Environmental Engineering Reference
In-Depth Information
12.1
INTRODUCTION
vegetation are extremely rare, by far not enough to
be used as a template for restoration. Therefore the
concept of
potential natural vegetation
(PNV;
Tüxen 1956) can be used. It refers to the vegetation
that would occur under recent site conditions but
without the impact of humans, expressed in terms of
recent vegetation types. It therefore offers restoration-
ists a valuable tool, allowing them to identify and map
sets of site conditions suitable for plant growth in terms
of vegetation types, especially in areas where the
natural vegetation has disappeared more or less com-
pletely. In other words, the concept can be used as a
system of reference. Most important: because the PNV
is a refl ection of the site conditions, with changing site
conditions it changes simultaneously. The
Map of the
Natural Vegetation of Europe
(Anonymous 2003 , scale
1:2.5 million) is the most comprehensive PNV map for
Europe. Land with site conditions favourable for tem-
perate forests, mentioned there as 'mesophytic decidu-
ous broad - leaved and mixed coniferous/broad - leaved
forests' (unit 'F') represents an area of 24.6% of the
whole of Europe from the Atlantic Ocean to the Ural
Mountains. In the central part of Europe, however, it
corresponds to more than 80% of the area.
Not only have large parts of the former forest area
been converted into farmland and urban land in recent
centuries, but also, compared with the PNV, the tree
species composition of about two thirds of the remain-
ing forest stands has been radically modifi ed (see also
section 12.2). For example, in large parts of central
Europe, European beech (
Fagus sylvatica
) has to a large
extent been replaced by conifers, mainly Norway
spruce (
Picea abies
) and Scots pine (
Pinus sylvestris
).
According to the tree species composition, only less
than one quarter of the recent forest stands can be
interpreted as near-natural, but they are indeed
managed. Only very few, small and remote forest
stands can be interpreted as ' primeval ' like the Urwald
Rothwald in the Austrian Calcareous Alps, and even
these are infl uenced by indirect human impact such as
climate change. Nevertheless, (1) native tree species,
(2) dead wood providing habitat for numerous insects,
fungi and animal species and (3) a wide range of tree
ages are prerequisites for maintaining high natural
biodiversity
, and can also be present in forests under
management aimed at achieving near-natural forest
stands.
This chapter addresses the way 'back' from different
levels of converted ecosystems to temperate forests.
We start with forests that are only slightly affected by
Temperate forests occur mainly in western and central
Europe, eastern Asia and northeastern North America,
but also in small areas of the southern hemisphere
(e.g. in New Zealand, Tasmania and southern South
America). The climate in these regions is characterized
by (1) 4 to 8 months with average temperature of 10°C
or above, (2) precipitation of 500 to 800 mm, but also,
in some places, up to 2000 mm per year, and (3) 0°C
mean temperature for the coldest month (temperate
oceanic forests) or at least one month with average
temperature below zero, during which temperatures
sometimes drop to below − 10 ° C (Food and Agricul-
tural Organization (FAO) 2001a). While the world's
forests cover about 3.87 billion ha (29.6% of the
world's total land area), temperate forests make up
only 11% of the total (FAO 2001b,c). Most of the land
within this zone nowadays has been converted either
into agricultural land (59.5%) or urban areas/villages
and infrastructure. In this chapter we focus on the
forests of central Europe with a long history of over-
exploitation and degradation, followed by approxi-
mately three centuries of forest management. We also
refer to temperate forests in central China (see Boxes
12.1 and 12.2) with a similar human impact sequence
but on different time scales. Another approach, devel-
oped in a different context in North America, is out-
lined in Chapter 13 .
Forest restoration means: bringing a section of the
landscape back closer to a 'more natural' situation.
What is ' natural ' ? Bradshaw (2002) calls it ' original
ecosystem' and focuses on the two major attributes,
ecosystem structure
and
ecosystem function
,
with typically high values for both. Degradation
reduces values for both; natural
recovery
as well as eco-
logical restoration mark the way back to the original
stage. Remaining fragments of the formerly wide-
spread ecosystems could be used as a certain template,
but bear in mind that with changing environmental
conditions even ' pristine ' or ' original ' ecosystems will
change. Such templates can be called
reference
forests
(Dudley 2005 ), defi ned as 'carefully preserved
natural or near-natural forests that can provide infor-
mation about natural species' mix and ecology, that
can be used in planning and measuring the success of
restoration'. In the temperate zone such references
may be used (e.g. in North America; see Chapter 13).
In other parts of the world, like in central Europe and
temperate Asia, such remains of the former 'natural'
