Soil seed bank of Austrocedrus chilensis (D. Don) Pic. Serm. et Bizarri related to different degrees of fire disturbance in two sites of southern Patagonia, Argentina

URRETAVIZCAYA, M. F. ; DEFOSSÉ, G. E.

FOREST ECOLOGY AND MANAGEMENT

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2004 vol. 187 p. 361 - 372

0378-1127

 Austrocedrus chilensis (D. Don) Pic. Serm. and Bizarri, called locally ??cypress?? is among the most important native tree species that compose the temperate Subantarctic forests of Patagonia in Argentina. Cypress grows in either humid, mesic or dry sites, being fire the most important natural disturbance affecting these forests. Regeneration of cypress in burned areas is very slow, and may depend on a series of environmental and biotic factors, among which the availability of seeds could be extremely important. In this study, the dynamics of cypress soil seed bank was determined in two sites of its southern distribution area (Trevelin, 43 12 S, 71 31 W, and El Bolsón 41 59 S, 71 33 W), and for three levels of fire disturbance (treatments): undisturbed (UN); partially burned (PB) and completely burned (CB). For about two growing seasons and at tri-monthly intervals, soil seed bank was measured by both direct and indirect methods. In the direct method, seeds were visually classified in apparently viable seeds (AVS) and apparently non-viable seeds (ANVS). The indirect method implied germination of seeds in a germination chamber after being stored to mimic natural conditions and break dormancy. For both study sites and all dates, the UN treatment showed significantly higher (P < 0:001) seed numbers than the other two (PB and CB). Before the autumn seed rain, all seeds were classified as ANVS and did not germinate. After that seed rain, a high proportion of seeds appeared to be AVS in both sites, showing a similar trend for all treatments, although in dissimilar numbers (UN > PB > CB). The number of AVS steadily diminished during winter and spring, to practically disappear at the beginning of the summer, coinciding with the results obtained with the germination method. This behaviour confirms that cypress has a transient type of soil seed bank, and that the size of this soil seed bank is greatly influenced by the degree of fire disturbance. The amount of AVS that reached the soil at any particular site, however, might also be conditioned by several insects that parasite seeds before the seed rain. The transient soil seed bank and the low seed density found in PB and CB areas appears as important constraints for cypress regeneration.