https://doi.org/10.1177/0959683616678467 The Holocene 2017, Vol. 27(7) 1008–1018 © The Author(s) 2016 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0959683616678467 journals.sagepub.com/home/hol Introduction The west-to-east vegetation gradient of northern Patagonia (latitude: 40.5–44°S) is one of the most abrupt in the world. From the crest of Andes eastward, the region descends from high peaks >3000 m a.s.l. in elevation to forested foothills that support large glacially carved lakes and valleys and then steppe- covered plateaus below 800 m a.s.l. Storms following the path of the southern westerlies lose moisture on their passage over the Andes creating a strong rainshadow effect on the eastern side. As a result, precipitation drops from ~3000 mm yr -1 at the crest to ~500 mm yr -1 in the steppe, a distance of ~80 km (Garreaud et al., 2013). In northern Patagonia, paleoecological research over the last 30 years has been motivated by an interest in the role of past and present climate change in shaping vegetation and fire regimes (e.g. Bianchi, 1999; Heusser, 2003; Iglesias et al., 2014, 2016; Moreno, 2004; Whitlock et al., 2006). The vegetation history reconstructions rest on the relationship between present-day veg- etation and modern pollen rain. That is, if a fossil pollen assem- blage matches closely with a modern pollen assemblage, then the modern setting of that sample forms the basis for reconstructing the ancient vegetation and environment. A limitation of this approach is that pollen samples in lakes and wetlands integrate vegetation over broad areas, smoothing over local heterogeneities and site-specific biases in the pollen rain (Jackson and Williams, 2004). As a result, the composition and abundance of taxa in pol- len assemblages often differ from the plant communities that pro- duced them. Efforts to estimate vegetation–pollen relationships along the eastern flanks of the Andes are challenged by issues related to pollen identification, production, dispersal, and deposition. For example, the dominant species of Nothofagus in northern Patago- nian forests have indistinguishable pollen, and thus, evergreen and deciduous forest communities are not easily discriminated by pollen data. The same limitation holds for the Cupressaceae genera (Fitzroya, Pilgerodendron, and Austrocedrus) that grow from wet-to-dry settings and have similar pollen. Nothofagus and Cupressaceae are also large pollen producers, and their abun- dance in pollen samples tends to overrepresent their presence in the vegetation (Bianchi and Olabuenaga, 2006). Conversely, Interpreting modern and fossil pollen data along a steep environmental gradient in northern Patagonia Virginia Iglesias, 1 Flavia Quintana, 2 William Nanavati 1,3 and Cathy Whitlock 1,3 Abstract Vegetation reconstructions rest on modern vegetation–pollen rain relationships and deductive reasoning. Establishing this relationship is a nontrivial task because differences among pollen assemblages are not necessarily proportional to differences in vegetation. This task is particularly challenging in Patagonia, where some tree taxa have indistinguishable pollen, and pollen grains can be transported long distances. In this study, we describe the modern pollen of 48 lake and wetland samples from northern Patagonia (40.5–44°S) to better discriminate the major vegetation zones of the region through pollen analysis. Specifically, we focus on the performance of three methodological approaches, namely, pollen indicators, classification trees, and optimal thresholds of dissimilarity. As a proof of concept, we use the modern pollen–vegetation relationships to reconstruct the vegetation history at Laguna el Trébol (41.07°S; 71.5°W). Our results revealed that (1) pollen sums exceeding 260 grains ensured replicable vegetation reconstructions, (2) modern vegetation zones could not be separated solely by visual inspection of their pollen spectra, (3) the classification tree and optimal thresholds of dissimilarity permitted discrimination of most vegetation zones, (4) detection of nonanalog communities required use of pollen indicators or optimal thresholds of dissimilarity, and (5) vegetation at L. el Trébol was likely dominated by late glacial shrubland with no modern analogs in the study area (15,000–12,180 cal. yr BP), modern shrubland (12,180–6500 cal. yr BP) and mixed forest (6500 cal. yr BP–present). This study allows a more realistic understanding of the pollen–vegetation relationship and provides new tools for interpreting past vegetation in northern Patagonia. Keywords calibration, lake sediments, modern pollen rain, Patagonia, pollen sum, vegetation Received 3 August 2016; revised manuscript accepted 5 October 2016 1 Montana Institute on Ecosystems, Montana State University, USA 2 CENAC, Parque Nacional Nahuel Huapi, Argentina 3 Department of Earth Sciences, Montana State University, USA Corresponding author: Virginia Iglesias, Montana Institute on Ecosystems, Montana State University, Bozeman, MT 59717, USA. Email: virginia.iglesias@msu.montana.edu 678467HOL 0 0 10.1177/0959683616678467The HoloceneIglesias et al. research-article 2016 Research paper