Differences in intra-annual wood formation in Picea abies across the treeline ecotone, Giant Mountains, Czech Republic

KEY MESSAGE : Picea abies requires warming of both the above- and belowground parts of the tree for full resumption of cambial activity. Elevation-related decrease in growing season temperatures is a highly important factor in limiting tree growth in cold environments such as alpine treeline ecotones. In this study, we aimed to identify radial growth timing differences in Picea abies (L.) Karst. between the lower (timberline) and upper (treeline) parts of an alpine treeline ecotone. Over three growing seasons, soil and air temperatures were measured and phenology of wood formation was analyzed at two sites separated by 140 m of elevation in the Giant Mountains, Czech Republic. The results showed that there were two periods with significant differences in wood phenology between timberline and treeline. In the early part of the growing season, higher ambient temperatures at timberline led to higher number of cambial and enlarging cells here than at treeline. In the second part of the growing season, the bigger and/or more numerous tracheids at timberline than at treeline required more time for maturation. Significant delay in the beginning of wood formation at treeline in comparison to timberline was observed only in 2011, when soil was frozen markedly longer at treeline. We found that cambial activity significantly increased when soil temperature increased from near zero to a threshold temperature of 4–5 °C. We therefore suggest that for P. abies both the above- and belowground parts of the tree must be sufficiently warm for full resumption of cambial activity.