Linking Restoration and Ecological Succession (Springer ... - Inecol

70 Joe Walker and Paul Reddell
demonstrate the need for detailed process knowledge about succession when
trying to restore old landscapes following land-use disturbances.
4.2 Natural Retrogressive Succession
Succession theory in ecology had its beginnings in studies of vegetation changes
across sand dune chronosequences (Warming 1895, Cowles 1895). A progressive
build up to a maximum biomass and species richness were observed across
the dunes, and these changes were accompanied by gradual changes in the
physicochemical composition of the soils. The early vegetation work was contemporary
with the idea of geographical cycles proposed by Davis (1899) to
explain how landforms evolved over time, and his schema included renewal
and degradation. The vegetation observations led to the idea of a “climax community,”
that is, the progressive development toward an optimum expression of
vegetation for the climate and soils of a region. As ecological ideas developed,
the climax state was seen to fluctuate but it was generally regarded as in a
stable or dynamic equilibrium state. In terms of restoration actions, succession
trajectories toward the original “climax” state have been viewed as desirable.
Debate continued about the dynamics and trajectory of successional processes,
but gradually it was realized that natural post-climax states exist. Early studies
using pollen analysis in peat or mor layers in postglacial deposits (Iversen
1964) showed examples of ecosystems with permanently reduced productivity.
These so-called retrogressive successions were associated with leaching of the
soils during pedogenesis (natural retrogression) and man-made disturbances
(secondary retrogression). A clear demonstration of a post-climax natural retrogression
was described for an intact aeolian sand dune chronosequences at
Cooloola, Queensland, Australia by Walker et al. (1981) and Thompson (1981
and 1983). Dune building at Cooloola was episodic, and some parts of earlier
dune systems were not buried by subsequent wind-blown deposits. These exposed
parts were subjected to weathering, leaching, and erosion, and form an
age sequence that stretches over some 750K years (Thompson 1992). These
studies showed that for freely drained sites vegetation type, species richness,
standing biomass, and soil carbon accumulation varied with dune age. After a
period of biomass build up (progressive succession) seen in the youngest four
dune systems, vegetation biomass and the store of organic material in the surface
soils declined in the oldest three systems (retrogressive succession). Vitousek
and Reiners (1975), Hedin et al. (2003) and Wardle et al. (2004) have shown
for a series of very long-term chronosequences, that ecosystem decline is a
widespread phenomenon. However, the mechanisms controlling retrogression
vary between bioregions and with disturbance regimes and intensity.
The hypothesis developed from the Cooloola study (Fig. 4.1) relevant to planning
restoration actions is that pedologically young landscapes when disturbed
tend to recover toward the previous state, whereas old systems become “leaky”
or cannot recover critical system functions and trend toward a new system state
with lower biomass and complexity.
Old landscapes are common in Australia but also occur in most parts of the
southern hemisphere and in tropical areas. At a global scale old landscapes predominate,
yet many of the developments in succession theory were carried out in
pedologically young landscapes, and perhaps the same is true of attempts to develop
links with restoration activities. Like many areas in the tropics, Australia