Spatiotemporal Pattern of Primary Succession in Relation to Meso-topographic Gradients on Recently Deglaciated Terrains in the Patagonian Andes

Abstract

Vegetation in primary succession is influenced by multiple stochastic and environmental factors at different spatial and temporal scales. In this study we analyze the effect of meso-topographic heterogeneity on vegetation development following the retreat of Glaciar Seco in the southern Patagonian Andes. Composition and cover of algae, lichens, mosses, and vascular plants were recorded in 580 plots located in different topographic positions within a chronosequence of eight consecutive moraines. Sample plots were characterized by topographical and surface features. Spatiotemporal patterns in vegetation composition and their relationships to environmental factors were assessed by classification and ordination. We recognized eight communities that correspond to four major successional stages. The successional sequence is characterized by a physiognomic development from pioneer saxicolous lichens (first stage) to secondary colonizer lichens (second stage), followed by shrub colonization (third stage) and the development of Nothofagus spp. forests (fourth stage). Alternative successional trajectories on different topographic positions vary in the sequence of these four major successional stages, with the trajectories on the moraine ridge-top and base not going through some of the stages. A variance partition procedure shows that time since deglaciation and topographic position on the moraines account for comparable amounts of vegetation variance, emphasizing the importance of spatiotemporal analysis of vegetation development on heterogeneous landscapes. Broad trends in vegetation development follow environmental gradients. However, emergence and persistence of vegetation patterns can also be attributed to dynamic geomorphic processes such as moraine slope degradation affecting boulder distribution along the moraine foreslope. At the landscape scale, successional trajectories converge to a Nothofagus-dominated state, but significant variability remains in the understory due to the differential distribution of cryptogams along the moraine topographic gradient. Convergence is mostly related to the expansion of communities from more favorable sites towards the harsher moraine crest, but it is not a process of gradual deterministic changes along the different successional pathways.