coincidence of rainfall and a specific temperature range—the latter in particular will vary with soil, slope»
aspect, presence or absence of litter and surface stones, and proximity to rocks and other plants. Thus not only will there be a spatial variation in microclimates but, because of the variability of occurrence and intensity of rainfall, also a variation in time, depending on the particular temperature regime immediately after rain-falls. T h e occurrence therefore of a particular annual species in an arid region m a y be spasmodic both in time and place. Desert annuals and indeed the majority of pioneer species germinate best at alternating tempe-ratures [l].1 T h e perennial bladder saltbush (Atriplex vesicaria H e w a r d ex Benth.) o n the other hand germi-nates best at a low constant temperature and is inhi-bited b y temperatures higher than 30° C . [3]. Thus rain at a particular time m a y be favourable for winter or s u m m e r germinating species, alternating-tempe-rature or constant-tempealternating-tempe-rature germinating species,
depending on the season of the year, macro-, and micro-climatic factors.
VARIOUS FACTORS AFFECTING MICROCLIMATE The following examples indicate h o w the micro-environment within a small area m a y differ as a result of changes in soils, slope, and vegetation.
Soil Type
In Australian arid and semi-arid regions soil type has a marked effect on vegetation. O n e of the most striking examples is the sudden transition from desert sandplain to Mitchell grass downs at Rockhampton D o w n s , N . T . T h e reasons w h y one group of species grow o n one soil type and another with different Ufe forms on the adjacent one have never been precisely determined. The temperature and moisture relationships of the two soils mentioned will differ very mark-edly—one would expect that the moisture profile would differ markedly in the two soils—the sand would be wetted to a greater depth b y a particular fall of rain than would the clay. This is reflected in the vegetation—the sand grows trees and shrubs while the clay grows predominantly Mitchell grasses (Astrebla spp.) and is treeless.
S o m e idea of the temperature characteristics of soils of varying textures is given in Hutchings' data (personal communication, 1954) from Trangie ( N . S . W . ) , where the m e a n annual rainfall is 17 inches. T h e temperatures at 12 inches depth in three soils, the surface textures of which were sand, loam, and clay respectively, are shown in Table 1. All three soils carried a low, sparse cover of grasses and, although the species varied, the cover characteristics were similar.
The figures show that the sandy soil w a s warmer in the s u m m e r than the clay loam, the greatest
differ-2. The figures in brackets refer to the bibliography at the end of this chapter.
Natural phenomena and microclimate T A B L E 1. Temperatures at 12 inches in three soils of similar
colour (grey-brown) but of different textures. Records taken on same days at three periods at Trangie, N . S . W .
ence being 6° C . The loam was intermediate between the two extremes. It was to be expected that the differences in temperature between the sand and the clay loam near the surface would be even greater than that recorded at 12 inches and it is significant that Digitaria coenicola (F. Muell.) Hughes, which requires a high temperature for germination, predominates on the sandy soil. O n the other hand, Stipa fálcala Hughes, a low-temperature germinating species, is dominant on the clay loam [2],
Topography
The influence of topography on temperature within micro-environments has been studied in the sub-alpine region of N e w South Wales and at Canberra (Moore, unpublished data 1956). Although a sub-alpine envi-ronment differs markedly from an arid one, both experience wide diurnal ranges of temperature and the example given shows the modifying effect of topo-graphy on temperature extremes near the ground and the controlling effect of the frequency and limits of these extremes on the distribution of plant species.
T A B L E 2. Temperatures on clear nights at different altitudes in a small mountain valley. Temperatures recorded at a height of 2 ft. 7 in. at Currango, N . S . W . in 1953
1. Adjacent to small stream 18 in.
Altitude
Temperature records were taken in a small mountain valley approximately 4,200 feet above sea level and are shown in Table 2.
The nocturnal temperature inversions which m a y occur at all seasons of the year are the cause of the absence of trees at the lower elevations of the valley.
Experimental plantings of young trees were m a d e at each site and only those above the natural tree-line survived. The species used included Eucalyptus pauci-flora Sieber, E. stellulata Sieber, and E. rubida Deane and Maiden, which are the tree-line species at Currango.
Daily m a x i m u m temperatures were not recorded at all sites but thermographs at sites 2 and 3, i.e. immediately above and below the tree-line on the south-facing slope, showed that from March to September the daily m a x i m a were higher and the diurnal temperature ranges greater at site 3. Records for two cloudless days and nights are shown in Table 3.
T A B L E 3. Maximum and minimum temperatures 18 inches above ground level at two elevations at Currango, N . S . W .
There are large fluctuations in diurnal temperatures in Australian arid regions and, although the minima would not be as low as in the sub-alpine regions, the m a x i m a would be m u c h higher. Temperature inversions also occur on flat areas and in desert basins, and frosts are c o m m o n . The frequencies of temperatures above and below certain limits will undoubtedly have controlling effects on the germination and thus on the local distri-bution of plant species in desert regions.
Vegetation
In addition to the effects of soil, aspect, and slope, the climate near the ground is modified by the presence of vegetation. The shielding effect of a leaf canopy on incoming and outgoing radiation results in cooler day and warmer night temperatures than in the open.
Thus vegetation tends to narrow the diurnal tempe-rature range and this m a y result in more favourable temperature regimes for the germination of species inhibited by very high or very low temperatures.
However, competition for soil moisture in arid regions limits the utilization of these otherwise favourable habitats. In general desert perennials are widely spaced, but certain species have a characteristic group of annuals within their foliage cover zone for a few weeks following rain.
Climatology and microclimatology / Climatologie et microclimatologie T h e utilization of arid zone vegetation b y grazing
animals will increase the range of microhabitats but generally in a m a n n e r unfavourable for the germination and growth of the species initially present i.e. the removal of foliage b y grazing a n d trampling will tend to widen the range of microclimatic factors.
Phénomènes naturels et microclimat ( R . M . M o o r e ) . D a n s les régions arides, les éléments climatiques présentent des variations diverses de grande amplitude.
Ils dépendent étroitement de facteurs c o m m e la
contex-1. B A R T O N , L . V . Contr. Boyce Thompson Inst., vol. 8, 1936, p. 7.
2. BIDDISCOMBE, E. F.; CUTHBERTSON, E . G.; HUTCHINGS, R . J. Aust. J. Bot., vol. 2, 1954, p. 69.
T h e possibility of modifying microclimates artifi-cially is small without the aid of water and a m o r e effective dispersion and absorption of the limited rain which falls offers greatest scope for improving conditions for the growth of plants in Australian arid regions.
ture et la composition d u sol, la topographie et la végétation. L'auteur donne divers exemples de la façon dont la température de micromilieux peut varier selon la nature d u sol et la topographie.
3. B U R B I D G E , N . T . Trans, roy. Soc. S. Aust., vol. 69, 1945 p. 73.