Survival and growth of three boreal conifer species transplanted to warm sites: Implications for responses to global warming and extreme climatic events

Understanding the responses of boreal conifers to climate change is essential for future mitigation and adaptation. In this study, 3-year-old seedlings of three Japanese boreal conifers, including Sakhalin fir, Yezo spruce, and Sakhalin spruce, naturally found in Hokkaido, Japan, were transplanted in spring 2016 to a cool control and two warm (air-dried interior and humid coastal) sites. We investigated survival, height, and ecophysiological traits based on three parameters: stable carbon isotope discrimination (δ13C), specific leaf area, and leaf mass-specific nitrogen concentration ( N) of seedlings during experiments. The survival rates of two spruce species were gradually significantly lower in warm sites, while that of Sakhalin fir did not significantly differ among sites. The relative growth rate (RGR) of two spruce species was significantly lower in the warm-interior site than in both cool control and warm-coastal sites in 2018, although in 2017, the RGR of two spruce species was lower in the warm sites than in the cool control site. The less negative δ13C value in 2018 revealed that a vapor pressure deficit might decrease the spruces’ RGR via stoma closure in the warm-interior site. We found that Sakhalin fir would be less sensitive to climate change than the two spruce species.

Methods
Three boreal conifers (Sakhalin fir, Sakhalin spruce, and Yezo spruce) are distributed in natural forests in Hokkaido, Japan. Seeds of three conifers collected from the University of Tokyo Hokkaido Forest (UTHF), Furano, central Hokkaido, Japan.  Saplings of three boreal conifers were grown in the nursery in the UTHF. In May 2016, 144 saplings (~12 cm in height) of each species were transplanted at the three sites: CBA, CCB, and FRN. 432 saplings (three species × three sites × 48 individuals) were planted.