According to fossil records, Araucariaceae represents the most ancient family to expand in both hemispheres during the Jurassic and to later decline with angiosperm development. Araucariaceae are currently relictual and Araucaria araucana in Chile is classified in the CITES Appendix I. Despite its high interest for biodiversity and conservation, little is known about the effect of Nothofagus species competition on its distribution. We hypothesized that soil marginal conditions related to volcanic activity or poor nutritional sites are the major environmental controls of Araucaria distribution, ensuring the long-term survival of the species in coexistence with Nothofagus spp. Thus, the aim of this research was to investigate and quantify the effect of climate and soil conditions as well as volcanic activity on the actual distribution of A. araucana. We used presence/absence data for Araucaria in 52,841 photo-interpreted forests over Chile, coupled with geological and climatic information, to obtain Araucaria distribution models. Fifty-five plots were established coupling pure Araucaria and Nothofagus stands and mixed stands in order to investigate the factors controlling the presence of Araucaria vs. Nothofagus species at the local scale. An inventory of trees and measurements of soil chemical and physical characteristics in the laboratory were done for each plot.
Results/Conclusions
Summer evapotranspiration and precipitation coupled with the presence of volcanic substrates can significantly explain the distribution of Araucaria araucana at the national scale with a prediction success of 93%. Thus, the presence of A. araucana is not just determined by climatic conditions associated with precipitations and temperatures, but also by soil conditions associated with recent volcanic and metamorphic geological substrates. Analysis of variance carried out with the local dataset (n=55), showed a significant difference in the water storage capacity of soil between Araucaria and Nothofagus stands. Thus, within its geographical range, the presence of Araucaria araucana is strongly related to sandy and gravelly soils with low water storage capacity, developed in recent volcanic substrates in the Andean Range, whereas Nothofagus species are dominant on soils characterized by fine particle size with higher water availability. By regularly creating new soils characterized by xeric conditions, volcanism in the Andean Range is probably the main determinant of long-term conservation of Araucaria araucana populations in natural South American forests.