THE CHANGING EFFECTS OF COMPETITION ACROSS A CLIMATE GRADIENT: PATTERNS OF SURVIVAL FOR THREE SUGAR MAPLE POPULATIONS

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

Climate gradients influence tree survival directly, ultimately limiting the extent of species’ ranges, but may also influence tree ranges indirectly by affecting the intesity of competition encountered across a species’ range. This study examines the effects of climate and competition on survival of young sugar maple across its range. Seedlings from three populations were planted at the St. Croix Watershed Research Station in Minnesota and at nine other sites along a transect from Ontario to Arkansas that encompasses the climate gradient of the sugar maple range and extends beyond range margins to the north and south (fig. 1). Research plots were located in forest understory and gaps, and surrounding vegetation was removed in half of the plots. Seedling survival and growth is predicted to decrease in the presence of neighbors, especially in the south where other woody species are expected to be most competitive, and to increase from north to south in the absence of neighbors as a function of climate. Preliminary results seem to support these initial expectations. 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.

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.