The relative roles of environment and history in controlling large-scale species distributions are important not only theoretically, but also for forecasting range responses to climatic change. Here, we use atlas data to examine the extent to which 55 tree species fill their climatically determined potential ranges in Europe. Quantifying range filling (R/P) as realized/potential range size ratios using bioclimatic envelope modelling we find mean R/P ¼ 38.3% (±30.3% SD). Many European tree species naturalize extensively outside their native ranges, providing support for interpreting the many low R/Ps as primarily reflecting dispersal limitation. R/P increases strongly with latitudinal range centroid and secondarily with hardiness and decreases weakly with longitudinal range centroid. Hence, European tree species ranges appear strongly controlled by geographical dispersal constraints on post-glacial expansion as well as climate. Consequently, we expect European tree species to show only limited tracking of near-future climate changes.

The relative importance of contemporary climate and history as controls of geographical diversity patterns is intensely debated. A key example is the controversy over the extent to which temperate tree distributions and diversity patterns reflect postglacial dispersal limitation. Here, we focus on Central and Northern Europe, and show that recent estimates of tree migration rates < 100 m year(-1) imply that many species have probably not reached equilibrium with climate in this region. We then demonstrate that geographical accessibility from glacial refuges explains 78% of the geographical variation in the region's tree diversity and is a much stronger diversity predictor than climate. Finally, we show that realistic estimates of migration rates can be derived from the observed tree diversity pattern by assuming it to be purely dispersal driven. In conclusion, the tree diversity pattern in Central and Northern Europe could, to a large extent, be a result of postglacial dispersal limitation.

Aim This study uses a high-resolution simulation of the Last Glacial Maximum (LGM) climate to assess: (1) whether LGM climate still affects the geographical species richness patterns in the European tree flora and (2) the relative importance of modern and LGM climate as controls of tree species richness in Europe. LocationThe parts of Europe that were unglaciated during the LGM.Methods Atlas data on the distributions of 55 tree species were linked with data on modern and LGM climate and climatic heterogeneity in a geographical information system with a 60-km grid. Four measures of species richness were computed: total richness, and richness of the 18 most restricted species, 19 species of medium incidence (intermediate species) and 18 most widespread species. We used ordinary least-squares regression and spatial autoregressive modelling to test and estimate the richness-climate relationships. ResultsLGM climate constituted the best single set of explanatory variables for richness of restricted species, while modern climate and climatic heterogeneity was best for total and widespread species richness and richness of intermediate species, respectively. The autoregressive model with all climatic predictors was supported for all richness measures using an information-theoretic approach, albeit only weakly so for total species richness. Among the strongest relationships were increases in total and intermediate richness with climatic heterogeneity and in restricted richness with LGM growing-degree-days. Partial regression showed that climatic heterogeneity accounted for the largest unique variation fraction for intermediate richness, while LGM climate was particularly important for restricted richness. Main conclusionsLGM climate appears to still affect geographical patterns of tree species richness in Europe, albeit the relative importance of modern and LGM climate depends on range size. Notably, LGM climate is a strong richness control for species with a restricted range, which appear to still be associated with their glacial refugia.

The influence of dispersal limitation on species ranges remains controversial. Considering the dramatic impacts of the last glaciation in Europe, species might not have tracked climate changes through time and, as a consequence, their present-day ranges might be in disequilibrium with current climate. For 1016 European plant species, we assessed the relative importance of current climate and limited postglacial migration in determining species ranges using regression modelling and explanatory variables representing climate, and a novel species-specific hind-casting-based measure of accessibility to postglacial colonization. Climate was important for all species, while postglacial colonization also constrained the ranges of more than 50 per cent of the species. On average, climate explained five times more variation in species ranges than accessibility, but accessibility was the strongest determinant for one-sixth of the species. Accessibility was particularly important for species with limited long-distance dispersal ability, with southern glacial ranges, seed plants compared with ferns, and small-range species in southern Europe. In addition, accessibility explained one-third of the variation in species' disequilibrium with climate as measured by the realized/potential range size ratio computed with niche modelling. In conclusion, we show that although climate is the dominant broad-scale determinant of European plant species ranges, constrained dispersal plays an important supplementary role.

Climate is often singled out as the primary range limiting factor at large scales, while other environmental factors, notably soil, are thought to predominate at smaller scales. However, the postglacial migrational lag hypothesis controversially suggests that many species are strongly dispersal-limited and still expanding from their ice age refugia. We investigated the importance of postglacial migrational lag, climate, and soil as range determinants for 47 widespread forest plant species across nemoral Europe (47.0Á60.08N, west of 24.08E) using regression modeling, information-theoretic model selection, multi-model inference, and variation partitioning. Migrational lag was represented by a measure of accessibility to recolonization from ice age refugia.Twelve species were largely ubiquitous and not analyzed further. For the remaining species, there was strong support for climate, soil, and accessibility to postglacial recolonization. Accessibility accounted for a small to moderate amount of variation, but its model-averaged regression coefficient was stronger than those for climate or soil for 11 species and the second or third strongest coefficient for an additional 10 species. The resulting odds ratios were greater than one for 33 out of 35 species, i.e. adjusted for climate and soil, prevalence for the far majority of the 35 non-ubiquitous species increased with increasing accessibility. There were no differences among growth forms in the importance of accessibility. In contrast, compared to non-boreal species, accessibility had little importance for species with wide boreal distributions, as expected from their more widespread, northern glacial distributions.In conclusion, even the ranges of many widespread forest plant species are probably still moderately to strongly limited by postglacial migrational lag. Therefore, although species ranges are also strongly influenced by climate, we cannot expect most forest plant species to closely track the expected 21st century climatic changes.