Effects of a simulated heat wave on growth and photosynthesis of Quercus ilex L. and Arbutus unedo L. seedlings

Drought-induced mortality of seedlings and young plants is critical for the assessment of forest vulnerability in Mediterranean environments where increasing frequency of extreme events, such as heat waves, are expected. Adult plants of Mediterranean shrub and tree species show very plastic responses to fluctuating climate parameters due to physiological adjustments, as well as to the occurrence of structural hydraulic traits (throughout the whole root-stem-leaf system), which allow adaptation and survival under limiting conditions. However, compared to adult plants, seedlings and young plants are more vulnerable to environmental constraints. Species-specific sensitivity to stressors during the early stages of seedling establishment is expected to affect vegetation dynamics and forest productivity. Within this framework, there is increasing interest in understanding if, how, and to what extent, saplings are able to tolerate prolonged water deficit combined with high temperature regimes. The aim of this study was to evaluate the effect of a simulated heat wave (prolonged water stress combined with high temperatures) on growth and eco-physiological behaviour of one-year-old plants of Quercus ilex and Arbutus unedo. The experiment was conducted under a plastic tunnel where rooted cuttings were transplanted in April and subjected to two irrigation treatments (100 and 50% water reintegration). After 115 days, irrigation was suspended for three weeks (maximum registered temperature), and later restored. Mortality, plant growth (including plant height, morphological parameters, biomass) and eco-physiological traits (e.g. leaf gas exchanges, plant water potentials and photochemistry) were analysed. Although in both species, pre-acclimation to water shortage allowed better surviving capability, Q. ilex and A. unedo showed different adaptation mechanisms. Understanding the behaviour of young tree and shrub species under severe fluctuations in climate parameters furnishes useful information for the management of not only natural ecosystems but also urban green areas.