Fjölrit RALA - 05.12.1999, Síða 47

OSVALDO A. FERNÁNDEZ ET AL.
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environmental gradients (Jobbagy et al. 1996). North-eastern slopes can be drier than
uplands which may be due to the protection of these slopes íforn strong westerly
winds prevailing in the area, which are stronger in summer, and that results in a more
favourable thermic balance on these slopes as compared with uplands (Coronato and
Bertiller 1996). North-eastem slopes are sensitive places for land degradation which
eventually may have a restricted restoration potential imposed by physical changes in
the upper soil and limited soil water availability (Coronato and Bertiller 1996).
In this region, vascular plant species can be grouped in three growth-form based
groups: shrubs, grasses and forbs (Aguiar et al. 1996). Shmbs include evergreen and
deciduous species which are taller than 0.5 m and without a well-developed main
stem. Grasses are mfted, and have the C3 photosynthetic pathway and stiff green
leaves all year round. Forbs include annual and perennial evergreen or deciduous spe-
cies which are mostly dicots. Shrubs and grasses differ in their strategies to cope with
limited soil water availability, an important constraint to plant growth in Patagonia.
Shrubs mainly use resources írom lower soil layers (Soriano and Sala 1983,
Femández and Paruelo 1988) and rely primarilv on winter recharge of deep soil water.
The tussock grasses, however, use resources mainly from the upper soil layers with
their root systems (Soriano and Sala 1983, Soriano et al. 1987). Grasses and shmbs
are less numerous than forbs but constitute most of the aboveground production
(Golluscio et al. 1982, Femández et al. 1991). Total plant biomass sharply decreased
with decreasing precipitation along an aridity gradient in westem Patagonia, but be-
lowground biomass decreased at a lower rate than aboveground biomass, resuiting in
increased root:shoot ratios (Schulze et al. 1996). The depth ofthe soil horizon (0.50-
0.80 m) that contained 90% of the root biomass along this gradient was similar for for-
ests and grasslands, but was shallower in the desert (0.30 m) (Schulze et al. 1996).
Shrubs and grasses play a main role in community functioning by forming patches
(Soriano and Sala 1986, Aguiar et al. 1992, Aguiar and Sala 1994) and controlling
community dynamics (Sala et al. 1989, Mazzarino et al. 1996). Vegetation patches
constrain ecosystem functioning by determining the spatial pattem of soil organic
matter, soil texture, nutrient cycling and water dynamics (Aguiar and Sala 1994, Maz-
zarino et al. 1996). Tussock grasses (0.2 m in height) and hemispheric shmbs (0.6 m
in height) are arranged in two kinds of stmctural patches, one formed by scattered tus-
socks interspersed with bare-soil areas and the other made up of shmbs each tightly
surrounded by a dense ring of grasses (Soriano et al. 1994). This difíerent vegetation
structure creates microsites of different suitability for seedling establishment (Aguiar
et al. 1992, Aguiar and Sala 1994, Soriano and Sala 1986). Bare-soil areas are more
favourable microsites for seedling establishment than are microsites close to individ-
ual plants. Most recmitment takes place in less favorable microsites (Aguiar and Sala
1994) because patterns of recruitment are predominantly determined by seed avail-
ability, not by seedling establishment (Aguiar and Sala 1997). Recruitment pattems
near existing vegetation tend to maintain and reinforce the current spatial heterogene-
ity (Aguiar and Sala 1997). Also, seedling establishment of desirable forage grasses in
bare soil patches can be limited by summer drought, grazing and wind (Defossé et al.
1997).
Natural vegetation in Patagonia has been used for sheep production since the end of
the last century (Morrison 1917). The stocking rate varies from 400 to 1500 heads per