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Volcanic eruptions have a significant effect on ecosystems, including soil and its fungal communities. Due to the eruption of the Puyehue-Cordón Caulle in 2011, thick tephra deposits accumulated in several Nothofagus pumilio forests in Northern Patagonia. The tephra deposition affected plant establishment and development during the post-eruption period. In this work we analyzed rhizosphere fungal communities associated with N. pumilio seedlings at three study sites sampled three years after the eruption, two with a thick tephra layer (50 cm) and a third site with no tephra deposition. Denaturing gradient gel electrophoresis (DGGE) was performed for the different fungal communities (Ascomycetes, Basidiomycetes and Total fungi). We found clear shifts in rhizosphere fungal communities, especially for Basidiomycetes and Total fungi. Site-related characteristics, such as type of substrate, light intensity and the existence of understory and previous mycelial networks, seemed to be the main factors determining the structure of these fungal communities. Given their sensitivity to different types of environmental change, these microbial communities could be used as bioindicators. Specifically, Basidiomycetes and total fungi seem to be better bioindicators of environmental changes than ascomycetes. The results shown here contribute to the understanding of how fungal communities respond to disturbance, an important issue in the evaluation of how ecosystem processes might be affected in areas commonly subjected to geological risks, such as volcanism. This information is also relevant to the planning and successful implementation of restoration activities based on seedling optimization by rhizosphere fungi.

The 2011 Puyehue-Cordón Caulle volcanic eruption left vast areas of Nothofagus pumilio forests covered by high volumes of tephra, which buried the understory. This event offered a unique opportunity to analyse the impact of recent tephra deposition and the main environmental factors affecting the growth of N. pumilio seedlings. To this end, three study sites were selected and sampled three years after the eruption, two with a thick tephra layer (50 cm) but with different tree conditions: i) most trees alive or ii) most trees dead, and a third site, iii) with minimal tephra deposition. From each site, five N. pumilio 18-month-old seedlings were collected, morphometrically measured (shoot, epicotyl and main root length; shoot, epicotyl and root collar diameter), and environmental factors were also registered (light intensity, soil pH and conductivity, available P, bulk and particle soil density, and porosity). The main results indicated that three years after eruption the site with tephra deposition and dead trees (the most near to the crater) had the biggest seedlings, the lowest pH and the highest values for available P and light intensity. This temporally unique geological event showed that the regeneration potentiality of N. pumilio is high when environmental factors allow direct incident light on a substrate with high mineral content, even when the environment has previously been subjected to a significant degree of disturbance. Given the recurrence of volcanic events in northern Patagonia, this subject merits further long-term study.

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