Environmental Engineering Reference
In-Depth Information
the presence of one cow per ha virtually eliminates recruitment entirely.
Simulations of the model described above by Kienast et al.
(1999) coni rm
these results since they also demonstrated that high browsing pressure
does reduce recruitment and does alter the forest structure considerably,
leading to high rates of tree mortality and more open forests. The spatial
model developed by Weber et al.
(1998) was used to determine the ef ects
of grazing intensity and grazing heterogeneity applied to the southern
Kalahari and shows that high levels of grazing lead to shrub invasion.
Jeltsch et al.
(1997) also reported that when grazing intensity reaches a
critical level, shrub cover increases, drastically lowering the productivity
of the range.
High levels of disturbance by wild or relatively unmanaged introduced
animals are also thought to af ect ecosystem productivity. The study of
long-term grazing-vegetation interactions using palaeovegetation data in
Ireland show that reasonably high populations of giant Irish deer imposed
a high pressure on shrubby vegetation and had a profound ef ect on the
change in vegetation communities from juniper scrub to grassland in
Ireland during the Late-glacial Interstadial (11 000-12 000 BP) (Bradshaw
and Mitchell, 1999). In Galicia, Spain, Hernandez and Silva-Pando
(1996) report a decline in the abundance and diversity of shrub species
after a period of three years' grazing by red (
Cervus elaphus
) and roe deer
(
Capreolus capreolus
). Jane (1994) considered the long-term ef ects of
browsing by red deer (
Cervus elaphus
) on mountain beech (
Nothofagus
solandri
) in New Zealand and concluded that the impact of high deer
densities on vegetation remains and can persist for many decades. She sug-
gests that in critical high altitude areas, large reductions in deer densities
were required to trigger the regeneration necessary for tree replacement.
Intensive levels of herbivory may reduce plant productivity, survival,
reproduction and growth (Fay and Hartnett, 1991; Fox and Morrow,
1992; Relva and Veblen, 1998). In a long-term experiment to evalu-
ate the impact of domestic livestock on tree species Hester et al.
(1996)
manipulated sheep stocking density and season in an upland broadleaved
woodland in Cumbria, UK. They observed that growth and survival to
the sapling stage was negatively correlated with grazing intensity, and
suggest that, apart from plots grazed at the lowest animal densities, only
a small proportion of saplings will attain canopy height. Other studies
from around the world implicate browsing by domestic livestock as a
cause of poor tree species recruitment (Kingery and Graham, 1991). Van
Hees et al.
(1996) employed an exclosure to determine the impact of roe
(
Capreolus capreolus
)
and red deer (
Cervus elaphus
) on beech (
Fagus silvat-
ica
), pedunculate oak
(
Quercus robur
) and silver birch (
Betula pendula
) in
the Netherlands. They showed that browsing reduced sapling abundance,
