Upward and downward colonization of vegetation following the retreat of the Aneto glacier, since the Little Ice Age

Glacier surface is decreasing worldwide and deglaciated land provides a unique opportunity to study plant colonization and primary ecological succession in alpine ecosystems. Most previous research on the topic has been conducted in strongly glaciated regions hosting the most important glacier systems. Contrastingly, little research has been conducted in thermic mountains with relict glaciers near extinction like those of the Pyrenees. The highest peak in the Pyrenees, Aneto, hosts a glacier that has dramatically shrunk in recent decades, and is currently restricted to upper slopes near the ridgeline. To understand plant succession and colonization in a system representative of relictual thermic glaciers, we conducted vegetation surveys along the deglaciation chronosequence of the Aneto glacier, from the Little Ice Age (LIA) to date. We identified vascular plant species present in 60 plots at 6 different sites along the deglaciation gradient, recording 65 different species in total. Plant composition was different between the deglaciated area and nearby control surveys conducted in adjacent unglaciated areas where diversity was higher. Moreover, plant diversity decreased towards recently deglaciated areas, and plant composition became more stochastic. Interestingly, we found evidence that colonization in the lower part of the deglaciated slope was based on the species pool of non glaciated areas below the glacier and colonization of the upper part was also strongly influenced by downward colonization from species native to the ridges. Consequently, although the altitude is not the main driver of the colonization process along the chronosequence, it has a relevant role in determining the species source.