Year-to-year variability in xylem and phloem traits of co-existing <i>Fraxinus ornus</i> , <i>Quercus pubescens</i> and <i>Ostrya carpinifolia</i>

The xylem and phloem anatomy of co-existing tree species provides valuable information on how different tree species face climate change and adjust their vascular structure to local weather conditions. We examined and compared annual ring widths and conduit size in earlywood and early phloem in Fraxinus ornus, Quercus pubescens and Ostrya carpinifolia in a sub-Mediterranean site during the period 2019-2021. The selected xylem and phloem traits were correlated with monthly weather conditions (precipitation and temperature). We found that phloem increment widths and conduits in earlywood and early phloem in the studied tree species showed different trends in terms of interannual variability and in relation to local weather conditions. In F. ornus, May conditions affected xylem traits, while June conditions phloem traits. In Q. pubescens, winter and March precipitation was related to phloem development. In O. carpinifolia, xylem ring width was positively correlated with June precipitation, while early phloem conduits were negatively affected by April temperature. Only two consistent patterns were detected across the species and years studied: wider xylem increments compared with phloem increments, and wider earlywood vessels compared with early phloem sieve tubes. Statistically significant differences were observed among species across all years for the size of xylem and phloem conduits and the hydraulic conductivity of earlywood vessels, which indicates great differences in the calculated hydraulic conductivity among the tree species. To summarize, hydraulic conductivity of earlywood vessels in Q. pubescens was on average for all 3 years 10.4-times and 114-times larger than in F. ornus and O. carpinifolia, respectively. High interannual variability and species-specific sensitivity of xylem and phloem traits to precipitation and temperature confirm high plasticity and different radial growth strategies of the studied tree species to ensure optimal functioning under local weather conditions.