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Arid and semi-arid rangelands of argentina
1994a, Branch 1995), both of which contribute to the impact of vizcachas on plant
communities (Branch et al. 1996).
An additional unpredictable environmental factor in this region are natural, acci-
dental, and sometimes provoked fires. They occur mainly during the hottest part of the
year when they can get out of control and cover many thousand hectares. Busso
(1997) has recently reviewed some beneficial and damaging effects of fire on the Cal-
denal ecosystem.
Fire and grazing history influence the dominance of a particular species group in
any given area (Distel and Bóo 1996). Stipa clarazii and P. ligularis would dominate
vegetation under exclosure or light grazing conditions. These species are sensitive to
grazing but highly competitive in the absence of grazing, and are thought to be part of
the pristine vegetation (Cano 1988, Moretto and Distel 1997). They have been re-
placed by other desirable perennial grasses more tolerant of grazing like S. tenuis and
P. napostaense at sites where moderate grazing has been continuous. All these desir-
able perennial grasses, however, have been replaced by undesirable grass species (e.g.,
S. gynerioides, S. tenuissimd) when exposed to heavy, continuous grazing. Sites
dominated by S. tenuis and P. napostaense have been converted to a scrubland with a
diverse herbaceous layer under conditions favouring shrub seedling establishment and
of availability of shrub propagules, lack of fire or low fire frequency and heavy con-
tinuous grazing. Under these circumstances, S. tenuis and P. napostaense are domi-
nant grass species, and P. caldenia, P. flexuosa, C. microphylla and L. divaricata are
dominant woody species. If mismanagement persists, undesirable perennial grasses
replace desirable ones, and annuals become common during spring within a woody
layer of Prosopis, Larrea and Condalia species. Plant communities may retum to a
more desirable and productive species composition from a more degraded stage of
both the soil and vegetation if the intensity and duration of fire and/or grazing are
properly managed, and availability of established individuals or diaspores of the desir-
able species is appropiate (Distel and Bóo 1996).
Another consequence of grazing and fire mismanagement, and plowing of non ar-
able marginal lands with subsequent abandonment, has been the unabated advance of
Geoffroea decorticans on the native understory during the last 60 years. This small
tree is very aggressive, and multiplies by seeds and vegetatively from its gemiferous
(adventicious) roots forming an almost monospecific continuos shmbland layer. Pri-
mary productivity under the canopy of this species is very poor (Anderson 1977).
During the First World War woody species were overused as an energy source to mn
engines of vapor trains. At present, harvesting of some shrubs (e.g., Condalia micro-
phylla) for fuelwood for barbecue in the cities is one of the preferred uses of woody
vegetation.
The eastem part of the Caldenal is at the fringe of the arable humid or sub-humid
Pampas which run up to the Atlantic Ocean. Plowing of this marginal land for crops is
a permanent temptation for land owners. Most of the time, this is translated into poor
grain crop, leaving behind an impoverished ecosystem in terms of plant quality and
soil erosion. Wind and water erosion continue to be a major cause of soil degradation
in the province of La Pampa within the Caldenal. Approximately 160xl03 ha are