Environmental Engineering Reference
In-Depth Information
when conditions for growth are worse; and the location
of the thinning line varies remarkably little for different
species and growing conditions.
There is only about a sixfold range of intercepts,
a minimum variation considering the enormous span of
mean plant weights. The rule fits not only trees but also
shrubs, herbs, ferns, and mosses, whose average masses
span more than 10 OM. This means that a monospecific
even-aged stand with 100,000 stems/ha has trees aver-
aging no more than about 0.2-0.4 kg and adding up to
a maximum of a bit over 40 t, whereas 100 trees/ha av-
erage 6 t/tree with a mass of more than 600 t/ha (fig.
3.12). Economies of scale mean that every tenfold con-
centration of photosynthesis brings roughly a 32-fold
increase in the average plant size as more phytomass is
accumulated in stands of fewer but larger plants, result-
ing in large, long-lasting structures that require durable
construction (dense wood) and effective protection
(barks or defensive chemicals) against heterotrophs.
The self-thinning rule has been recurrently questioned,
and Lonsdale (1990) even concluded that there is no ev-
idence to support it: the slope is much more variable than
previously claimed, and the straight lines demarcating the
M
max
are the exception rather than the rule. In contrast,
Enquist, Brown, and West (1998) found a strong corre-
lation between maximum plant density and average plant
mass for autotrophs whose mass ranged over 11 OM,
from Sequoia to Lemma (duckweed). Their slope for
the allometric relation between M
tot
and r was
0.325
rather than
0.5. (fig. 3.12), but their analysis strongly
confirms the existence of the self-thinning rule as plants
grow and proliferate until checked by the availability of
energy and nutrients.
