Environmental Engineering Reference
In-Depth Information
New Zealand found that the highest levels of bird diversity were recorded
in disturbed productive forests while undisturbed mature forests contained
less bird diversity, though they were populated predominantly by native
bird species. Sternberg et al. (2000) conducted a four-year study on the
response of a Mediterranean herbaceous community to grazing manage-
ment in north-eastern Israel. Contrasting dif erent grazing treatments they
found that low and high grazing regimes reduced herbaceous diversity but
that moderately grazed areas increased diversity.
The study of long-term grazing-vegetation interactions (10 2 -10 6 years)
using palaeovegetation data permits the reconstruction of vegetation and
herbivore abundance and associations. Data from Jutland in Denmark
from the Holocene about 5000-7000 years ago suggests that large forest
herbivores did not have a signii cant inl uence on regional forest structure
(Bradshaw and Mitchell, 1999). This is because either large predators
held populations at modest levels, or the diversity of grazing species held
populations at stable, low populations of individual species.
Moderate levels of insect herbivory may actually increase productivity.
For example, Holling (1978) carried out an experiment on the defoliation
of balsam i r ( Abies balsamea ) by spruce budworm. The larvae result in the
death of mature trees aged 55-60 years though young trees are unaf ected.
Saplings grow rapidly after mature forest is damaged, and the forest is
restored by its juvenile population. In the short term there is a shortfall
in the production of timber, but over the longer term wood production
remains unaf ected. In fact, production rates of the juvenile forest remain
above that of the mature forest because in a mature stand, most trees
have passed their rapid-growth phase. Mattson and Addy (1975) reached
similar conclusions in their study on the ef ects of forest tent caterpillars
on aspen.
French et al. (1997) conducted a study on the development of Scots
pine in the Cairngorm mountains in Scotland and found that recruitment
is possible provided grazing/browsing pressure remains at a low level.
Similarly Sun et al. (1997) evaluated the ef ects of cattle grazing and seed-
ling size on the establishment of Araucaria cunninghamii in a silvo-pastoral
system in north-east Australia. They report that grazing did not cause
unacceptable mortality due to the fact that the tree has prickly needles,
which prevented browsing by cattle. They recommend that grazing does
not af ect recruitment and can begin immediately after tree planting pro-
vided that a moderate stocking rate is used. Elsewhere, modelling has
also shown that plant populations may be little af ected by low levels of
herbivory. Kienast et al. (1999) used a succession model - FORECE − to
assess the long-term dynamics of alpine forests in Central Europe. They
report that moderate levels of browsing posed no threat to the long-term
Search WWH ::




Custom Search