Fine-root morphological and growth traits in a Turkey-oak stand in relation to seasonal changes in soil moisture in the Southern Apennines, Italy

We investigated the effects of seasonal changes in soil moisture on the morphological and growth traits of fine roots (<2 mm in diameter) in a mature Turkey-oak stand (Quercus cerris L.) in the Southern Apennines of Italy. Root samples (diameter: <0.5, 0.5-1.0, 1.0-1.5, and 1.5-2.0 mm) were collected with the Auger method. Mean annual fine-root mass and length on site was 443 g m(-2) (oak fine roots 321 g m(-2); other species 122 g m(-2)) and 3.18 km m(-2) (oak fine roots 1.14 km m(-2); other species 2.04 km m(-2)), respectively. Mean specific root length was 8.3 m g(-1). All fine-root traits displayed a complex pattern that was significantly related to season. In the four diameter classes, both fine-root biomass and length peaked in summer when soil water content was the lowest and air temperature the highest of the season. Moreover, both fine-root biomass and length were inversely related with soil moisture (p<0.001). The finest roots (<0.5 mm in diameter) constituted an important fraction of total fine-root length (79 %), but only 21 % of biomass. Only in this root class, consequent to change in mean diameter, specific root length peaked when soil water content was lowest showing an inverse relationship (p<0.001). Furthermore, fine-root production and turnover decreased with increasing root diameter. These results suggest that changes in root length per unit mass, and pulses in root growth to exploit transient periods of low soil water content may enable trees to increase nutrient and water uptake under seasonal drought conditions.