Restoration of dry grasslands and heathlands - Restoration Ecology - page 100

infrastructure', i.e. large-scale tending of flocks of live-
stock (Poschlod et al . 1996). Hence, dispersal is supposed
to become an important constraint in restoration.
This does not only hold for seeds, but also for gene
exchange by pollen between populations (Kwak et al .
1998). A complication here is the finding that species
of different trophic levels (plants, herbivores/pollin-
ators and predators/parasitoids) show an increasing
positive correlation between fragment size and spe-
cies richness (Steffan-Dewenter & Tscharntke 2002).
This implies that common insects will become more
and more dominant (Kwak et al . 1998). Therefore,
specialized relationships between trophic levels may
be disturbed in small fragments with subsequent
problems in restoration.

ively exploited arable fields and pastures created
mesotrophic and eutrophic soil conditions. Authorit-
ies in charge of management cannot reduce the
atmospheric deposition from precipitation. However,
dry deposition can be reduced by maintaining a low
canopy. In the Netherlands, atmospheric deposition
of nitrogen compounds ranges from between 50 and
170 kg of N ha −1 yr −1 in forests and between 30 and
50 kg of N ha −1 yr −1 in grassland and heathland. Critical
nitrogen loads for heathland and grassland are below
20 kg of N ha −1 yr −1 (Bobbink et al . 1998). Reducing
the nutrient availability in the soil is a challenge.

8.3.5 Invasive species

Even when removed as adult plants, invasive species
may leave a legacy that makes long-term restoration
of the site difficult. This legacy may be in the form
of a long-term persistent soil seed bank that is often
larger in a species' new home than in their native hab-
itat. Another legacy may be chemical alteration to the
habitat, such as elevated nitrogen mineralization by
nitrogen fixation (D'Antonio & Meyerson 2002). Atmo-
spheric deposition may enhance this latter problem.

8.3.3 Soil organisms

Many plant species depend on mycorrhizal fungi.
However, soil organisms disperse very slowly, and hence
are a constraint for restoration (Baar 1996, Van der
Heijden et al . 1998) as indicated above for seed dis-
persal. Grasslands dominated by Sporobolus wrightii
once covered riparian floodplains in the south-
western USA, but were reclaimed for agriculture.
Inoculation of the grass with arbuscular mycorrhizal
fungi can benefit restoration efforts in abandoned agri-
cultural fields in semi-arid regions (Richter & Stutz
2002). Also, other soil communities may be important.
Disturbed sites that have been planted with native
perennial species to facilitate restoration revealed
that the soil microbial community composition differed
from that of relic native stands: it was closer to
that of grasslands composed on invasive, non-native
species (Steenwerth et al . 2002). In a microcosm
experiment, soil herbivores such as nematodes forage
particularly on grass species dominant in early eutro-
phic successional stages of restoration, and hence
may accelerate the establishment of later successional
target species in restoration of species-rich meso-
trophic grasslands (de Deyn et al . 2003).

Box 8.1 When maintenance of grassland
or heathland fails: can forests be
restored easily?

Ancient forest species cannot rely on a long-term
persistent seed bank for recovery after reclamation to
arable field or pasture (Bossuyt et al . 2002). Dispersal
is another constraint. Although a large number of
forest species will be able to colonize new forest, some
typical forest species have low rates of colonization.
Hence the complete restoration of the understorey
requires a time period of over a century (Bossuyt &
Hermy 2000). A third point may be competition.
Recently established forests on former agricultural land
with a relatively high nutrient status and a relatively
open canopy provide excellent habitat conditions for
competitive species such as Urtica dioica and Rubus
fruticosus . Therefore, competitive exclusion can be an
explanation for poor establishment of characteristic
forest species (Butaye et al . 2002). Restoration of
forest will be dealt with further in Chapter 10 in this
volume.

8.3.4 Eutrophication

The atmospheric deposition and transformation of
oligotrophic dry grassland and heathland into intens-