Grasslands of the World.pdf - Disasters and Conflicts - UNEP

Grasslands of Patagonia 141
TABLE 4.3.
Vegetation shifts under grazing in Patagonia .
Biozone
Semideserts
Shrub-grass
steppes
Grass- shrubs
teppes
Grass
steppes
Shrub
steppes
Vegetation transitions under grazing ( 1)
From To
Nassauvia glomerulosa (forage
dwarf shrub)
+ Poa dusenii (palatable grass)
Mullinum spinosum (forage shrub)
+ P. ligularis (palatable grass)
Festuca pallescens (palatable grass)
+ M. spinosum (forage shrub)
N. ulicina
(unpalatable dwarf shrub)
Senecio sp. (unpalatable shrub)
+ Stipa humilis (unpalatable grass)
Senecio filaginoides (unpalatable shrub)
+ M. spinosum (forage shrub)
+ Stipa spp. (low palatability grasses)
F. pallescens (palatable grass) F. pallescens (palatable grass)
+ M. spinosum (forage shrub)
+ Senecio sp. (unpalatable shrub)
F. pallescens (palatable grass) Acaena sp. (low palatability dwarf
shrub)
F. gracillima (low palatability grass) Nassauvia sp. (unpalatable dwarf shrub)
+ Stipa spp. (unpalatable grasses)
Schinus sp. (palatable shrub)
+ Prospidastrum sp. (palatable
shrub)
+ Stipa tenuis (palatable grass)
Chuquiraga avellanedae (low
palatability shrub)
+ Stipa tenuis (palatable grass)
Grindelia chiloensis (unpalatable low
shrub)
Chuquiraga avellanedae (low
palatability shrub)
Reference
Bertiller, 1993a
EVOLUTION OF PATAGONIAN GRASSLANDS OVER THE LAST
40 YEARS
Patagonian vegetation is generally described as having few adaptations to cope
with grazing by domestic ungulates, since the entire region is thought to have
evolved under conditions of light grazing by native ungulates (Milchunas, Sala
and Lauenroth, 1988). Although this notion has recently been challenged by
Lauenroth (1998), there is general consensus that vegetation throughout most
of Patagonia has been modified significantly by sheep over the last century,
particularly in the last 40–50 years (Golluscio, Deregibus and Paruelo, 1998;
Paruelo et al., 1993).
Deterioration of grazed vegetation has usually been demonstrated by
replacement of palatable grasses by unpalatable woody plants (Bertiller,
1993a). The severity of plant life-form replacements varies among biozones ,
depending on abiotic constraints within each ecosystem (Sala, Lauenroth
and Golluscio, 1997; Perelman, León and Bussacca, 1997). The process of
plant species replacement has been described for most of Patagonia ’s biozones
following the conceptual model of “states-and-transitions” proposed
by Westoby, Walker and Noy Meir (1989) for non-equilibrium rangeland
ecosystems (Table 4.3). According to this model, plant communities shift
between alternative steady states rather than progressing in a linear manner
toward a predictable climax . Shifts in vegetation composition (transitions)
are produced by particular combinations of biotic and abiotic stressors. In
Bonvissuto et al.,
1993; Fernández and
Paruelo, 1993
Paruelo and
Golluscio, 1993
Bertiller and Defossé,
1993
Bertiller and Defossé,
1993
Oliva and Borrelli,
1993
Nakamatsu et al.,
1993
Rostagno, 1993
NOTES: (1) The order in which species appear is associated with their status in the plant community.
SOURCE: The table is a synthesis of state-and-transition catalogues developed for different biozones of Patagonia and
compiled in Paruelo et al., 1993.