Seeds banks of desert annuals in an aridity gradient in the southern Atacama Desert

摘要

*Background/Question/Methods* Annual plant communities are important components of the biodiversity found in the coastal southern Atacama Desert in Chile. Moreover, they are an important economic resource for the human communities living in that area. These plant communities develop, after heavy rainfall episodes, a phenomenon locally known as the “blooming of the desert”. Although the minimum rainfall thresholds for these plants to emerge are relatively well known, little is known about their seed banks, its composition, dynamics and variation across the latitudinal aridity gradient from south to north. This system is interesting to study species coexistence, as well as, to look for exotic plants invasion, since previous studies have shown that exotic annual plants decreases in importance northwards. We hypothesize that, as a result of the gradient, we should find more seeds of annuals in the southern part because of the increase of exotic species. The lowest diversity will be found in the northern limit, while the highest in the middle part of the gradient, coincident with a natural protected area, and less influence of exotics. In terms of seed size and viability, we should find larger and more viable seeds in the southern limit of the gradient, probably contrasting with smaller and more dormant seeds to the north. To test these hypotheses we selected 8 sites along the gradient and collected soil samples after seed set in December 2008 for seed bank determination. *Results/Conclusions* According to our hypothesis we found more seeds in the southern part of the gradient. The northern seed banks (Rodillo and Cisne) are characterized by their low number of seeds and low species richness, according with their more extreme aridity. The central and southern seed banks have more than two times higher number and species richness than the northern ones. Southern seed banks (Lagunillas, Romeral and Punta Choros) contain more exotics’ seeds; meanwhile central seed banks (Pajonales, Carrizal Bajo and Los Bronces) have less seeds, and less exotics’ seeds. These results suggest that the increased seed number in the southern portion of the gradient could be due to an increased number of exotic species, rather than an increased diversity of native annuals. More viable and larger seeds were found in the southern sites, meanwhile, smaller seeds were found to the north indicating the use of a cautious opportunism strategy by these species. This is the first report on seed banks composition of this Atacama Desert area.