Response of northern populations of black spruce and jack pine to southward seed transfers: Implications for climate change

A variety of responses to climate change have been reported for northern tree populations, primarily from tree-ring and satellite-based studies. Here we employ provenance data to examine growth and survival responses of northern populations (defined here as those occurring north of 52oN) of black spruce (Picea mariana) and Jack pine (Pinus banksiana) to southward seed transfers – a space for time substitution that affords insights into potential climate change responses by these important northern tree species. Based on previous work, we anticipated relatively flat response curves that peak at much warmer temperatures than those found at seed source origin. These expectations were generally met for growth-related responses, with peak growth associated with seed transfers to environments 2.2oC and 3.6oC warmer than seed source origin for black spruce and jack pine, respectively. These findings imply that northern tree populations harbor a significant amount of resilience to climate warming. However, survival responses told a different story, with both species exhibiting reduced survival rates when moved to warmer and drier environments.  Together with the growth-based results, these findings suggest that the warmer and drier conditions expected across much of northern Canada under climate change may reduce survival, but surviving trees may grow at a faster rate up until a certain magnitude of climate warming has been reached. We note that all relationships had high levels of unexplained variation, underlining the many factors that may influence provenance study outcomes and the challenges in predicting tree responses to climate change. Despite certain limitations, we feel that the provenance data employed here provide valuable insights into potential climate change outcomes for northern tree populations.

Methods
Black spruce provenance data were obtained from measurements on a portion of the Canadian Forest Service’s (CFS) long-term black spruce provenance trial, which originally incorporated 202 seed sources across 34 test sites in Canada and the United States.  Measurements of height and diameter of all surviving trees at each test site were made in 2003, at 33 years of age from seed. These data were obtained through a collaborative research agreement with William H. Parker at Lakehead University, Thunder Bay, Ontario. In total, 192 seed sources at 18 test sites in Canada and one test site in Minnesota were included in the measurement.

Jack pine provenance data were obtained from measurements on a portion of the CFS 255 series rangewide provenance trial, which consisted of 99 seed sources planted in various combinations at test sites across eastern Canada, the United States, and Europe. During the summer of 2005, at 39 years of age from seed, all 16 remaining viable test sites in Canada and the United States were remeasured. We obtained these data through a collaborative research agreement with William H. Parker at Lakehead University, Thunder Bay, Ontario.