PLANT RESPONSES TO DROUGHT STRESS: HOW TRADITIONAL AND INNOVATIVE METHODS CAN HELP TO MAINTAIN HEALTHY GREEN AREAS WHILE LIMITING WATER CONSUMPTION

The effects of climate changes will be exacerbated in the urban environment which is already characterized by specific microclimatic conditions, such as temperature 3- 5°C higher than the surrounding countryside ("urban heat island”), low-quality and impermeable soils that impose several stresses on plants. Also, the multiple interactions between water stress, increased temperature (diurnal and nocturnal) and increased atmospheric CO2 is particularly interesting and controversial. Plant tolerance of combined shade and drought, typically considered irresolvable due to trade-offs in morphology and physiology, potentially determines important vegetation patterns, especially as drought spells are becoming more severe worldwide. Existence of inverse correlations between ecological requirements of species involves the ad hoc hypothesis that being tolerant to a certain environmental factor involves a cost such that the plant cannot adjust simultaneously to multiple environmental stresses. Selection of the right species is therefore a primary requirement for successful tree planting. To this regard, pre-conditioning of nursery trees can be a supplementary tool to improve tree performances in the different urban environments. It has been shown that acclimation in the nursery to mild stress resulted in the production of hardened nursery stock, better able to tolerate transplanting and other environmental stress during establishment.