Connectivity dynamics since the Last Glacial Maximum in the northern Andes: a pollen-driven framework to assess potential migration

We provide an innovative pollen-driven connectivity framework of the dynamic altitudinal distribution of North Andean biomes since the Last Glacial Maximum (LGM). Altitudinally changing biome distributions reconstructed from a pollen record from Lake La Cocha (2780 m) are assessed in terms of their changing surface and connectivity within the study area. The upper forest line (UFL) ecotone lodged during much of the time around 2000 m (LGM), 2400 m (ca. 14-8 ka), 2800 m (ca. 8-3 ka), and 3550-3600 m (modern time). This resulted in a four-fold increase of the area covered by mountain forest (Andean and sub-Andean), a decrease of 96% of páramo, and a disappearance of permanent snow. Upslope migration of the UFL of 20 vertical m yr-1 and more, as inferred from the pollen record, was spatially assessed: reduced surface area, dispersal limitation, reduced connectivity, and extirpation of the subpáramo biome during a few centuries is shown. The study area includes abundant higher mid-range altitudes (2600-3400 m), with a steep reduction of available surface area and increased dispersal distance in the high and low altitudes. In this range, each 100-m altitudinal rise of the UFL results in 20%-60% reduction of the surface area available for páramo and connectivity. The critical elevations where large biome surfaces start to disconnect depend on the elevation of lowest thresholds in the landscape and the elevation of summits. The 2500-3600 m elevation range is most dynamic in terms of geography and ecological species sorting; the 1000-1500 m interval is relatively stable and is permanently covered by Andean forest, making this interval less sensitive for monitoring climate change. When forests migrate to higher elevations, distribution nuclei of species are compressed, resulting temporarily in a higher species diversity. The species dissimilarity coefficient reflects rate of (ecological) change more adequately than the rate of palynological turnover, because the latter is much influenced by the lengths of the time steps between the pollen samples. Spatial analysis of site-specific dynamics provides exciting new insights into past vegetation dynamics, with potential for better understanding species-area distributions, distribution patterns of biodiversity, and conservation of mountain ecosystems