Climate change and Mediterranean ecosystems: plant and soil responses to UV radiation and water availability before and after a perturbation
- Autores: Laura Diaz Guerra
- Directores de la Tesis: Laura Llorens Guasch (dir. tes.), Dolors Verdaguer Murlà (dir. tes.)
- Lectura: En la Universitat de Girona ( España ) en 2017
- Idioma: español
- Tribunal Calificador de la Tesis: Salvador Nogués Mestres (presid.), Anna M. Romaní (secret.), Isabel Nogues Gonzalez (voc.)
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
In the Mediterranean basin, despite the current recovery of the ozone layer, reduction in cloud cover as a consequence of climate change is expected to enhance solar UV levels (UV-A and UV-B) decreasing precipitation amount over the coming years, especially during summer months. At the same time, drier and warmer conditions would favor the occurrence of more frequent and intense wildfires. At plant level, the potential effects of higher UV doses and diminished water availability on the biochemical, morphological, physiological and growth traits could alter the regeneration ability after a disturbance of Mediterranean resprouter species. Ultimately, variations in resprouting success among plant species in response to UV and water levels could have implications on the resilience of shrubland ecosystems, one of the most extensive Mediterranean-type terrestrial community in Europe. In addition, the role of UV radiation in the functioning of Mediterranean shrublands, particularly its effects on C and N pools in plants and soils, remains still unclear, with null information about its possible interactive effects with precipitation amounts and/or fire. The present study aims to contribute to the elucidation of the effects that projected changes in UV radiation and precipitation could have on Mediterranean resprouter species and shrubland ecosystems in the near future, taking into account the possible impact of the aerial plant biomass removal due to a disturbance (such as a fire) on modulating these effects.
In this context, the specific objectives pursued in this thesis were: to examine the effects of enhanced UV radiation (UV-A and UV-B) and diminished water supply, before and after a disturbance implying aerial biomass removal, on i) leaf content of phenols and antioxidant components, and ii) leaf morphology and physiology, plant growth, root reserves and, finally, resprouting capacity of two common Mediterranean resprouter species; and iii) to investigate the role of UV radiation (UV-A and UV-B) on C and N pools of the soil and dominant plant species of a Mediterranean shrubland before and after a fire, and whether these UV effects can be altered by the expected reduction in precipitation. To achieve objectives 1 and 2, an outdoor experiment involving UV supplementation combined with two levels of irrigation was conducted using seedlings of Arbutus unedo and Quercus suber, two sclerophyllous resprouter species, whose aerial biomass was removed during the study period. Several parameters associated to leaf biochemistry, morphology and physiology, plant biomass, root biochemistry and resprouting success were measured in seedlings and resprouts. To accomplish objective 3, a field experiment was conducted in a natural Mediterranean shrub community, where the levels of UV and water availability reaching the ecosystem were modified, performing an experimental fire around the middle of the study period. In addition to plant and litter cover, different parameters related to C and N pools were measured in the soil, and in the litter and leaves of A. unedo and Phillyrea angustifolia, two resprouter species largely dominant in the shrubland community.
As main results, responses of A. unedo plants to enhanced UV, especially UV-B, were mainly addressed to the reinforcement of the antioxidant machinery through increases in the levels of quercetins, ascorbate and glutathione coupled with an upregulation of catalase (CAT) activities. In contrast, Q. suber augmented UV-screening to face UV-B enhancement, avoiding UV-B penetration into leaf tissues via accumulation of kaempferols. This species was also sensible to enhanced UV-A, but, in this case, the response implied morphological adjustments to harden the leaves (smaller leaves with greater leaf mass per area (LMA)) coupled with a proportionally greater carbon allocation to roots (lower leaf to root biomass ratio). Concerning water supply, A. unedo appears to counteract water constraints by a stimulated antioxidant activity, as in response to enhanced UV-B, while the main response in Q. suber was a reduction in biomass allocation to leaves in relation to roots and to total biomass, likely as a strategy to minimize water loss by transpiration and increase water absorption by roots. The protective strategies displayed by both species were apparently efficient, since no reductions in root reserves or in resprouting success were found. However, the sensitivity of resprouts to UV radiation or water levels was notably different in relation to that of seedlings, being higher in A. unedo but lower in Q. suber resprouts. At ecosystem level, the effects of UV-A and UV-B exposure on C and N pools were contrasted and often exacerbated by reduced levels of precipitation. During the pre-fire period, UV-A exposure increased soil moisture, respiration and β-glucosidase activity, suggesting increased soil biological activity and C turn-over. The additional presence of UV-B decreased soil respiration, pH and β-glucosidase activity, and increased C concentration in P. angustifolia leaves, pointing to a lower soil microorganism activity probably slowing down C and N cycling. Under reduced rainfall, UV-B exposure also enhanced δ15N values in leaves and litter of A. unedo, suggesting higher nitrification and N losses by leaching in the soil. Most of the effects were only found before the fire, indicating a homogenizing influence of this perturbation.
To sum up, over the coming decades, changes induced by enhanced UV doses and diminished precipitation levels on C and N dynamics in Mediterranean shrublands would be in part driven by opposite effects of UV-A and UV-B on soil processes, modulated by low water availability in combination with plant species-specific responses to these factors, which, depending on the species, could be emphasized or mitigated after a disturbance (fire, clear-cut, grazing) occurs.
