Íslenskar landbúnaðarrannsóknir - 01.09.1982, Blaðsíða 60

58 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
manufacture food via photosynthesis and
to acquire nutrients and water via their
root systems. Of the factors that influence
the rate offood manufacture (photosynth-
esis), the amount of exposed leaf area is of
principal concern, for it is the parameter
most subject to manipulation and manage-
ment. In the following section, the import-
ance of delaying grazing until a minimum
leaf area is achieved will be discussed.
Grazing in early spring, before the plant
has attained a critical leaf area, may delay
and reduce plant productivity for the
remainder of the growing season and thus
reduce carrying capacity.
THE ROLE OF CARBOHYDRATE RESERVES IN PLANT FUNCTION
The foods manufactured by plant leaves in
excess of those used by the plant for
immediate growth and respiration are
stored for later use. Perennial plants
commonly store their photosynthetic pro-
ducts in the form of carbohydrates. These
carbohydrate reserves are stored in the
roots and stem bases of perennial her-
baceous plants, in the stems and roots of
woody plants, and in the seeds of annual
plants. These food reserves are used for (1)
respiration and maintenance of perennial
structuresduringdormancy, (2) theinitia-
tion of growth following winter dormancy,
(3) vegetative and sexual reproduction,
and (4) an emergency energy source
during times of stress (e. g., drought,
frosts, grazing, etc.). Thus, carbohydrate
reserve levels are of key importance to the
welfare ofthe plant (see review by Graber
et al. 1927; May 1960; Jameson 1963;
White 1973;TRLicAandSiNGH 1979).The
importance ofbelowground vascular plant
storage structures in tundra systems has
been acknowledged for some time (Bliss
1962, 1966, 1970; Mooney and Billings
1960; Scott and Billings 1964; DennA and
Johnson 1970; Wielgolaski 1972,; Webber
1974,1977, WEBBERand May 1977; Dennis
et al. 1978; Miller et al. 1978). Carbohy-
drate concentrations found in arctic tun-
dra graminoids are typically high, often
being much greater than those of many
alpine and temperate counterparts (Fonda
and Bliss 1965; McCown and Tieszen
1972). Seasonal trends and effect of clip-
ping on carbohydrate and nutrient levels of
various age classes of the grass Dupontia
fisheri at Barrow, Alaska are well
documented by Mattheis et al. (1976).
Figure 2 shows the generalized pattern
ofseasonal trends in carbohydrate levels in
storage organs known to occur in many
temperate zone plants. Three phases ofthis
seasonal curve are of particular interest:
(1) during the dormant season there is a
gradual decline in carbohydrate levels due
throughout thc annual cycle. Arlegt kolvetnajafnvaegi og uppskera
dæmigerðra grasategunda.