Environmental Engineering Reference
In-Depth Information
Figure 14.1
Conceptual bottleneck model representing the different successive processes of re-establishment in different
types of forest as an example, which can be impeded by a two-stage limitation (dispersal and environmental). The funnel-
shaped lines indicate the decreasing number of individuals through mortality and predation. First, dispersal limitation causes
lower seed occurrence on the soil in a site to be restored (arrow A), second, environmental limitations can cause the funnel to
narrow even more across different life stages (arrow B); it depends on the species and specifi c land-use history of the site to be
restored. Modifi ed from Baeten
et al
. (2009) . Reproduced by permission of Wiley - Blackwell.
persal mode mechanisms were predominant in the
local community before
fragmentation
of the land-
scape. Low-intensity farming entailed an enormous
dispersal of seeds as a result of livestock grazing,
cutting and manure application (Poschlod & Bonn
1998) and from holding barns (Losvik & Austad 2002).
Current intensive farming in north-western Europe
strongly reduces this seed dispersal pathway as a result
of fencing and regulation of rivers and brooks
(Poschlod & Bonn 1998). Remnants of the once exten-
sive dry grasslands and heathlands are separated in the
highly fragmented landscapes of contemporary
Europe, and are no longer interconnected by fl ooding
water and large-scale tending of fl ocks of livestock for
transport of seeds (Ozinga
et al
. 2009 ; see Box 14.1 and
Figure 14.2). Hence, failure of short-distance dispersal
through fragmentation, and of long-distance dispersal
due to the loss of connecting vectors, has become an
important constraint to restoration efforts. This holds
not only for seeds but also for gene exchange by pollen
between populations (Kwak
et al
. 1998 ).
14.3.3
Dispersal in time: seed longevity
Plant species that had disapperared from the estab-
lished vegetation can survive in the soil
seed bank
as
a memory of the past. Especially after changes in
habitat quality and land use, it is important to know
which species have a long-term persistent seed bank.
Only those species can re-establish during restoration.
Soil seed bank analyses from a range of plant commu-
nities in dry grassland and heathland in north-western
Europe show that these communities can only partly
