Update: From acorns mighty micropockets grow

Abstract

ISSN 1948-6596 news and update update From acorns mighty micropockets grow Dispersal processes may be critical in determining rates of species range shifts under future climate change. Travis Marsico, Jessica Hellman and Jeanne Romero-Severson, writing in the Journal of Biogeography, shed new light on the future by examining the past shifts in Oregon oak since the Last Glacial Maximum (LGM) (Marsico et al. 2009). They find that it doesn't take a village, it just takes a single tree. This helps support an emerging new view of how plant range shifts are realized when climate shifts rapidly. Until recently, prevailing concepts of bio- logical response to rapid warming in North Amer- ica and Europe were of large, monolithic range shifts in which species totally vacated habitat of waning suitability, moving to new, completely dis- junct range. This view began to change in 1995 when Matt McGlone and Vera Markgraf pre- sented a starkly contrasting model of vegetation shift in the Southern Hemisphere. In New Zealand, McGlone found recolonization of the South Island to be much more rapid than could be accounted for by long-distance dispersal (McGlone 1995). In southern South America, Markgraf documented vegetation change keeping pace with extremely rapid climate 'flickers' (Markgraf and Kenny 1995). Both authors suggested that micropockets of vegetation could explain the very rapid observed changes in dominant vegetation. Such micropock- ets of vegetation were very plausible in these southern settings where no large continental ice sheets existed. Evidence has since grown that micropockets may be important in Northern Hemisphere set- tings as well (recent literature is briefly reviewed by Marsico et al.). Tree remains very close to the boundary of the ice sheet have been found in pa- leorecords of both Europe and North America. Pollen signatures that were previously assumed to be from long-distance blow-in are now being rein- terpreted as possible evidence of vegetation mi- cropockets. If the growing consensus around micro- pockets is correct, the implications for vegetation response to future, human-induced climate change is profound. Patches of vegetation of a few tens of meters on a side might provide source material at rapidly expanding range margins. Long-distance dispersal might establish micro- pockets of vegetation that would serve as step- ping-stones for additional long-distance dispersal. Marsico et al. show that in Oaks in the Pa- cific Northwest, dispersal was primarily from indi- viduals on the leading edge of the range. Chloro- plast DNA markers were remarkably homogenous in their samples. At 16 of 22 sites all individuals sampled shared a single chloroplast haplotype. Having identical chloroplast haplotype indicates a common maternal ancestor, so these populations were established from single colonizations or mul- tiple colonizations by seeds from the same tree (which seems much less likely). The implication is that single trees are criti- cal for establishing populations in newly suitable range. Micropockets of vegetation would have ample sources of seeds for range shifts, if all it takes is a single tree. Marsico et al. found much more diversity in nuclear DNA, indicating that once single colonizations occur, populations grow and are diversified by wide pollen flow from many individuals. Colonists from micropockets and di- versity from broader populations may be a key mechanism in rapid range shifts as rapid human- induced climate change gathers force. Markgraf, V. & Kenny, R. (1995) Character of rapid vegetation and climate change during the late- glacial in southernmost South America. In Hunt- ley, B., Cramer, W., Morgan, A.V., Prentice, H.C. and Allen, J.R.M. (eds.) Past and Future Rapid Environmental Changes: The Spatial and Evolu- tionary Responses of Terrestrial Biota, pp. 81- 102. Springer-Verlag, Berlin. Marsico, T.D., Hellmann, J.J. & Romero-Severson, J. (2009) Patterns of seed dispersal and pollen flow in Quercus garryana (Fagaceae) following post-glacial climatic changes. Journal of Bio- geography, 36, 929-941. McGlone, M.S. (1995) The responses of New Zealand forest diversity to Quaternary climates. In Hunt- ley, B., Cramer, W., Morgan, A.V., Prentice, H.C. frontiers of biogeography 1.1, 2009 — © 2009 the authors; journal compilation © 2009 The International Biogeography Society