Tobias Walter Miller

Climate warming lengthens the productive season in extratropical forests. Yet there is inconclusive evidence on whether this in turn elicits enhanced tree growth and forest carbon sequestration. In this study, we used cellular observations of conifer wood formation spanning boreal to semiarid forests to quantify the relative importance of cambial growth rate versus season length in driving annual cell production, the key process underlying woody growth. We found that rate is the primary driver of annual cell production. Furthermore, whereas the growing season lengthens with annual temperature, growth rate exhibited an optimum at ~6°C, beyond which increasingly dry conditions compromise wood production gains. The negative impact of warming on intra-annual growth dynamics can thus largely offset the benefit of longer growing season on future forest carbon storage.

Key message: Phloem cell production was less influenced by environmental factors than xylem cell production. The moment of maximum number of conducting phloem cells occurred at the end of the growing season. The understanding of the seasonality of phloem production, its dependence on climatic factors and potential trade-offs with xylem cell production is still limited. This study determined key tree-ring phenological events and examined the dynamics of phloem and xylem cell production of Norway Spruce (Picea abies(L.) Karst) by sampling microcores during the growing seasons 2014 and 2015 along an elevational gradient (450 m, 750 m, 1250 m a.s.l.) in south-western Germany. The onset of phloem formation preceded xylem formation at each elevation by approximately 2 weeks, while cessation showed no clear differences between the stands. Maximum rates of xylem and phloem cell production were observed around the summer solstice, independent of elevation. No linear pattern was found in the occurrence of phenological events along the elevational gradient. Phloem formation appeared to be less sensitive to environmental conditions since no difference was found in the number of produced sieve cells between the 2 years of study, whereas the ratio of xylem to phloem cells was significantly smaller in the year 2015 with summer drought. The total number of conducting, non-collapsed phloem cells did not culminate as expected at the time of the potential maximum assimilate production, but at the end of the growing season. Thus, interpretation of phloem formation should not be limited to the function of assimilate transport but should follow a more holistic view of structural–functional relationships of conductive tissues and tree physiological processes.