LACK OF ANNUAL PERIODICITY IN CAMBIAL PRODUCTION OF PHLOEM IN TREES FROM MEDITERRANEAN AREAS

Abstract

Annual periodicity of cambium production of xylem and phloem cells has rarely been compared in trees from different environments. We compared the structure of cambium and the youngest xylem and phloem increments in four tree species, Fagus sylvatica, Picea abies, Pinus sylvestris and Pinus halepensis, from nine temperate and Mediterranean sites in Slovenia and Spain. In Picea abies, Pinus sylvestris and Fagus sylvatica from temperate locations in Slovenia, xylem and phloem growth ring boundaries could be identified. In Fagus sylvatica growing at two elevations on Moncayo mountain, Spain, phloem increment consisted of only early phloem. In Pinus sylvestris from the same two sites, growth ring boundaries were not as clear as in temperate Slovenian sites. In some cases we could identify phloem growth ring boundaries but in others it was very doubtful, which could be explained by collapse of the outermost early phloem sieve cells. In Pinus halepensis from all sites, we could only distinguish between collapsed and non-collapsed phloem, while phloem rings could not be identified. Widths of the youngest phloem and xylem annual increments could only be compared when phloem increments could be clearly defined, as with Picea abies, Fagus sylvatica and Pinus sylvestris from temperate sites. The visibility of the growth ring boundary in phloem was not related to the width of annual radial growth. The correlation between xylem and phloem ring widths was high, but moderate between the number of dormant cambial cells and xylem ring and phloem ring widths. Based on the structure of the youngest phloem increments, we concluded that there is no typical annual periodicity in cambial production of phloem cells in trees from certain Mediterranean sites. This may be due to continuous yearlong cell production and the absence of true cambium dormancy, at least on the phloem side, under mild winter conditions.