Seasonal changes in the width and structure of non-collapsed phloem affect the assessment of its potential conducting efficiency

Quantitative phloem anatomy is increasingly used in dendroecological studies since recent evidence shows that phloem traits are a valuable indicator of the morphological and physiological strategies of tree performance in different environments. To better understand intra-annual variations in non-collapsed phloem (NCPH) width and structure in three temperate species (Picea abies, Fagus sylvatica, and Quercus petraea), we investigated phloem samples repeatedly taken during the growing season, using light microscopy. The diameter and area of sieve elements were measured in early and late phloem to estimate their conducting potential. The seasonal dynamics of phloem formation and the collapse of sieve elements significantly affect the structure and width of the NCPH. In combination with sieve element characteristics, they determine the conducting potential of the NCPH, which is not constant throughout the growing season. Although the three species differed in terms of wood porosity, the seasonal structural variations of the NCPH followed a similar pattern. At the onset of the growing season, the phloem increment of the previous year was crucial for the function and accounted for over 80% of the NCPH, whereas at the end of the growing season, the phloem increment of the current year constituted the majority of the NCPH. These results indicate that sampling time should be considered when comparing quantitative phloem anatomy data from different laboratories, indicating the need for a uniform sampling protocol and methodology. Otherwise, the assessment of the conducting potential of phloem sieve elements is difficult to compare.