Íslenskar landbúnaðarrannsóknir - 01.09.1982, Side 62

60 ISLENZKAR LANDBÚNAÐARRANNSÓKNIR
(1950) and Spetzman (1959) noted a direct
loss of reproductive primordia because of
preferential grazing by deer, sheep and
caribou in northern latitudes. Indirect
supression of sexual reproduction may also
occur as a result of grazing which lowers
plant vigor and causes a depletion of stored
energy and nutrient reserves (Archer and
Tieszen 1982). Inthis regardTiKHOMiROV
(1959) and Smirnov and Tokamakova
(1971, 1972) observed that lemming graz-
ing suppressed flowering in several arctic
graminoids. In northern environments
where sexual reproduction is already
severely limited by abiotic factors, such
grazing induced losses either direct or
indirect, may be of some significance.
From a plant community standpoint,
excessive early spring grazing may also
confer a competitive advantage upon less
desirable species. Species escaping early
spring grazing will gain an advantage over
species subjected to grazing since the
former can maintain shoot and root growth
(Mueggler 1972, 1975; Archer and
Detling 1982). This early spring competi-
tive advantage may then carry over
throughout the growing season resulting in
a decrease in the relative productivity of
desirable forage species. If the desirable
species are kept at a competitive disadvan-
tage long enough, plant community com-
position will eventually be altered in favor
of the less desirable species. The recovery
of plants from setbacks induced by grazing
may be quite slow, especially in northern
latitudes where growing seasons are short
and cool, soils are young and poorlý
developed, and nutrient availability is
limited. Menke and Trlica (1983)
observed that abusively defoliated shrubs
required more than one year of rest and
Cook and Child (1971) found that desert
plants required more than seven years of
nonuse for recovery of vigor when defoli-
ated to the extent that vigor was mod-
erately reduced. Archer and Tieszen
(1982) working in the Alaskan arctic and
Trlica et al. (1977) working in the
shortgrass steppe observed that heavily
grazed graminoid tillers required two or
more years of rest to attain carbohydrate
and biomass levels in storage organs
comparable to those of undefoliated
plants.
Finally, it should be pointed out, that
changes in the carbohydrate status of
various storage organs are not necessarily a
direct function of time of season in arctic or
Alpine plants. Haloway and Ward (1965)
noted that in Colorado’s alpine areas the
date of initiation of various phenological
stages varied considerably from site to site,
and from year to year at a given site. Thus,
phenological stage is likely a better indi-
cator ofrange readiness than is date ( West
and Gasto 1978).
Few specific details are available regar-
ding the seasonal trends in shoot and root
growth and carbohydrate levels in impor-
tant Icelandic range plants. Even less is
known about how grazing at various times
of the growing season affects these trends
and how long it takes a plant to recover
from grazing at various frequencies and
intensities at various times in the growing
season. At this point, we can only speculate
using the results of studies conducted in
other parts of the world. F uture research on
Icelandic rangelands would do well to
address these questions directly if retrog-
ression of Icelandic rangelands (Thor-
steinsson et. al. 1971; Mitchell 1979) isto
be arrested.