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III. Characteristics of Growth and Climatic Tolerance
FIG.4. Annual growing pattern of Pinus radiata at Mount Burr, South Australia,
together with data for soil temperature measured at a depth of 3 inches at 9.00 A.M.,
rainfall, and potential evapotranspiration calculated using Thomthwaite's formula
(Pawsey, 1984; Ruiter, 1984).
winter are also found in California. On the other hand, Will (1964)
observed that in New Zealand the tree grew for almost the whole year.
This difference is probably due to climate (see under Section 111, C ) .
The range of latitudes for plantings of P . radiatu is 32" to 46", and
mainly about 37", the northern Hemisphere having fewer forests. The
tree flourishes in Mediterranean and warm temperate climates (see Fig.
S), It has been very successful in New Zealand, where day length rather
than soil or air temperature is said to limit the growth during winter
MilIett ( 1944) at Stromlo, Australia, concluded that among climatic
phenomena, rainfall as a single factor was most important in determining
growth. Prescott and Lane Poole (1947), while pointing out that annual
rainfall of 30 inches or more was desirable, quoted excellent results from
areas of winter rainfall receiving 22 to 25 inches annually (see also
Kessell and Stoate, 1938). Prescott and Lane Poole concluded that the
seasonal distribution of rainfall was important, a well-distributed rainfall being undesirable. Experience in New Zealand has shown that this
is not quite correct. It is true, however, that hot damp summer conditions
are not favorable since they encourage fungal attack, but in spite of this,
Days of Frost and Distribution
FIG. 5. Monthly rainfall and mean monthly maximum and minimum temperatures at locations in different countries in which Pinus rudiutu is grown, with some
data on the number of days of frost per year. (After Scott, 1960.)
a certain amount of summer rain, or at least fog or moist conditions, is
In giving the climatic limitations of the species for Argentina, Golfari
(1959) quoted an absolute minimum temperature of -7°C. ( 19°F.).
Pryor (1947) found that damage was caused by low temperatures, frost,
hail, and particularly snow. On the other hand, high temperatures and
drought brought on needle cast and damage to the leaders. The range
of climatic conditions for the growth of P. radiata is thus wide, but
danger of damage from such influences as moisture stress, fungal attack,
or insect activity may increase with deviations from the climatic patterns
shown in Fig. 5.
Soil Factors Restricting Growth
Previous to 1918, the view was held in Australia and elsewhere that
the demands of P . rudiutu could be satisfied by all soils down to the very
poorest. This mistaken idea was responsible for the establishment of
plantations on infertile soils in which growth was either extremely
unsatisfactory or a complete failure. While modern fertilizer practice has
enabled a reasonable forest to be grown on some of these sites, many
problem areas remain where close proximity to timber markets would
make even a modest growth rate an economic success. Soil factors which
restrict growth are those which appear to give a poorer rate of growth
than expected from the terrestrial climate.
The rate of growth of a forest can be expressed mathematically as a
function of many factors. Often some of the factors used are themselves
mutually related. Because of this, even though partial regression procedures which in effect hold other factors constant, may isolate a significant
association between one particular factor and growth, as yet no causal
relationship can be inferred. This is because there still exists a possibility
that the significant factor may be further related to an underlying causal
factor which may not have been examined or even measured. To be certain that a significant association is causal, independent evidence is necessary. Sometimes such independent evidence may come from carefully
controlled experiments; in other cases it may be quite obvious. This
argument does not detract from the use of correlation techniques, but it
points to the caution necessary when interpreting the results of
A. LIMITEDVOLUMEOF EXPLOITABLE
The volume of soil which is available for root exploration may be
limited by physical or chemical factors.
I . Soil Depth and Stones
P . radiata is frequently planted over a rolling landscape with better
results on the lower sites and valley bottoms than on the higher areas.
There are many reasons for such differences; among these are dissimilarities in genesis which generally make the valley soils more fertile and
greater in profile depth. Of these how important is the depth of the soil
profile? Probably it is important only insofar as it provides ( 1 ) a nutrient
and moisture source of sufficient magnitude and capacity to supply the
demands of good growth and ( 2 ) a secure anchorage against windthrow.
Glasshouse work meeting these conditions can give trees of 6 feet or
more in height in quite small containers.
In Argentina, Vidal (1961) has reported poor results on soils less
than 18 inches deep, and in South Australia and Argentina a comparison
of soils defined by soil surveys has shown that the growth of P. radiuta
is poorer where the soils are shallow or have impenetrable horizons of
hardpan or concretionary material (Beckmann, 1964; Barrett and Garbosky, 19.60). However, in California trees as high as 60 to 70 feet are
found with as little as 6 inches of surface soil on weathering granite
rubble (Scott, 1960), but roots would penetrate into the parent material
and growth would be very slow.
Jackson (1965) has presented a table for the allocation of different
species to sites at Hawke’s Bay, New Zealand, taking into account rainfall, soil type, and profile depth. On red loams, a rainfall of 35 inches
per annum and a soil depth of 2% feet are required for P. rudiatu. It
would be desirable to examine the relationship between mean annual
increment, depth to the least permeable horizon, and rainfall for P.
radiata in a similar way to Jackson’s study for slash pine ( P . elliottii
Little is available on the rooting habits of P. radiuta. In California
roots penetrate only to depths of about 2 feet even in good soil, but far
deeper penetrations in clay soils (Bowen, 1964; Raupach, 1967) and in
sands have been found in South Australia. Windthrow is common for
this species on poorly structured shallow soils.
The volume of exploitable soil is sometimes limited severely by stones.
For example as much as 70 percent by weight of some lateritic podzolic
soils supporting P. radiata in South and Western Australia are retained
by a 2-mm. sieve. This is considered further in Section V, D below,
2. Chemical Barriers
The soil water regime will be discussed under Section IV, B, but
reduced conditions and high salt content in localized zones of soil give
rise to barriers which are more chemical than physical. Depressed growth
has been reported on sites with poorly drained heavy clay (e.g., Laughton, 1937; Weston, 1958), root development probably being restricted to
the non-clay layer. Solonetz soils have also been observed to be uncertain sites (Woods, 1955); here the salt content of the clay layer below
the sandy A horizon may be toxic under particular seasonal conditions.
Although clay layers have been said to restrict growth, high quality
trees are commonly observed in parts of Australia where the clay content
is 60 to 90 percent, whereas in Argentina Barrett and Garbosky (1960)
reported an upper limit of about 40 percent in the B-horizon. Clearly a
number of other soil properties should be considered along with clay
content in defining growth limits.