Tree Migration-Rates: Narrowing the Gap between Inferred Post-Glacial Rates and Projected Rates

Angelica Feurdean,H John B Birks,Heike Lischke,Shonil A Bhagwat,Katherine J Willis,Thomas Hickler,David Nogues-Bravo

doi:10.1371/journal.pone.0071797

Abstract

Faster-than-expected post-glacial migration rates of trees have puzzled ecologists for a long time. In Europe, post-glacial migration is assumed to have started from the three southern European peninsulas (southern refugia), where large areas remained free of permafrost and ice at the peak of the last glaciation. However, increasing palaeobotanical evidence for the presence of isolated tree populations in more northerly microrefugia has started to change this perception. Here we use the Northern Eurasian Plant Macrofossil Database and palaeoecological literature to show that post-glacial migration rates for trees may have been substantially lower (60–260 m yr–1) than those estimated by assuming migration from southern refugia only (115–550 m yr–1), and that early-successional trees migrated faster than mid- and late-successional trees. Post-glacial migration rates are in good agreement with those recently projected for the future with a population dynamical forest succession and dispersal model, mainly for early-successional trees and under optimal conditions. Although migration estimates presented here may be conservative because of our assumption of uniform dispersal, tree migration-rates clearly need reconsideration. We suggest that small outlier populations may be a key factor in understanding past migration rates and in predicting potential future range-shifts. The importance of outlier populations in the past may have an analogy in the future, as many tree species have been planted beyond their natural ranges, with a more beneficial microclimate than their regional surroundings. Therefore, climate-change-induced range-shifts in the future might well be influenced by such microrefugia.

Highlights

Estimating rates of tree migration is critical for understanding how species range distributions are shaped by past expansion and contraction, and how species might respond in the future to climate and land-use changes
We find that post-glacial migration rates for trees may have been substantially lower than those estimated by assuming migration from southern refugia only
Our analysis shows that: 1) the spreading-rate estimates from northern refugia are substantially lower than those that assume colonization only from the south, and 2) early-successional trees (Betula, Pinus, Alnus) migrated faster than mid- and late-successional ones (Picea, Abies alba, Quercus, Carpinus betulus, Fagus sylvatica) (Fig. 2, Table 2)

Summary

Introduction

Estimating rates of tree migration is critical for understanding how species range distributions are shaped by past expansion and contraction, and how species might respond in the future to climate and land-use changes. Migration rates of European tree species in response to past climate changes have generally been estimated by assuming that these species persisted during the last glacial maximum (LGM) in southern Europe (southern refugia) with their northernmost distributions at approximately 40–45uN latitude [3,4,5,6]. It has often been assumed that trees dispersed rapidly (100–1000 m yr–1) via long-distance dispersal in response to climate warming during the early post-glacial [3,7]. Theoretical, and modelling studies have shown that long-distance dispersal could explain rapid migration rates [7,9,10], the possibility of such a rapid migration-capacity has been challenged [11]

Methods

Results

Conclusion