Environmental Engineering Reference
In-Depth Information
Box 12.2 Afforestation in the Loess Plateau in central China
Afforestation in the Loess Plateau (Schmitt
et al
. 2002;
Fischer
et al
. 2006, 2007) is essential to reduce soil
erosion and bring this area back to a path of ecosys-
tem health and resiliency. Aspects to be considered
include:
• Loess has excellent water storage capacity, but pre-
cipitation in the region is limited. Native species are
adapted to these conditions thanks to their relatively
low levels of water consumption as compared, for
example, to black locust (
Robinia pseudacacia
), intro-
duced in China from North America via Europe. This
latter species has been intensively used in the Loess
Plateau for afforestation in recent decades, with mixed
results. For young trees there is enough stored water
available in the soil for establishment, but older trees
quickly use up underground water supplies, leading to
a 'dried soil layer' (Wang
et al
. 2006), reaching down
5 m and more. As a result, many well-planted
Robinia
stands are now dead or dying.
• The regional pool of native woody plants of the
Loess Plateau includes over 500 species (e.g. species
from the genera mentioned in Box 12.1). Here as else-
where, indigenous species are better adapted to the
local ecological conditions than introduced species,
and this advantage can and should be used in affor-
estation and restoration efforts.
• Remaining secondary forests in remote areas
include only a relatively small number of forest types
(forest communities). The most important site factor
determining the distribution of these forest communi-
ties is water availability, which in turn is strongly
affected by slope exposition and inclination (e.g. steep
and very dry south-facing slopes as opposed to gentle
north-facing ones). Because of the omnipresent loess
cover, the differentiation of site conditions regarding
substrate and soil is minor, and the limiting factors
relevant for afforestation and restoration are easy to
detect in the fi eld. Thus, detailed site-specifi c planning
can be developed according to the patch mosaic of
local siteconditions.
• For native species such as
Quercus liaotungensis
,
Acer truncatum
,
Prunus sibirica
,
Pinus tabulaeformis
and
Platycladus orientalis
, experimental plantings to
scientifi cally test establishment and early growth have
shown good rates of establishment (A. Fischer
et al
.
2007).
Afforestation and restoration, however, face several
problems:
• Only very few native species have to date been used
for restoration purposes. For most species, nursery
material is not available. There is also little quality
control for production of plant material, to date, or for
genetic provenances of seed material used.
• Forest restoration preferentially is done, and is most
needed, on steep slopes. Such steep slopes, however,
have long been used for grazing in the past. Conse-
quently, as soon as grazing is halted, so as to open
the way for afforestation, a more or less thick cover of
grasses and herbs reappears, thereby reducing the
chances for establishment of planted trees and shrubs,
not only due to competition, but also because this
natural vegetation provides excellent habitat for small
wild mammals such as mice and rabbits, which feed
on the planted seeds and young trees.
Tree and shrub plantings not only help slow and
ultimately prevent soil erosion, there also are a range
of economic products of direct use value that result.
Timber takes several decades until it can be harvested.
Other valuable products such as fruits, leaves, oils and
ingredients, however, can be harvested within a few
years after planting. The social and cultural benefi ts of
these plantings is also important: growing human
population and rising living standards lead to increas-
ing demand for recreational areas and facilities. Enjoy-
ing forest lands, learning about forests (eco-tourism)
and other nature-based leisure activities is in increas-
ingly important activity in a country with only limited
forest land.
The results of restoration measures during the last
two decades in China are impressive: the annual loess
load of the Yellow River in the period 1998-2007 was
approximately one quarter of the loess load in the
period 1955 to 1969 (unpublished Chinese Survey
Reports 2007).
