Spruce shows greater sensitivity to recent warming than Douglas‐fir in central British Columbia

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AbstractThe effects of climate change on tree growth in ecosystems limited by cold temperature remain unclear. In some cases, growth may increase, but because warmer temperatures can increase water stress, growth may also decline. As climate changes in the sub‐boreal spruce (SBS) zone in central British Columbia, hybrid white spruce (Picea engelmannii × glauca)—a dominant but drought‐intolerant species of this region—is predicted to decline over much of its range, while species such as Douglas‐fir (Pseudotsuga menziesii var. glauca) are likely to replace spruce. To see whether these shifts are already evident, we examined recent trends in basal area increment (BAI) of these two species over the past 50 yr (1961–2010) and explored relationships with climate variables in the SBS zone across a range of sites varying in water availability due to differences in climate and soil moisture regimes. In addition, we analyzed wood δ13C during this period and root carbohydrate reserve (NSC) concentrations at the end of this period as indicators of water and carbon stress, respectively, to help interpret growth trends and climate relationships among sites. Spruce BAI was negatively associated with spring and/or summer temperature on all sites, but growth was never negatively correlated with temperature in Douglas‐fir. For both species, lower BAI on the driest site was accompanied by higher δ13C and lower root NSC reserve concentrations, particularly starch, consistent with drought stress reducing water and carbon availability for growth. However, there was no indication that the negative growth response to temperature was more pronounced on drier sites, as the only growth decline in spruce occurred on a mesic site with lower δ13C. We conclude that rising temperatures are likely to negatively impact the growth of spruce more than Douglas‐fir in the SBS zone, supporting predictions that spruce will decline and be replaced by other species like Douglas‐fir. And while rising temperatures may have already reduced growth on some sites, these declines are not clearly related to increasing drought stress but may relate to effects on the timing of snowmelt.