Environmental Engineering Reference
In-Depth Information
with high nature conservation interest are still found
in Europe, it is increasingly diffi cult to maintain them
as a result of changes in land use (Veen
et al
. 2009 ).
North American tallgrass
prairies
expanded greatly
beginning about 5000
BP
(Knapp
et al
. 1998 ). From
that time until about 3000
BP
, the region's climate was
hotter and drier than at present, favouring grassland
development as woodlands retreated. The origins of
tallgrass prairies and the roles of climate, grazing
animals and human activity are still not completely
understood. Humans have been part of the prairie
biome on that continent ever since before the last gla-
ciers retreated from the Midwest. Prairies were home to
Native Americans for thousands of years prior to Euro-
pean settlement. The Native Americans managed the
prairies according to their needs and abilities, and delib-
erate fi res played an important part in shaping prairie
vegetation. Fire was used to facilitate travelling and
hunting, to stimulate new plant growth for game and
to reduce fuel loads near habitations. Nowadays, large
areas of former prairie that were destroyed through
conversion to agriculture, intensive grazing or urbani-
zation, have been degraded through fi re suppression,
drainage or other alterations (Knapp
et al
. 1998 ).
Having indicated the main characteristics of the
relevant low-intensity farming systems, in the next
section we will discuss the changes in land use, namely,
intensifi cation and abandonment. Then, we will discuss
the results of techniques to overcome the constraints
for restoration. Some such as growing crops deal with
only the nutrient status of the soil; others such as
mowing, grazing, burning, sod cutting and topsoil
removal have effects on both reduction of nutrients
and prevention of succession to shrubland. Finally, we
will discuss whether such techniques must be carried
out by nature conservation agencies in designated
reserves or also outside reserves carried out by farmers
according to agri - environment schemes.
(3) reduction in plant species richness as a result of
seeding agricultural crops, fertilizer application and
soil homogenization. Further increase in soil fertility is
the result of background
eutrophication
, which is
derived from surrounding fertilized agricultural areas
and atmospheric deposition, especially of nitrogen
(Verhagen & van Diggelen 2006). This has been de-
monstrated clearly in the United Kingdom through the
analysis of long-term land use surveillance data (Smart
et al
. 2005). Indeed background eutrophication was
identifi ed as one of the drivers leading to biotic homo-
genization at a regional scale (Smart
et al
. 2006 ). Plant
communities that occur on very nutrient-poor soils,
such as those of dry grasslands and heathlands, are
particularly vulnerable to this indirect nutrient addi-
tion (Bobbink
et al
. 1998 ). This atmospheric deposi-
tion, which has increased since the 1980s (Smart
et al
.
2005), makes it harder to (1) maintain the commu-
nities on nutrient-poor soil where they already exist
and (2) restore these communities after exploitation
(Marrs 2002 ).
The second type of change occurs where marginal
agricultural areas either have been
abandoned
with
no active management or now receive reduced man-
agement in one way or another. On such sites,
succes-
sion
occurs with shrub and tree encroachment and
hence a decline in the area of grassland and heathland.
Well-documented examples of the decrease of habitats
of the type covered in this chapter include heathlands
(Webb 1997 ), calcareous grasslands in north - western
Europe (WallisDeVries
et al
. 2002 ), wooded meadows
in Sweden (Mitlacher
et al
. 2002) and tallgrass prairies
in the United States (Samson & Knopf 1994). In addi-
tion, European coastal sand dune grasslands have also
decreased as a result of the decline of the natural her-
bivores, namely, rabbits in the 1950s (Jones
et al
.
2008), agricultural abandonment and urbanization.
Abandonment coincides with bush encroachment
accompanied by a decrease in plant species richness.
For instance, on the alvar grasslands in Öland, Sweden,
the number of plant species characteristic of the alvar
community has declined continuously as cover of
Juni-
perus communis
shrubland increases to become the
dominant species (Rejm á nek & Ros é n 1992 ). Associ-
ated fauna is also affected. In Sweden, insect species
dependent on plant species of nutrient-poor soils
declined, whereas those dependent on plant species
growing on nutrient-rich soils increased during the
past 20 years (Öckinger
et al
. 2006). Up to 20% of
European butterfl ies are threatened (Bourn & Thomas
14.2
THREATS
Grasslands and heathlands have been subject to at
least two different types of change that reduce their
conservation value. The most obvious change is where
land use
is changed through so - called ' reclamation ' ,
actually exploitation, where the aim is to increase the
agricultural productivity of the area via 'agricultural
intensifi cation'. This process resulted in (1) habitat loss
and fragmentation, (2) an increase in soil fertility and
