Impact of climate on growth of Acer saccharum (sugar maple) at the prairie-forest border in western Minnesota

Background/Question/Methods

Performance of Acer saccharum at the edge of its current range may be predictive of the species’ response to near-term climate change.    We used tree rings to establish growth patterns for A. saccharum at three sites along a SW-NE transect spanning the historical prairie-forest border in western Minnesota. Our study sites span a moisture and temperature gradient approximately equal to predicted climate change for the state during the remainder of the 21^st century. Sampled A. saccharum populations established in the 1850s at the time of European settlement and mean diameters of sampled trees were similar across all three sites.  In a prior study, we found that Quercus macrocarpa (bur oak) in our study transect showed decreasing sensitivity to drought with time in the 20^th century. Here, we specifically asked how A. saccharum growth responds to drought and whether this drought-growth relationship might also be changing with time. To that end, we employed only conservative detrending methods to preserve low frequency variation.

Results/Conclusions

Sampled A. saccharum showed a strong response to summer drought at all three sites during 20^th and early 21^st centuries (correlation with PDSI ranged from 0.33-0.55).  Based on raw ring widths, growth since 2000 at our hottest, driest site has been less than 60% of growth for similar sized trees at our wettest site. To expand our comparisons, we used recently published chronologies from the central portions of Minnesota to extend our transect further to the NW and more firmly into the range of A. saccharum. There is a clear pattern of decreasing drought sensitivity from the SW to the NE ends of the expanded transect. Unlike our previous work with Q. macrocarpa, we note that A. saccharum drought sensitivity appears to be increasing with time (post-1950 correlations between growth and PDSI ranged from 0.42-0.58).  We have argued that decreasing drought sensitivity may buffer Q. macrocarpa populations at the prairie forest border against retreat in the face of near-term climate change. In contrast, given established links between slow growth and increased mortality risk, the results we report here suggest that A. saccharum populations at the current ecotone may be more vulnerable.