Vegetation development and succession - Fundamentals of the Physical Environment - page 653

Growth form r species k species

Stability Poor Good

Table 21.3 Energy changes through succession

Attribute Early Late

P/R ratio > < 1 1

P/B ratio High Low

B/E ratio Low High

NPP High Low

Food chains Linear Webs

ratio (the ratio of gross production to biomass) decreases during succession. The B/E
ratio (the ratio of biomass to a unit of energy flow) is the inverse of the P/B ratio, and is
called the biomass accumulation ratio. It increases from close to 1 in the annual herb
stage to 2-5 in the perennial herb-low shrub stage, to 5-10 in the high shrub-young
forest stages, to 30-50 in climax forests. Thus biomass in relation to productivity reaches
a maximum in the climax. Net primary production (NPP) does not reach a maximum in
the climax, however. It is highest in the early stages of succession, when the vegetation
consists of fast-growing species. In the climax vegetation the biomass shows a stable,
steady state.

THEORIES OF CLIMAX VEGETATION

The concept of climax vegetation arose at the beginning of the twentieth century. At a
time when soil scientists were working with the idea of 'zonal soils', and climatologists
were defining 'climate regions', ecologists started to conceptualize a stable type of
natural vegetation which would be in complete equilibrium with climatic and soil
conditions. The theory was put forward that, with no human interference, the end point of
succession would be a self-sustaining and self-perpetuating community. This community
would be the one which could compete most successfully in the prevailing soil and
climatic conditions. The US ecologist Frederick Clements proposed the term climatic
climax vegetation , defined as 'vegetation in stable equilibrium with climate and soil,
given undisturbed conditions and free soil drainage'. Clements is credited with advancing
this monoclimax theory , or climatic climax theory . He developed the concept of the
plant community as an 'organism' which followed a sequence of stages as it developed
into a mature state. The mature state or 'climatic climax' would be in equilibrium with
the regional climate and the zonal soil, provided there was relatively long-term stability.
All communities would reach this end point through plant succession, no matter what had
been the initial starting point. Thus a psammosere and a hydrosere in a given region
would ultimately reach the same steady-state vegetation. In southern Britain both would