Understanding range limits: Climate, competition, and patterns of survival for three sugar maple populations experimentally planted from Arkansas to Ontario

Background/Question/Methods

Forests are vulnerable to climate change, but long-term effects are uncertain. Climate gradients influence tree survival directly, ultimately limiting the extent of species’ ranges, and may also influence tree ranges indirectly by affecting the intesity of competition encountered across a species’ range. This study examined the effects of climate and competition on survival of young sugar maple across its range. Seedlings originating from populations in northern, central, and southern portions of the range were planted at ten sites along a transect from Ontario to Arkansas that spans the range of sugar maple and extends beyond range margins to the north and south. Research plots were located in forest understory and gaps, and surrounding vegetation was removed in half the plots. Seedling survival and growth were predicted to decrease in the presence of understory neighbors, especially in the south where other woody species were expected to be most competitive, and to increase from north to south as a function of climate when understory neighbors were removed.  Responses to canopy competition were hypothesized to be the reverse (most negative in the north, positive in the south).  In addition, regionally local populations of sugar maple were hypothesized to exhibit highest growth and survival.

Results/Conclusions

As hypothesized, sugar maple seedling survival was low at northern sites but remained high beyond the southern range limit of the species, suggesting range-limiting climate constraints only at northern range margins. Overstory canopy competition had a significant effect on survival at sites beyond the southern and northern range margin; in general, seedlings with less light competition (gap plots) experienced higher survival, but the southern population at the southernmost site had significantly higher survival in shaded plots, indicating a facilitative effect.  Slight but significant increases in survival also occurred for some populations and sites when understory competition was reduced (clip plots).  The three sugar maple populations responded differently to treatments across sites.  For example, while all populations experienced reduced survival at the northernmost site, the northern (and presumably most locally adapted) population had much higher survival than the other populations. We found that climate effects may interact with competition to influence survival of woody seedlings, and these effects may be population-specific.  These results suggest that plant distributions may be sensitive to biotic interactions as well as climate, which may interact to influence range shifts in the context of global climate change.