V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
217
for taking the samples for this calculation. Even light grazing causes a
marked reduction in productivity. Naturally, the species composition of
the grassland changes with increasing rainfall. For more humid climates,
larger and more productive grass species (e.g., Aristida spp., Eragrostis
spp.) are found compared with those growing in less humid climates.
Calculations based on the values obtained by Loneragan (Walter, 1964,
p. 312) show that the linear relationship between annual rainfall (in the
range of 500-1500 mm) and production is also true for the Eucalyptus
forest area in Southwest Australia. The annual production of dry matter
in the leaf litter per square meter of soil surface is directly proportional
to yearly rainfall. This relationship was also found for the total leaf litter
surface.
For grasses of Southwest Africa it may also be assumed that the transpiring plant surface is proportional to the dry matter produced above
ground and therefore is also proportional to annual rainfall.* Plant density
of Larrea divaricata was found to increase directly with annual rainfall in
the Sonora and Mojave desert (Woodell et al, 1969, p. 40).
From these considerations the following important conclusion can be
drawn: The unit of the transpiring plant surface receives about the same
amount of rainfall in arid and in humid regions. In other words, the water
supply of an individual plant per unit of its transpiring surface is not significantly less in arid climates than it is in humid regions.
The chief response of vegetation of arid regions against water loss is
reduction of the transpiring plant surface per unit of land area. Of course,
other adaptations are required to endure a long drought season. They are
mostly of a morphological and anatomical nature.
With decreasing density of the plant cover, the plants are farther apart.
Each plant has more soil area available and the plant has a larger root
system which can develop more extensively horizontally. This indicates
another significant feature: Root systems become larger as the climate becomes increasingly more arid, when comparing plants of similar life forms
of arid and humid areas, and the ratio of shoot mass to root mass decreases
(see Table I ) .
In deserts with a large proportion of succulents and hygrohalophytes,
the contribution of the underground parts to the whole phytomass is low
(62% for deserts with hygrohalophytes). The ratio is very high in the tun* This rule also holds true for plantations in arid zones, e.g., olive trees are cultivated in Tunisia in regions with 200-800 mm annual rainfall. Correspondingly, the
tree density increases from about 20 trees per hectare to about 80 trees per hectare.
Since the olive harvest per tree (about 20 kg) is about the same in arid regions as
in humid, proportionality exists between olive yield and amount of annual rainfall
(Le Houérou, 1959, p. 32, Table III).
217
for taking the samples for this calculation. Even light grazing causes a
marked reduction in productivity. Naturally, the species composition of
the grassland changes with increasing rainfall. For more humid climates,
larger and more productive grass species (e.g., Aristida spp., Eragrostis
spp.) are found compared with those growing in less humid climates.
Calculations based on the values obtained by Loneragan (Walter, 1964,
p. 312) show that the linear relationship between annual rainfall (in the
range of 500-1500 mm) and production is also true for the Eucalyptus
forest area in Southwest Australia. The annual production of dry matter
in the leaf litter per square meter of soil surface is directly proportional
to yearly rainfall. This relationship was also found for the total leaf litter
surface.
For grasses of Southwest Africa it may also be assumed that the transpiring plant surface is proportional to the dry matter produced above
ground and therefore is also proportional to annual rainfall.* Plant density
of Larrea divaricata was found to increase directly with annual rainfall in
the Sonora and Mojave desert (Woodell et al, 1969, p. 40).
From these considerations the following important conclusion can be
drawn: The unit of the transpiring plant surface receives about the same
amount of rainfall in arid and in humid regions. In other words, the water
supply of an individual plant per unit of its transpiring surface is not significantly less in arid climates than it is in humid regions.
The chief response of vegetation of arid regions against water loss is
reduction of the transpiring plant surface per unit of land area. Of course,
other adaptations are required to endure a long drought season. They are
mostly of a morphological and anatomical nature.
With decreasing density of the plant cover, the plants are farther apart.
Each plant has more soil area available and the plant has a larger root
system which can develop more extensively horizontally. This indicates
another significant feature: Root systems become larger as the climate becomes increasingly more arid, when comparing plants of similar life forms
of arid and humid areas, and the ratio of shoot mass to root mass decreases
(see Table I ) .
In deserts with a large proportion of succulents and hygrohalophytes,
the contribution of the underground parts to the whole phytomass is low
(62% for deserts with hygrohalophytes). The ratio is very high in the tun* This rule also holds true for plantations in arid zones, e.g., olive trees are cultivated in Tunisia in regions with 200-800 mm annual rainfall. Correspondingly, the
tree density increases from about 20 trees per hectare to about 80 trees per hectare.
Since the olive harvest per tree (about 20 kg) is about the same in arid regions as
in humid, proportionality exists between olive yield and amount of annual rainfall
(Le Houérou, 1959, p. 32, Table III).