Growth of Norway spruce (Picea abies L.) saplings in subalpine forests in Switzerland: Does spring climate matter?

Limited knowledge is available about the growth of Norway spruce saplings related to spring climate, especially snow duration, in subalpine forests. The primary goal of this research was to quantify the relative importance of the direct and indirect drivers of growth for spruce saplings in subalpine forests that are related to spring climate. Two related, subordinate goals were to compare the statistical power of different growth indices for saplings, and to evaluate differences among model accuracy and strength of predictors for saplings stratified into three height classes. During the 2002 and 2003 field seasons we conducted an observation study in the subalpine forests of Davos and Sedrun, Switzerland. We measured growth and local light availability, spring climate, and snow environment for 634 spruce saplings. Tree size, spring snow duration, overstory canopy cover and elevation were among the most important variables related to annual height growth (R2 = 0.35), primary growth (of the main stem and upper branches, R2 = 0.44), and crown volume growth (R2 = 0.80) of saplings. In addition, the presence of black snow mold (Herpotrichia nigra) was important for height and primary growth. Crown volume growth of trees stratified into height classes explained more variability than height or primary growth models, with R2-values of 0.43 for trees 10–30 cm tall (A), 0.32 for trees 31–60 cm (B), and 0.51 for trees 61–130 cm (C). Primary growth models for A and C-size class trees had moderate R2-values of 0.37 and 0.36, respectively. Growth of A-size trees was primarily related to canopy cover and spring snow duration. While for B-size trees it was difficult to capture aboveground growth, this group responded more to the presence of snow mold than the other groups. Growth of C-size trees was correlated well with the availability of direct light, snow duration, and slope orientation. We conclude that in addition to the other environmental predictors mentioned, spring climate and especially snow duration matter for the growth of Norway spruce saplings in subalpine forests.