POST FIRE SUCCESSION IN NORTH-WESTERN PATAGONIA GRASSLANDS: SPATIAL OCCUPATION AND WATER USE STRATEGIES

GHERMANDI L.; BRAN D.; GUTHMANN N.; GITTINS C.

Simposio; 46 IAVS Symposium; 2003

Grasslands in North-western Patagonia are subjected to frequent wildfires. Stipa speciosa and Festuca pallescens tussocks, under a semiarid climate with winter rain concentration and summer water stress, dominate this region. Water is a very important resource in this environment and plants compete for its use depending on their different forms and stages of life. The fires interfere with this water use patterns changing the competition roles. A big wildfire (16.000 hectares) occurred in January 1999, after a very dry year (363 mm vs. 842 mm, annual precipitation average) in grassland of north-western Patagonia. The post-fire spatial occupation of vegetation was monitored in November 1999, 2000 and 2001. All species were grouped in three functional types related to depth of roots and consequent soil water utilisation: annual / biannual herbs and grasses and shrub?s seedlings (superficial roots: until 20 cm); perennial herbs and grasses (intermediate roots: 20-50 cm); shrubs (deep roots: 50-150 cm).The total cover recovery was 36% in the first, 42% in the second and 84% in the third growing season. If we discriminate the functional groups we find that the relative cover of the plants with superficial roots was 56% in 1999, 17% in 2000 and 7% in 2001. This response could be related to high precipitation in late winter (181 mm in August 1999 vs. 99 mm, August precipitation average), which was utilized by fugitives annual species and seedlings of Fabiana imbricata shrub, which explore the same soil layer. The functional group of intermediate roots rapidly increase its cover in the second post-fire year (43% in 1999, 79% in 2000 and 74% in 2001) principally due to the contribution of perennial tussock resproutes which respond to a great increase in winter precipitation (334 mm in 1999 vs. 615 mm in 2000). In the second and third post-fire season the seeders F. imbricata and Acaena splendens incorporate to this group due to the rapid elongation of their roots. The last group (50-150 cm) increase very fast in the last season studied fundamentally due to the contribution of resprouter Mulinum spinosum spherical shrub that explore the deeper soil profile where the water table winter replenishment is produced (634 mm in 2001 vs. 435 mm, average winter precipitation). This was also observed in the unburned control site where interannual cover fluctuation was also very important (52% in 1999, 75% in 2000 and 89% in 2001) and shrubs was the group mainly responsible of the cover increase.The main conclusion is that the fluctuations between years in the precipitation generate different responses in the total vegetation cover and in functional groups cover, depending on what months reach more precipitation and, consequently, which soil layer is involved. The post-fire recovery of grasslands is strongly dependent on the weather pre and post event. The increased frequency of ENSO phenomenon in the context of the global climatic change influence the post-fire community recovery and our knowledge of the differential climatic responses of the functional vegetation groups is very important for the understanding of the post-fire Patagonian grassland succession.