Plasticity of seasonal xylem and phloem production of Norway spruce along an elevational gradient

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.