Effect of drought and fire on root development in Quercus pubescens Willd. and Fraxinus ornus L. seedlings

This paper casts light on the events taking place in the root system when seedlings of two deciduous woody Mediterranean species, Quercus pubescens Willd. and Fraxinus ornus L., are stressed by drought or fire or both. Stress treatment mimicked the conditions that often occur in natural Mediterranean ecosystems during dry summers. Allocation of resources to the root system is affected as shown by the fact that taproot biomass decreases under stress conditions only in Fraxinus ornus whereas lateral roots undergo a decrease in length, dry weight, number of apices in both species. An increase in electrolyte leakage from roots of both species during treatments suggests that morphological variations in the root system are associated with damages occurring in root tissue. All the effects observed are reversible if a critical threshold of stress duration is not reached but recovery starts when the aboveground parts are fully replaced. The alteration of a number of root traits suggests that both drought and fire affect root growth and root turnover. Differences in recovery patterns between the two species are observed and attributed to specific tolerances of the root systems. The knowledge of the events taking place at root level might help to understand better the tolerance mechanism to drought or fire characterizing species living in Mediterranean ecosystems.