Íslenskar landbúnaðarrannsóknir - 01.09.1982, Side 58

56 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
Fortunately, however, there seems to be
a growing awareness and understanding
among stockmen that untimely grazing
can have a detrimental eífect upon both
range and animal productivity. As a result,
many districts are gaining better control
over turnout dates. This article will
discuss:
1. why proper turnout dates are important
to plant productivity;
2. why proper turnout dates might opti-
mize financial returns in pasture and
rangeland systems; and
3. research needs and priorities in these
areas.
EFFECTS OF GRAZING ON PLANT PRODUCTIVITY
It is generally acknowledged that the
ability of a plant to survive in a given
habitat depends upon four basic and
interrelated factors:
1. the ability to synthetize and store food
for maintaining plant functions;
2. the ability to form vegetative structures
for renewal of top growth;
3. the ability to maintain a healthy root
system; and
4. the ability to produce reproductive
organs (Stoddart et al. 1975).
Each of these factors is interrelated and
dependent upon there being sufficient
foliar tissue to acquire the energy needed
for plant growth and development. It is
well known that different plant species
react differently to environmental stress,
including grazing. Archer and Tieszen
(1980) for example, describe various
strategies observed for graminoid and
evergreen and deciduous dwarf shrub
lifeforms in an Alaskan arctic tundra with
regard to either tolerating or avoiding
defoliation. Their results suggest that the
ability ofvarious plant lifeforms to tolerate
defoliation (Fig 1) is related to a variefý of
factors, such as the amount of non-
photosynthetic supportive tissue and the
longevity, photosynthetic rates, and pat-
terns of development ofleaf tissue. Because
of the wide array of responses of plants to
defoliation, a detailed account of the
physiological processes ofkey range plants
is needed to form a basis upon which
management decisions can be made. Gaps
in our knowledge of range plant physiology
125 - rb
100
75
3 2
<1!
50

25
0
1 Tussock Graminoid margsprota grös
2 Single - Shoot Graminoid einsprota grös
3 Deciduous Shrub lauftré - runnar
4 Evergreen Shrub Sígrænir runnar
4
n
Ist Season 2nd Season
1. ár 2. ár
Leaf Material Regrown Following Defoliation
Endurvöxtur eftir blaöskerðingu
FIGURE 1.
Foliar regrowth of four Alaskan arctic growth
forms folowing a single, 100% defoliation imposed
in late June. (1) Eriophorum vaginatum, (2) Carex
aquatilis, (3) Salix pulchra, (4) Ledum palustre.
Endurvöxtur fjögurra jurtategunda frá Alaska eftir 100% blað-
skerðingu, seint í júní.