The effects at ecosystem level of recurrent environmental stresses in Mediterranean Climate regions

The Mediterranean regions are a climate hotspot where the effects of anthropogenic climate change are expected to appear more rapidly and impact the areas more harshly compared with other regions. These regions are particularly affected by increasing temperatures and decreasing precipitations. Summer heatwaves, combined with severe drought spells and high solar irradiance, have become more frequent because of climate change. These events are threatening both the diversity and the survival of Mediterranean forests, causing a loss in biodiversity, through a modification in the composition of native plant communities and facilitating biotic invasion of exotic species, which are more competitive in these new habitats. Furthermore, drought stress, high temperature stress and the intraspecific competition between plants could lead to a variation in Biogenic Volatile Organic Compound (BVOCs) emissions, secondary metabolites that play different roles in plants. Firstly, these compounds have a key antioxidant function (quenching Reactive Oxygen Species), thus improving ozone and drought tolerance, while also acting as pollinator attractors and repellents for dangerous herbivorous insects (contributing to the taste and odour of different plant tissues). In addition, these compounds could modify the ozone and the Secondary Organic Aerosols (SOAs) contents, playing an important role in the air quality. Lastly, some of these compounds, in particular some terpenes, may also provide multiple healthpromoting benefits for humans, thanks to their use in different types of industries, such as pharmaceutical, nutraceutical, and cosmetic industries. In this work the first aim was to develop a fast and easy-to-handle analytical methodology to sample BVOCs using solidphase microextraction (SPME) fibres at the canopy level. An improvement of BVOCs adsorption from SPME fibres was obtained by coupling the fibres with fans to create a dynamic sampling system (DBSS) and the results obtained showed high efficiency and sensitivity of SPME fibres, reducing sampling time. Afterwards, these DBSS devices were used for three years to seasonally monitor the changes in BVOC emission of a Quercus ilex (holm oak) coastal forest in two stands characterized by two levels of tree mortality estimated by crown defoliation assessment. In addition, another study was conducted in south-eastern Australia and focused on the invasiveness of an aggressive invasive native species: Pittosporum undulatum. Our results, in both studies, showed the efficiency of this new device. In addition, in the study regarding the high mortality of holm oak in Tuscany the results suggested that terpene emissions from Mediterranean forests would be modified by an increase in Q. ilex dieback, with important consequences for the atmospheric chemistry and functioning of this forest ecosystem. In the study, regarding the alterations on composition of Eucalypt forests due to the invader Pittosporum undulatum, our results suggested that the invasiveness of P. undulatum might be linked to the biosynthesis of compounds that play a protective role against abiotic stresses. In conclusion, the DBSS technique shows great potential applications in several conditions and studies, both in natural environments (such as those presented in this thesis), as well as in urban environments, to monitor air quality and plant emissions. However, it is necessary to mention that the next step in this field of research would be to develop a reliable quantification procedure, required to further improve the DBSS sampling strategy, which will then provide quantitative data on BVOCs emitted in the environment.