Íslenskar landbúnaðarrannsóknir - 01.09.1982, Síða 59

SPRING GRAZING 57
are currently limiting the accuracy and
precision with which we can predict
rangeland productivity. These gaps also
represent “unknowns“ with regard to
predicting and assessing the degree to
which key range plants can tolerate biotic
and abiotic stresses while maintaining
their productivity on a sustained-yield
basis.
Croppingofvegetation by animals alters
normal plant growth and development.
Removal of photosynthetic tissue by graz-
ing animals lessens the capacity of the
plant to manufacture foods required for
growth, maintenance and reproduction.
In perennial plants, stored food reserves
are diminished as the plant recovers from
grazing ( Ward and Blaser 1961; Donart
and Cook 1970; White 1973; Smith 1975;
Buwai and Trlica 1977). As a result, the
phenological development of the plant
may be retarded and its reproductive
capacity reduced. These are some direct
effects of grazing that affect plant pro-
ductivity and vigor. Removal ofleaf tissue
by grazers also has some indirect effects
upon plant productivity. Typically, grazed
plants reorganize carbon and nutrient
allocation patterns following defoliation in
order to replace the foliage lost to herbi-
vores. While such reorganization facili-
tates the regeneration of photosynthetic
tissue, it is generally done at the expense of
root growth and activity (Davidson and
Milthorpe 1966; Evans 1972; Hodgkinson
and Baas Becking 1977; Chapin and Slack
1979; Archer and Tieszen 1980).
Impaired root growth and activity follow-
ing grazing will reduce the ability of the
defoliated plant to extract nutrients and
water from the soils. In areas where
nutrients are limiting, impaired root
growth resulting from grazing may be
more detrimental to the defoliated plant
than the loss of leaf tissue. In fact, grazing
simulations at Barrow, Alaska suggest that
leaf growth following defoliation may
deplete nutrient reserves more than
carbohydrate reserves (Chapin 1977).
Fortunately, however, many range
plants can withstand a certain amount of
grazing without being adversely affected
(Strickler 1961; Vickery 1972; Chapin
and Slack 1979; Detling et al. 1979;
McNaughton 1979;TiEszENand Archer
1979). However, the amount of grazing a
given plant species or population can
tolerate over the long term will depend
upon the complex interaction of many
factors. The frequency, intensity, and time
of season of defoliation are of paramount
importance, as is the stage of plant growth
(see review by Trlica and Singh 1979). A
mature plant, for example, can withstand
more frequent and intense grazing bouts
than can a young plant whose root system
is poorly developed and whose levels of
strored foods are low. As a result, seedlings
and new vegetative propagules are parti-
cularly vulnerable to grazing ( Vallentine
et. al. 1963; Reynolds and Martin 1968).
However, any plant, when grazed, will
become weakened and eventually die if
grazed to frequently or too intensely at a
critical stage in its life cycle or at a critical
point in the growing season. The phy-
siological processes by which the indi-
vidual plant grows and the morphological
changes that occur over the course of the
growing season are then an important
consideration when managing grazing
systems.
The survival of plants in grazing systems
is directly related to their ability to