Identifying floristic diversity from the pollen record in open environments; considerations and limitations

Abstract

Abstract Pollen analysis provides a key technique for the reconstruction of post-glacial vegetation. Modern observation of the plant-pollen relationship, through the use of surface pollen studies, is an instrumental process for informing these reconstructions. However, results from surface pollen studies suggest that the diversity of pollen assemblages is rarely a faithful representation of the diversity of the local vegetation. Pollen diversity at high altitude is determined by both the composition and density of the local vegetation and long-distance pollen. This paper examines the relationship between floristic and palynological diversity using surface samples collected from alpine communities found on three glacier forelands (Austerdalsbreen, Storbreen and Grjotbreen) in the Jostedalsbreen-Jotunheimen region of Norway. Several diversity indices indicate the complexity of the relationship between palynological and floristic diversity. This is accentuated in open, high altitude, sparsely-vegetated sites, where palynological diversity is a measure of the diversity of the long-distance pollen assemblage rather than the diversity of the local vegetation. Even at sites on older moraines at Storbreen and Austerdalsbreen, where the vegetation is denser and more diverse, the relationship is not simple but depends on pollen production of the taxa that dominate the local vegetation. On Storbreen foreland, for example, the diversity of small, insect-pollinated plants growing locally is poorly represented in the pollen assemblage which is dominated by long-distance arboreal pollen. The surface pollen results were compared with fossil pollen data from a palaeosol at Memurubreen in Eastern Jotunheimen. DCA ordination indicates that the fossil pollen spectra are distinct from the modern samples, due to differences in the pollen assemblages, probably related to lower treelines in the past associated with climatic cooling or human impact. The palaeosol samples are most similar to modern samples from heath and snowbed communities, found on older terrain at Grjotbreen and Storbreen, and the diversity indices also show strong similarities with these communities. The potential for improvements in methodology to increase resolution is considered.