Quantifying height growth and monthly growing degree days relationship of plantation Taiwan spruce

The future of the endemic Taiwan spruce (Picea morrisonicola) under climate change is of great concern. It is the southernmost species of the genus and its current distribution is limited to high altitudes of Taiwan. As a first step toward assessing the impact of future temperature changes on the species, we quantified the effects of past monthly growing degree days (GDD) on the height growth of plantation Taiwan spruce based on nonlinear mixed-effects growth analysis. Our results showed that past GDD variations had both positive and negative effects on the height growth of the species. July of the preceding year had the greatest influence on current year height growth. An increase in the mean GDD level of the current May would also promote height growth. In contrast, a warmer previous November or current January had negative effects on height growth. If the established height growth–GDD relationship holds, the influences of climate change on Taiwan spruce height growth will depend on the timing of the temperature increases, as well as on the trees current growth stages. Our results suggested that a warmer climate would have a greater influence on trees that are still in the early stages along the height growth trajectory. The established height growth–GDD relationship will be a keystone for developing models assessing how Taiwan spruce responds to climate change.