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

Chapter 4 Retrogressive Succession and Restoration on Old Landscapes 81
The spatial spread of “successional” stages replaces time with space and in
most cases only two or three steps in the succession are needed in any one
landscape facet. In a landscape with high surface run-off, the first step is to
install an interceptor drain to remove excess water and store it in local dams.
Perennial pastures are the precursor of shrubs and trees that are established
along the drain (Fig. 4.5c). Facet A is the lowest part of the landscape and
initially requires restoration with halophytes but as the water table is lowered
by actions upslope and leaching of salt, true halophytes can be replaced over
time by salt-tolerant trees or taller shrubs. The lower slopes (B) and (C) are
the production areas and rotate between crop production and perennial grazing
pastures. Steeper slopes are less suited to crop production and are placed under
permanent pastures, while the steep upper slopes and crests are planted to trees
as wildlife corridors and to reduce recharge from these areas.
The proportions of the different plant communities can be estimated using
relatively simple and well-understood models of plant water use. Dunin (2002)
has shown how the species ratio can be calculated for a farm in Western Australia.
In this case, he estimated that 12% trees, 30% lucerne, and 58% annual
crops would on average result in only a small leakage of water to the ground
water table to maintain stream flows.
4.6 Secondary Rainforest Successions on Old Tropical Landscapes:
Symptoms of Declining Site Nutrient Capital
Old, highly weathered soils cover almost three quarters of the humid tropics
(Kauffman et al. 1998, Sanchez 1976). Many of these areas naturally support
tall-stature, biologically complex rainforests where the apparent lushness of the
vegetation belies the inherent infertility of the substrate (Richards 1952). It is
typical in these systems on older landscapes that a high proportion (50–90%)
of the total site nutrient capital (i.e., the total amount of nutrient in all pools that
are potentially available to the biota) is held in the biomass (Nye 1960, Jordan
1982, Medina and Cuevas 1989) and that the vegetation has well-developed
mechanisms for the acquisition and retention of nutrients (Grubb 1977, Stark
and Jordan 1978, Janos 1983). Whittaker (1970) succinctly summarized the
functioning of tropical rainforests on these highly weathered soils as “a relatively
nutrient rich economy perched on a nutrient poor substrate.”
Tropical forests on old landscapes in which most of the site nutrient capital
is stored in the biomass are particularly vulnerable to high intensity anthropogenic
disturbances (e.g., intensive timber extraction, burning, and clearing)
and there is an extensive literature on the problematic nature of their regeneration
(Sanchez 1976, Lovejoy 1985, Sim and Nykvist 1991). In these situations, not
only is much of the site nutrient capital directly removed by the disturbance, but
the relatively tight, biologically controlled nutrient cycles are disrupted further,
exacerbating nutrient leakage from the landscape. Poor natural regeneration
characterized by arrested succession and/or lack of complex structural development
is a particularly common feature of these disturbed areas. In essence,
disturbed tropical forests on old landscapes provide examples of where anthropogenic
activities have accelerated the process of retrogressive succession.
Perhaps the most striking illustration contrasting tropical forest succession
in young and old landscapes comes from work on secondary successional