Absence of a persistent seed bank in Nothofagus pumilio (Fagaceae) in Tierra del Fuego, Chile

The soil seed bank, allowing a plant species to maintain a fraction of its seeds dormant until conditions are adequate for germination and establishment of seedlings, is considered an adaptation for facing spatial and temporal environmental variation. Theory suggests that various factors favor, while others disfavor the evolution of the dormancy in the persistent soil seed bank. In this paper we determine whether the tree species, Nothofagus pumilio (Poepp, et Endl.) Krasser (lenga), forms a persistent soil seed bank, given that it exhibits several characters predisposing it to form a persistent seed bank, while other characters disfavor its formation. The potential for persistent seed bank formation was evaluated through seed burial experiments and examination of soil cores in two contrasting environments with respect to microclimate and soil type: the forest floor (450 m elevation) and the alpine zone (700 m elevation) on cerro Balseiro, Tierra del Fuego, 54 degrees S, Chile. Nothogagus pumilio forms a transient seed bank during part of the summer and winter (february-september), but lacks a persistent bank (maintenance of seed viability for more than one year). Within the forest habitat, 69-76% (depending on soil depth) of the seeds germinated at the beginning of the spring, the remainder being totally inviable at this stage. In the alpine zone, germination was lower (7%), but viability of the ungerminated seeds (19%) was significantly higher than in the forest site. No viable seeds were found in soil cores taken in the two sites seven months after seed dispersal. The results for both sites suggest that storage conditions and germination characteristics an important factors determining the potential for soil seed bank formation. We propose that extensive seedling banks, persisting up to 25 years in Nothogagus pumilio, are an alternative to persistent seed bank formation in a forest habitat where soil conditions for seed storage are unfavorable.