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

154 Richard J. Hobbs, Anke Jentsch, and Vicky M. Temperton
the onset of restoration measures, to help assess what the limiting factors for
restoration are, and how one could alter the filter mesh to allow certain species to
establish.
This is where the issue of disturbance comes in. Disturbances modify community
assembly in two ways. Firstly, disturbances change environmental filters
such as nutrient availability, and secondly, they act on plant traits as filters of
species (biotic) assembly in their own right (White and Jentsch 2004). Thus,
disturbance can be not only the cause of degradation, but at appropriate scales
and magnitude can also be a direct tool for restoration managers wanting to restore
appropriate ecosystem dynamics. An example here would be the restoration
of species-rich calcareous grasslands on nutrient-poor sites, where active
disturbance in the form of sheep grazing or mowing of the grassland, keeps
the ecosystem from accumulating nutrients and from undergoing succession to
shrub- and woodland. Another example would be the restoration of speciespoor,
resource-limited inland sand dunes, where, at least in some situations, anthropogenic
disturbances in the form of military maneuvers or top-soil removal
keep parts of the ground bare for seedling establishment and again prevent the
ecosystem from accumulating nutrients and undergoing succession to shruband
woodland (Jentsch and Beyschlag 2003).
7.4 Succession—Not One-Way
Although there are various conceptual models in existence, succession is widely
acknowledged to be a continuous, though often stepwise, process of species
turnover with varying speeds and trajectories (see Chapter 1). Thus, the application
of knowledge about temporal dynamics in ecosystems is the fundamental
approach in successional theory that can be linked to restoration. We explore the
idea that temporal dynamics in ecosystems are the product of two interacting
factors: continuous versus discrete processes (Jentsch and White, unpublished
data). Continuous processes include gradual accumulation of biomass and nutrients,
as the system moves through progressive successional stages. Discrete
processes include the occurrence of disturbance, which can cause rapid transitions
between different ecosystem states or suddenly reset the successional
clock. In addition, disturbance can change continuous processes such as colonization
or extinction of indigenous species to sudden events such as rapid
invasion of an alien species. Thus, restoration managers have two different
options for modifying ecosystem dynamics at restoration sites: manipulating
continuous processes (succession) or making use of discrete events (disturbance).
While most successional sequences seem to follow a progressive accumulation
of biomass, successional studies on very long-term chronosequences or
pollen sequences indicate a decline phase in succession in the absence of a
major disturbance (Iversen 1969, Wardle et al. 2004). This mirrors earlier considerations
of vegetation dynamics as more cyclical than unidirectional (Watt
1947, Remmert 1991, van der Maarel 1996), and is included in more recent
conceptualizations of ecosystem dynamics (Holling 1986 and 1993). These
ideas of cyclic dynamics implicitly include a decline phase which is followed
by a recovery phase, either following disturbance or after the slow release of
accumulated biomass and nutrients.