Geoscience Reference
In-Depth Information
Plate 21.5
Tropical deforestation is not new. Forests at 1,500 m near Eliya, Sri Lanka, were cleared by British tea growers in
the 1850s to produce tea gardens. Note that the trees planted to give shade are exotics from South America and Australia.
Photo: R. T. Smith
2
Both mycorrhizal and non-mycorrhizal roots produce
phosphatase enzymes which improve phosphorus
nutrition by hydrolysing organic-phosphorus com-
pounds. Increases in atmospheric carbon dioxide stim-
ulate phosphatase production and hence phosphorus
availability.
emerging that tropical forests make up a significant
component of the terrestrial sink for atmospheric CO
2
(Lloyd
et al
. 2001).
CONCLUSION
3
Roots are able to exude citrate and oxalate acids
which are capable of solubilizing organic phosphorus
compounds in the soil and increasing the uptake by
plants of inorganic phosphorus in the soil solution.
Root exudation is energetically demanding, and will
increase with improved plant carbohydrate status.
The International Biological Programme (IBP), which
commenced in 1964, stimulated much data collection
and ecological stocktaking in all the world's biomes. The
aim has been to solve the fundamental ecological equa-
tions dealing with productivity, biomass, nutrient status
and energy assimilation. Rates of photosynthesis in
different biomes vary with light intensity, temperature,
moisture and soil nutrient content. Thus latitude is a
great determinant of productivity on land through its
effects on radiation, temperature, moisture and the length
of the growing season. By contrast productivity in oceans
is much more closely linked with the availability of
nutrients. The productive zones in the oceans occur not
One can conclude that plants have several means of
increasing the rate of phosphorus mineralization and
uptake, even when soil phosphorus levels are low. As
increases in atmospheric carbon dioxide should increase
plant carbohydrate status, one should not assume that
nutrient-poor tropical forests will not be able to respond
to higher CO
2
levels. In fact more and more evidence is
