Agriculture Reference
In-Depth Information
Individual strategies
Trees usually have much deeper root systems than grasses (see,
e.g.,
Singh and
Srivatsava, 1984; Fournier, 1991). Their root systems are often differentiated into a lateral-
superficial system specialised for absorbing nutrients from decomposing litter and the
upper organic soil and tap roots that penetrate deeply into the soil searching for water
(Figure III.14). In grasslands, most roots are concentrated near the surface. In the humid
savannas of Lamto (Côte d'Ivoire) for example, César (1971) collected 78 % of root
biomass in the first 20 cm. In shrub savannas, the root distribution showed a bimodal
pattern with the first mode corresponding to grass roots close to the surface (65 % in the
upper 20 cm) and shrub roots comprising a second mode at a 30-40 cm depth.
The maximum depths at which roots have been found exceed 61 m, for
Juniperus
monosperma
roots from underground mine workings. The maximum radial extent of tree
roots is more than 50m from the stem and 30m has been recorded for a number of species
(Stone and Kalisz, 1991).
Nutrient availability
In highly nutrient-limited soils the superficial roots may form mats up to 40 cm thick
which grow into the litter layers. Mycorrhizal fungi link the roots and the litter
layers ensuring direct nutrient reabsorption from the decomposing leaves (Stark and
Spratt, 1977). This root mat has proved to be highly efficient in the interception of
nutrients and, in a South American rainforest, 99 % of a radioactively-labelled solution
of phosphate sprayed on its surface was taken up by the plants (Stark and Jordan, 1978).
Dynamics of water in the soil profile
The distribution of deep roots appears to be closely related to soil water status and par-
ticularly the depth to the water-table. In two oxisols of the equatorial forest, Humbel
(1978) showed that root penetration was markedly reduced where water tables occurred
at shallow depths; in well-drained soils, roots normally explore much deeper soil strata.
In the Banco tropical rain forest (Figure III.20), the presence of a water-table in the low-
lying areas of the valley bottom limits the extension of coarse roots to 40 cm in depth
