STEM XYLEM FEATURES IN 3 QUERCUS (FAGACEAE) SPECIES ALONG A CLIMATIC GRADIENT IN NE SPAIN

Stem xylem features in two evergreen Quercus species (Q. coccifera and Q. ilex) and a deciduous one (Q. faginea) were analysed along an Atla ntic-Mediterranean climatic gradient in which rainfall and winter cold experience strong variation. Mean maximum vessel diameter, vessel den sity, vessel element length, xylem transverse sectional area, Huber va lue (xylem transverse sectional area per leaf area unit), theoretical leaf specific conductivity (estimated hydraulic conductance per leaf a rea unit) and total leaf area were determined in 3-year-old branches. Q. faginea presented the widest vessels and the highest theoretical le af specific conductivity while Q. coccifera showed the lowest total le af area and the highest Huber value. Studied features did not exhibit significant correlations with mean minimum January temperature in any species but did show significant relationships with rainfall. In Q. co ccifera, mean maximum vessel diameter, vessel element length and theor etical leaf specific conductivity increased with higher rainfall while vessel density decreased, Mean maximum vessel diameter and total leaf area in Q. ilex increased with precipitation whereas variables of Q. faginea did not show any significant trend. Results suggest that aridi ty, rather than minimum winter temperature, controls stem xylem respon ses in the studied evergreen species, Q. faginea traits did not show a ny response to precipitation, probably because this species develops d eep roots, which in turn makes edaphic and topographic actors more imp ortant in the control of soil water availability. In response to aridi ty Q. coccifera only exhibits adjustment at a xylem level by reducing its water transport capacity through a reduction of vessel diameter wi thout changing the amount of xylem tissue or foliage, whereas Q. ilex adjusts its water transport capacity in parallel to the foliage area.