Efecto de las sequías extremas en especies arbóreas mediterráneas y su modulación por el microclima y la fragmentación
Extreme drought frequency has increased over the last decades in semiarid areas, such as the Mediterranean region. Moreover, water scarcity in summer which historically affects Mediterranean ecosystems is expected to intensify over the next few years due to ongoing climate change. Water stress produced in those ecosystems has favoured coexisting species that have several water regulation strategies as well as ecophysiological mechanisms which allow species to adapt and recover from an increase in the frequency and intensity of disturbances. However, information about the efficacy of those different adaptations and responses to frequent and recurrent droughts is lacking. Besides, the relevance of many factors able to modulate species response to drought; amplifying or attenuating its effects, as well as resilience and resistance processes in the short term, are still unknown. The general objectives of this thesis were: (1) understanding the functional response to drought of coexisting Mediterranean forest species and with contrasting strategies, to cope with extremely dry periods; (2) quantifying the modulation in the response of species to extreme droughts by microclimatic conditions, and (3) assessing the role of forest fragmentation on the ecophysiological response of species to drought. In order to address these objectives, the main ecophysiological variables as well as the timing and intensity of drought were assessed. Moreover, several tracers of the microclimate and habitat fragmentation were considered as possible factors modulating drought responses in Mediterranean forest species. The study was carried out in a Mediterranean forest ecosystem where coexisting tree species have different evolution histories and drought tolerances (Quercus ilex subsp. ballota (Desf) Samp., Quercus faginea Lam. y Pinus nigra J.F. Arnold subsp. salzmannii (Dunal) Franco). Holm oak (Quercus ilex), one of the most representative tree species in the Mediterranean ecosystems, was selected to address the fragmentation effect on drought modulation the selected species was Our results point that the response of Mediterranean forest species to drought events is highly complex, and they differ in their potential ability to cope with extremely dry periods according to species¿ water regulation strategy. Drought-avoiding species such as P. nigra had a certain disadvantage in the carbon balance when events are extreme, which could be accentuated under more frequent extreme events. Microclimate and fragmentation may be significant attenuating factors of extreme droughts and modify the severity of their effects in the study species. Moreover, these factors may facilitate tree recovery after drought events. The timing and intensity of drought, as well as its interaction, were factors that significantly aggravated the impact of water stress on the study species. The timing of the drought was equally or even more important than early drought intensity. Very dry springs or early summers degraded tree functional response, especially in P. nigra. Both Quercus species were more resistant, but not more resilient, in their response to drought than P. nigra. Despite the high drought intensity and obvious symptoms that species were near their tolerance limit, the studied tree species neither declined in growth nor died. This corroborates the high resilience of Mediterranean ecosystems and point out that climatic dryness has not yet reached irreversible levels for species functionality at our study sites. However, there is a real possibility that repeated extreme droughts modify these patterns and initiate the decline and death of individuals of some species, signaling that the limit for irreversible recovery seems to be not so far away. The weakness of Mediterranean forests may cause a long term change in species abundance patterns, as drought-sensitive species are replaced by drought-tolerant species. In turn, species shift may cause changes in water and carbon balance of the whole forest ecosystem.
Tesis Doctoral leída en la Universidad Rey Juan Carlos de Madrid en 2017. Directores de la Tesis: Fernando Valladares Ros e Ismael Aranda García
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