Comparing stem growth of strip-bark and whole-bark growth morphologies in a subarctic conifer (Pinus banksiana), Yellowknife, Northwest Territories

Generally, organisms living near the periphery of their range experience increased environmental stress compared to individuals of the same species growing at the core of their geographic distribution. The greater environmental stress at the ecological limits of a species distribution normally leads to greater sensitivity to growth-limiting factors. For some tree species, this can lead to the development of unique growth morphologies such as strip-bark or partial cambial dieback. Previous studies have concluded that the growth differences induced by CO₂ fertilization effects in high elevation strip-bark trees makes them unsuitable for climate reconstructions. However, comparisons of strip-bark and whole-bark growth morphologies at high latitude and low elevation sites are limited. This study compares the growth of strip-bark and whole-bark jack pine (Pinus banksiana Lamb.) trees near Yellowknife, Canada. Annual ring widths from trees of each morphological type were measured, and the results indicate the growth of the two populations are statistically different from one another prior to the late 19th century. After the late 19th century, growth of the two populations remains statistically different, but the average mean growth of strip-bark trees increases 44% after 1891 while average mean growth of whole-bark trees decreases nearly 11%. Climate sensitivity of the two populations was tested using monthly and seasonal climate indices from 1943 to 2015 and results indicate the two populations share similar responses to climate. The results of this research suggest these strip-bark and whole-bark trees share similar climate-growth relations at inter-annual timescales, however, their growth trends diverge at decadal to centennial timescales. The timing of cambial dieback was dated to a short time interval in the early 19th century which coincided with a period of cool summer temperatures across the northern hemisphere. Increased growth in strip-bark trees after ∼1891 could be related to CO₂ fertilization effects, although further studies into intrinsic water use efficiency would be helpful in assessing this possibility more accurately.