Phenological variation in xylem and phloem formation in Fagus sylvatica from two contrasting sites

Xylem and phloem formation, as well as cambium and leaf phenology, and their relation to weather factors, were studied from 2008 to 2010 in beech (Fagus sylvatica L.) trees from two sites in Slovenia with different elevations and weather conditions: Panška reka (PA) (400 m a.s.l.) and Menina planina (ME) (1200 m a.s.l.). During the vegetation periods leaf phenology and dynamics of xylem as well as phloem formation were monitored. Leaf unfolding, onset of cambial cell production and increased number of active phloem cells occurred simultaneously for each site: in mid-April at PA and in the first week of May at ME; all three events were positively related to temperature in the first part of the growth season. Maximum rate of xylem cell production occurred at PA from 20 May until 9 June and about two weeks later at ME. Maximum phloem cell production occurred more than 1 month earlier at both sites. Cessation of xylem and phloem cell production was observed at PA around 19 August and around 10 days earlier at ME. Differentiation of the last-formed xylem cells was concluded by mid-September at both plots. The differences in xylem and phloem formation phases were smaller in the second part of the growth season and can be ascribed to similar temperatures at both plots. Year to year variability of the observed phases was not statistically significant but the differences between the sites were. Phloem formation seems to be less subjected to fluctuations in environmental conditions since the growth ring widths were comparable at both sites. Temperature and growing degree days before the occurrence of most of the observed phenological phases significantly differed between the sites. This demonstrates that the observed differences in xylem and phloem formation between the sites can be attributed to high intra-specific plasticity of beech.