UNDERSTANDING SUGAR MAPLE RANGE LIMITS: CLIMATE AND BIOTIC INTERACTIONS ALONG A NORTH-SOUTH GRADIENT

Rachel C. Putnam and Peter B. Reich
University of Minnesota

Tree ranges are shaped primarily by climate but also by factors such as biotic interactions. This study examines the relative importance of climate and biotic interactions in affecting growth and survival of young sugar maple across its range. My central hypothesis is that the range of sugar maple is limited by climate in the north and competition in the south. Sugar maple seedlings from three populations have been planted at the St. Croix Watershed Research Station in Minnesota and at nine other sites in a transect from Ontario to Arkansas that spans the range of sugar maple and extends beyond range margins to the north and south. In addition to planted seedlings, naturally occurring sugar maple seedlings and competing tree seedlings will also be studied at sites within the range of sugar maple. Plots are located in forest understory or gaps, and neighboring vegetation has been removed in half of the plots. In the coming two years I will measure seedling survival, growth, and photosynthetic response. I expect seedling survival and growth to decrease in the presence of neighbors, especially in the south where heterospecific saplings are expected to be most competitive, and to increase from north to south in the absence of neighbors as a function of climate. This study will examine factors that determine current range limits, identify population differences in response to climate and competition, and lend insight on how sugar maple may respond to novel climate conditions.

Figure 1. Range of sugar maple (Acer saccharum), in green (adapted from Little 1971). Colored diamonds indicate site locations where seedlings have been planted, and black dots indicate counties of origin for the seeds from which seedlings were grown.

Suggested Reading

Loehle, C. 1998. Height growth rate tradeoffs determine northern and southern range limits for trees. Journal of Biogeography 25:735-742.