Fine root growth of Quercus pubescens seedlings after drought stress and fire disturbance

Post-fire resprouting is an important process in the Mediterranean climate regions of the world and involves considerable rearrangement of biomass allocation. We have investigated the morphological changes occurring in the fine root portion of Quercus pubescens seedlings growing in controlled conditions in which fire disturbance is superimposed on drought-stressed plants. We measured the absolute (length, number of apices) and relative (specific root length and root tissue density) morphometric traits of fine roots, and the biomass and water content of the main plant compartments (leaves, shoot, taproot and lateral fine roots). Initially, soil drying significantly increased the fine root standing mass and decreased the specific fine root length irrespective of the fire, but fine root biomass declined after a critical length of time. Fire significantly decreased the above-ground biomass and its water content notwithstanding the drought stress interruption. On the contrary, time, water supply and fire disturbance factors showed significant interaction effects for the plastic morphological traits, namely, length and number of apices. In fact, fire reduced and postponed the peak of root growth in terms of the thinnest fine root fraction (0.0\u20130.5 mm diameter) and number of apices. These findings indicate the advantages of shedding over maintaining the roots under a condition of severe drought. Indeed, shedding makes the overall reduction of the root system more functional, and induces a partial increase in water particularly in the thicker fraction of the fine roots (0.5\u20132.0 mm). Shoot removal by fire seems to lessen and prolong the acclimation process to drought, but the decrease in non structural carbohydrate reserves appears to impede the recovery process at least after persistent drought