ROOTS - The Soil - page 47

plants (especially crop plants) have the smallest proportion of their mass as roots, with
often as little as 10 percent of the total being below ground, although 15-25 percent is
more usual for trees and the value may be as much as 40 percent. The actual mass of
roots varies enormously between various types of vegetation. For example, a soybean
crop may have less than one tonne (1 t = 1000 kg) of roots per hectare (100 m x 100
m), while wheat tends to have over 1 tonne per hectare. Under grasslands there is very
much more root, typically 10 to 20 tonnes per hectare, often making up four-fifths or
more of total plant mass, and since these roots are generally very fine, the length of
root in a given soil volume can be astonishingly high: over 100 centimetres of root
per cubic centimetre of soil or around a million kilometres of root under a single field
of grass. The mass of roots in forest ecosystems depends very much on soil type, spe-
cies and (for plantations) the age of the stand, but can be as high as 200 tonnes per
hectare. The range of figures that have been reported is shown in Table 3 .
T ABLE 3
Root production in various ecosystems. The data are taken from various sources and
are estimates which should be treated with some caution because of the difficulty of
extracting roots quantitatively from soil.
These figures suggest that herbaceous plants such as grasses devote much more
of their resources to growing roots than do trees and other woody plants, which tend
to have rather low below-ground biomasses in relation to what is above ground. It is
also possible, however, to estimate the proportion of the annual production that plants
commit to above-ground and below-ground growth ( Table 3 ). Whereas grasslands de-
vote resources to roots and shoots in about the same proportion as their biomasses,
trees appear to send proportionately much more below ground, since forests rarely
have much more than 40 percent of their biomass in roots but always commit at least
this much (and sometimes as much as 70%) of production to root growth. There are
two reasons for this discrepancy: one is that many fine roots are short-lived and are
replaced by new rootlets during the growing season, and the other is that a significant
part of the production going below ground in many tree root systems does not actually
go to the roots at all, but to fungi which live in asociation with them. These are called