Environmental Engineering Reference
In-Depth Information
The populations of several ungulates in the temper-
ate zone are often larger than expected without human
impact. They play a certain role in forest ecosystems,
for example wild boars (
Sus scrofa
) activate seeds buried
in the soil ('soil seed bank') by digging activity. How-
ever, grazing by wild deer species may be problematic
due to their selective damage on tree regeneration (e.g.
Abies alba
in central Europe, and
Tsuga canadensis
and
Thuja occidentalis
in North America). A balance has to
be found between game animal density and the require-
ments of native tree species regeneration.
The ecological roles of large grazers such as aurochs
(
Bos primigenius
) and tarpan (
Equus przewalskii gmelini
),
which were hunted to extinction centuries ago, are not
well known. Although there are many contemporary
examples of open woodland resulting from several
thousands of years of domestic cattle grazing in forests
(Olff
et al
. 1999), it is hard to extrapolate the results to
forest ecosystems of former times because important
ecosystem elements are now missing (e.g. large preda-
tors such as wolf, lynx and brown bear).
Although major human impact on temperate forests
in southern South America (e.g.
Nothofagus pumilio
) is
relatively recent compared to the situation in Europe
and Asia, the problems are similar to what prevailed in
Europe circa 200 years ago: conversion to pastureland
and farmland, and overutilization. Improved manage-
ment methods in Chile and Argentina are needed (Gea-
Izquierdo
et al
. 2004 ; Newton & Featherstone 2005 ),
and much research is now taking place.
The main goal of Continuous Cover Forestry is to
work 'as close to nature as possible'. It is possible to
keep the ecological quality of forest stands high even
while they continue to be exploited for timber and non-
timber products (Conservation in Figure 12.1) and
before they need any restoration measures.
species. This is due to low overall
resilience
and a lack
of ecological self-regulation. For example, the amount
of dead wood, which serves as an important resource
and habitat for many functionally important groups of
organisms (see section 12.3.1), is quite low: between 3
and 12 m
3
h a
− 1
in many European countries, with the
maximum of circa 20 m
3
h a
- 1
in Austria (Parviainen
2009). This has many impacts on
ecosystem func-
tioning
and
biodiversity
.
In many European countries there is a growing
trend to bring such artifi cial forests back to a more
near-natural situation, which means increasing or
improving tree species biodiversity, tree layer structure,
age structure, content of dead wood and soil quality.
Changing demands of society, an enhanced level of
ecological understanding and the expected risks related
to climate change have forced the development of
strategies to improve secondary coniferous stands
(Hansen & Spiecker 2005). Two basic
ecological res-
toration
strategies for temperate forests can be distin-
guished (Figure 12.6):
• Conversion
:
changing tree species composition of
secondary coniferous stands into mixed broad-leaved
forests, usually by artifi cial regeneration in terms of
' advance planting ' in middle - aged stands.
• Transition
:
changing monolayered forest stands into
two- or multilayered stands, for example by moving
from an even-aged to an un-even-aged forest stand via
changes in the timber rotation period employed, or
even striving for management that allows a continu-
ous canopy cover through selective harvesting on
small areas, respectively avoiding extensive harvesting
on large scale (in particular clear - cutting).
There are some indications that mixed stands can be
more productive than monospecifi c stands (Bartelink
1999), thanks to complementarity in crown morphol-
ogy, shade tolerance, tree height and diameter dynam-
ics, rooting depth and/or phenology, on the one hand,
and/or to facilitation, which refers to positive interac-
tions and feedbacks among species, including syner-
gism and symbiosis, on the other hand. However, even
for the best-investigated mixture in central Europe,
namely Norway spruce and European beech, this ques-
tion is not as yet well understood (Pretzsch & Schütze
2009). According to Knoke
et al
. (2008) the trend
towards mixed forests seems due mainly to ecological
drivers, while sound economic analysis of mixed forests
is still rare. Mixed-species stands would be better able
to respond to disturbance factors than monocultures
(resilience). Moreover, there is substantial evidence
12.3.2 Ecological restoration: conversion
and transition of forest stands
Today most forests of the temperate zones of Europe
and Asia are managed and exploited, mainly for timber
production. As a result, they differ dramatically from
natural forests: on large areas there occur only one or
few tree species, and most trees are relatively young
and of similar age. Many such stands show increased
susceptibility to damage by windstorm, snow, ice,
drought, insects, fungi and possibly soil degeneration
as compared to forests composed of site-adapted
