Recent synchronous horticultural escape and naturalization of Magnolia tripetala north of its native range will give tree species a ‘head start’ on climate change

Background/Question/Methods

Plants grown in horticulture or occurring as adventives outside their native range can provide insight into species fundamental niche requirements that might not be evident from the native range, or realized niche, alone.  Such occurrences can also identify conditions that support individual survival, but do not currently sustain positive population growth (i.e., a species ‘tolerance niche’).  Further, in the context of rapid climate change, horticultural and adventive occurrences beyond current range edges might circumvent natural dispersal limitations and facilitate species range shifts.

To explore these concepts in the field, we investigated the history and structure of five newly discovered populations of naturalized Magnolia tripetala near horticultural sites in western Massachusetts, USA.  This tree species is native to the southeastern US, but has been grown horticulturally in the Northeast since the 1800s.  However, naturalization had not been widely documented in the region previously, raising the possibility that the species escape has been triggered by recent climate change.  With tree coring and life stage surveys, we asked whether the naturalized populations exhibited synchronous patterns of establishment and expansion, suggestive of climatic release and a shift from tolerance niche to fundamental niche conditions in the region. 

Results/Conclusions

Across the five sites, we documented 660 individuals, with populations ranging in size from 46 to 396 individuals, including seedlings, saplings, and reproductive trees.  Although horticultural specimens of M. tripetala have been present near the sites for many decades, the adventive populations showed clear evidence of recent, synchronized escape and naturalization.  Dated tree cores from the 10 largest adventive M. tripetala at each site showed the average age of establishment was 22.8 years (~1991), with the individual population means falling in a narrow range from 20.6 to 25.3 years (~1989-1993).  Three older trees (35-45 years) in 3 of the 5 populations suggested rare establishment of individuals prior to 1980s, but most individuals (88-96%) were seedlings and smaller saplings that have established since the 1990s.

Recent escape from old horticultural plantings is allowing M. tripetala to rapidly colonize newly-suitable habitat in the Northeast US, ~300 km beyond its native range.  Recent climate change appears to have released the reproductive potential of horticultural trees that had existed under ‘tolerance niche’ conditions for many decades, resulting in vigorous new naturalized populations.  It is unlikely that natural dispersal from the south would have allowed M. tripetala to reach this region anytime soon.