Íslenskar landbúnaðarrannsóknir - 01.09.1982, Side 61

SPRING GRAZING 59
to continued respiration; (2) at the begin-
ing of the growing season there is a
dramatic decline in reserve levels corres-
ponding to the onsetofnewgrowth; and (3)
this “spring drawdown” continues until a
certain leaf area is produced (Trlica and
Singh 1979). When the plant has reached a
critical leaf area the input of photosynthe-
tic foodstuffs exceeds maintenance and
growth expenditures and carbohydrate
reserves begin to accumulate as the plant
matures and enters dormancy. With the
onset ofdormancy, the cycle repeats itself.
It is during the spring drawdown period
that plants are susceptible to heavy graz-
ing. After winter dormancy and during the
initial flush ofspring growth, reductions in
stored carbohydrates of 50 to 75% have
been reported for mountain forage plants
(McCarty and Price: 1942; Mooney and
Billings 1960). Removal of the early
spring leaves at this time would delay the
incorporation of photosynthetic energy
and result in a futher depletion of food
reserves. Donart (1969) related carbohy-
drate reserve cycles of six mountain species
to growth and development and noted that
minimum reserve levels were reached
during early spring growth after approx-
imately 15% of the total annual growth
had been produced. It was further noted
(Donart and Cook 1970) that defoliation
of these herbaceous plants early in the
season was considerably more detrimental
than defoliation later in the season. If
grazing pressure is maintained throughout
the spring the plant may never recoup its
reserve losses and will enter winter in a
state ofreduced vigor. The result may be an
increased mortality rate during the winter,
especially among young age classes of
plants, or a delayed and reduced pro-
ductivity the subsequent growing season
(McCarty and Price 1942; Smitii 1964;
Mooney and Billings 1965; Owensby et
al. 1970; Archer and Tieszen 1982).
Also, early spring grazing likely alters the
reserve carbohydrate allocation patterns
within the plant such that energy
normally expended for root productions
is diverted to replacing leaf tissue lost to
grazers. As a result, root growth is often
suppressed and the ability of the plant to
take up water and nutrients is impaired
(Crider 1955; Davidson and Milthorpe
1966; Evans 1972; Chapin and Slack
1979). In this regard, Archer and Ties-
zen (1982) noted that early season
defoliations affected root initiation,
elongation, and biomass to a greater
extent than did defoliations imposed later
in the season. Reduced root growth, at a
time when the plant is normally growing
rapidly, may retard not only rate at
which recovery from defoliation may
occur, but may also reduce the soil hold-
ing capability of the plant. Also, early
turnout dates would allow grazing during
periods when soils are wettest and would
increase the likelihood of trampling dam-
age via destructive hoof action
(Thilenius 1975).
Reproductive vigor may also be sub-
stantially reduced as a result of excessive
defoliation. Vegetative reproduction in
graminoids, the primary mode of plant
propagation in most perennial grassland
systems, is often markedly reduced by
grazing (Troughton 1957; Ellison 1960;
Jameson 1963; ARCHERandTiESZEN 1982)
although, there may be a stimulation of
tillering if apical meristematic tissue is
disturbed (Younger 1972). Sexual repro-
duction may also be aífected. Pearsall