Polylepis mountain woodlands of central Argentina: Livestock rearing, fires and woodland distribution

RENISON, D.; CINGOLANI, A.M.; HENSEN, I.; TORRES, R.; ALINARI, J.

San Carlos de Bariloche

Taller; Taller de fragmentación, pérdida de biodiversidad, conservación y restauración de los bosques de Sudamérica.; 2006

Mountain forests are generally inaccessible, sparsely populated and often believed to be relatively pristine environments where human influence is negligible. However, this concept is being challenged and the issue of whether humans have, or have not, had a considerable influence remains controversial (Körner, 1999). As livestock rearing is the main economic activity in most of South American mountains and is almost always associated to burning for grass re-growth, since 1997 we have been studying the long and short term effects of livestock and fires on the development of Polylepis australis (Rosaceae) woodlands in Central Argentina. Here the human/natural influence dilemma on Polylepis woodlands is yet controversial to a certain extent, as livestock breeders, scientist and managers have differing opinions. P. australis is the dominant species of the woodland canopy, harbors a high richness of endemic species and is usually associated to deep ravines and rocky areas. Our main results suggest that the present distribution, degree of fragmentation and woodland structure is strongly influenced by human-use, which interacts in different ways with natural features (Renison et al. 2006). Fire scars were present in 70 % of our 146 study plots and were less frequent in deep ravines and rocky areas. Topography also influences the post-fire recovery capacity (Renison et al. 2002, 2006). When trees are burnt around 70% survive by re-sprouting forming multi-stemmed shrubs; however the high incidence of human caused fires does not allow the development of mature forests, which are almost absent. Uneven browsing by free ranging livestock results in the retreat of woodlands in accessible areas and microsites. When accessible, P. australis is so heavily browsed that it cannot regenerate (Teich et al. 2005). Livestock is also the main triggering factor of soil erosion (Renison et al. submitted) which in turn reduces P. australis reproductive output (Renison et al. 2004). Because both fires and livestock affect woodlands differentially according to topographic features, an association between topographic features and woodland distribution cannot be taken as evidence for a purely natural process with no human intervention, an error which has caused confusion in the general public, livestock breeders, scientist and managers. Implications: (1) livestock must be carefully managed to permit woodland regeneration and avoid soil erosion (Cingolani et al. 2003); (2) forestation with P. australis (Renison et al. 2005) can be considered as a restoration objective even when done out of deep ravines and rocky areas.