Seasonality of fine root dynamics and activity of root and shoot vascular cambium in a Quercus ilex L. forest (Italy)

We investigated the effects of seasonal changes in soil moisture and temperature on the morphological growth traits of fine roots (< 2mm in diameter) and vascular cambium activity of stems and coarse roots in a mature Quercus ilex L. stand in the South of Italy. Fine roots were sampled by a soil core method, and cambium tissues were carefully collected by hammer and chisel. Mean annual fine root mass and length were 115 gm−2 (live 45 gm−2; dead 70 gm−2) and 471mm−2 (live 244mm−2, dead 227mm−2), respectively. Mean diameter size of fine root necromass was higher than for fine root biomass. Mean specific root length (SRL) was 6.8mg−1 and turnover rate was 3 year−1. Fine root traits displayed a complex pattern related to season. In particular, biomass and length peaked in summer and late autumn. The summer maximum was characterized by an increase of the thinner part of the root population (smallest diameter size and highest SRL) and was mainly driven by soil temperature. Our results suggest that Q. ilex adopted an intensive strategy modifying the root length per unit mass, channelling carbon preferentially into the production of very fine roots. This allowed trees to exploit transient periods of low soil water content by accessing a greater soil volume and thereby facilitating nutrient and water uptake. The autumn maximum was characterized by an increase in mean diameter size of the fine root population (largest mean diameter size and lowest SRL). Thus, once precipitation sufficiently recharged soil moisture, it is reasonable to state that in addition to trees producing new roots, their percentage of very fine roots that did not die after the summer flush continued their growth in a radial pattern to function for starch storage. Shoot and root cambial activity strongly varied during the season from the winter minimum (4.8 shoot and 4.7 root cambial cell number) to three- and two-fold higher values measured during the summer maximum, and higher values but of lesser magnitude measured in autumn. Periods of cambial activity significantly matched fine root phenology. Matching these growth traits with soil temperature and water content within a natural stand of Q. ilex enables this species to survive the typically dry summer in the Mediterranean area, which is likely to become drier and longer given the increase in temperature expected this century.