Abstract
Key messagePhloem 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.
Highlights
Secondary growth in trees is the result of cambial activity that produces both acropetal conducting xylem cells and basipetal conducting phloem cells (Larcher 2003)
The present study investigated the phenology of phloem and xylem growth ring formation in mature Picea abies trees along an elevational gradient in the Black Forest
The onset of cell production did not differ for either the phloem or the xylem between LOW and MED, while a clear delay was observed at HIGH
Summary
Secondary growth in trees is the result of cambial activity that produces both acropetal conducting xylem cells and basipetal conducting phloem cells (Larcher 2003). Studies on the intra-annual dynamics of xylem formation have become quite common Trees (2020) 34:1281–1297 seasonal dynamics of phloem formation in several conifers and deciduous trees (e.g. Evert 1963; Alfieri and Evert 1968; Davis and Evert 1968; Tucker and Evert 1969; Davis and Evert 1970; Alfieri and Evert 1973; Alfieri and Kemp 1983), there are only a handful of studies from recent years dealing with the same topic. Annual phloem formation is crucial for tree survival and the functionality of the entire plant body, since in addition to assimilates, proteins, RNA and electrical wound signals, almost all endogenous plant hormones are transported in the phloem (Oparka 1999; van Bel 2003; Taiz and Zeiger 2006) and except for situations of unfavourable growth conditions sieve cells in conifers are functional mostly only for one to two years (Abbe and Crafts 1939; Holdheide 1951; Evert et al 2009). The function of the xylem is closely related to the phloem (Zimmermann and Brown 1980; Evert et al 2009; Woodruff 2014)
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