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

et al. 2002) because large amounts of nutrients have accumulated in the soil.
Nutrient balance sheets have shown that the annual removal of N and P through
haymaking is at most 3% (N) to 5% (P) of the soil nutrient pool (Bakker 1989),
and the amounts removed by grazing are very low (Perkins 1978, Bakker 1989).
One way to decrease nutrient supplies rapidly is to completely remove the
surface layer of topsoil (Marrs 1993, Verhagen et al. 2001). This approach has
been shown to reduce soil N, but it was less effective at reducing soil P (Marrs
et al. 1998b). This approach may also lead to a shift in the nutrients which
limit productivity; from co-limitation by N and P to limitation by N alone. The
consequences of such a shift are unknown but indications suggest that it might
favor grasses at the expense of herbs (Olde Venterink et al. 2003).
On organic-rich grassland soils the nutrient stocks are large, and the only
way to reduce fertility is to lower nutrient turnover rates. One way to lower the
N supply is to reduce oxygen availability by rewetting drained sites to lower
aerobic microbial activity (see Chapter 5). This may work in sites that have
not been drained intensively, but where drainage has been intense the N supply
remains high (Hauschild and Scheffer 1995). This is also generally the case
immediately after rewetting. Phosphorus availability is mainly controlled by
the adsorption and desorption of P on Fe-, Al- and Ca-complexes. Alteration of
these binding reactions when sites are rewetted with mineral-poor rainwater or
sulphate-rich water may even lead to increased P supply (Lucassen et al. 2004).
Other impacts are also possible; infiltrating rainwater can wash out potassium
(K) and K-limitation may develop (Eschner and Liste 1995, van Duren et al.
1997) instead of N- and/or P-limitation. Again, the consequences of such shifts
are unknown and manipulation of these factors is not easy.
The local species pool (cf. Zobel et al. 1998) is a second filter that affects
succession trajectories of grassland restoration. There are negative relationships
between the period of intensive land use and the degree to which species
reappear from the seedbank (Bekker et al. 1997), and between the degree of
landscape fragmentation and immigration of target species (Poschlod and Bonn
1998). Thus, the speed and course of succession are determined largely by the
period and intensity of previous land use on the site and in the surroundings.
Because of limitations on species colonization, steps need to be taken to
increase the colonization of target species. Successful methods included deliberate
mowing or grazing regimes where the machines or animals are moved
from species-rich target communities to species-poor restoration sites in order
to facilitate the transfer of seeds (Strykstra et al. 1997, Mouissie et al. 2006).
Alternatively, dispersal limitation can be overcome by deliberate addition of
propagules by a variety of methods; for example, by adding fresh hay cut from
species-rich reference fields (Hölzel and Otte 2003), adding top soil from donor
sites (Vécrin and Muller 2003), or even by transfer of complete turfs (Klötzli
1987, ˇSeffer and Stanová 1999, Antonsen and Olsson 2005). The obvious disadvantages
of these methods are that reference donor sites are affected severely
or even destroyed and these methods are very costly (Vécrin and Mueller
2003).
Unassisted succession in human-managed secondary grasslands (in contrast,
for example, to natural prairies) is not usually a tool of restoration. Continued
manipulation of succession in the form of maintenance management (Bakker
and Londo 1998) is essential for the persistence of seminatural grasslands.
Chapter 6 Manipulation of Succession 133