Paper del nínxol climàtic de l'espècie en el sistema insecte-hoste en episodis de sequera

Resumen

In the last decades, extreme weather episodes related to anthropogenic climatic change have enhanced the frequency and magnitude of biotic disturbances, causing worldwide mortality of forests. Particularly, changes in temperature and precipitation regimes are altering the relationships between host tree populations and their attacking insects as insect-host systems are highly sensitive to climate variation. Nevertheless, it is still not largely understood how climate interacts with the different components of the insect-host system and determines its response at different spatiotemporal scales. In this thesis, I aim to address whether the species climatic niche explains the response of coniferous host tree populations to drought and bark beetle disturbances, using niche modeling tools. In Chapter 2, I assessed whether the climatic suitability of both host tree and bark beetle species, and tree and stand attributes, explained tree mortality and stand infestation of Scots pine populations that experienced drought and beetle attack in the northeast of the Iberian Peninsula. At tree-level, mortality was determined by tree size and intensity of beetle attack in which smaller trees were more susceptible to die. At stand-level, infestation was negatively associated with tree density and basal area, and positively associated with tree richness. Notably, host tree populations located in historically suitable climatic areas were more susceptible to infestation, particularly when such locations were also suitable for bark beetle species. In Chapter 3, I analyzed the role of different indices of tree and beetle climatic suitability and forest attributes, in the dynamics of spruce beetle outbreak on Engelmann spruce populations during 14 years at the Southern Rocky Mountains. In this case, I estimated species historical climatic suitability, its variability, and the climatic suitability during the outbreak episode. The outbreak dynamics were analyzed from the cumulative overall response to the year-to-year patterns of outbreak initiation and spread. The results showed that host tree populations living in historically suitable climatic areas were more susceptible to experience an outbreak. In addition, for outbreak initiation the occurrence of suitable climatic conditions for beetle populations was more important than a severe loss of climatic suitability for the host trees during the outbreak episode. Year-to-year patterns revealed that low levels of outbreak initiation and spread were mainly determined by host climatic suitability, while massive amounts of outbreak initiation and spread were determined by beetle climatic suitability and forest attributes. Finally, in Chapter 4, I characterized the species climatic niche of different insect-host systems to assess the resistance of European coniferous forests to bark beetle disturbance. Overall and species-specific forest resistance to bark beetle attack and subsequent tree mortality were analyzed in relation to the distance to species climatic niche optimum, drought conditions, and forest attributes. The results showed that a confluence of predisposing factors determined the overall forest resistance to beetle attack. Monospecific forests close to both host tree and beetle climatic optimums were less resistant to beetle attack, especially when such forests experienced long-lasting moderate droughts. However, factors determining forest resistance to beetle attack varied among insect-host systems, likely due to critical feedbacks driving the dynamics of the attacking beetle species. Contrastingly, once attacked, forest resistance to subsequent tree mortality was driven by the occurrence of drought events of long duration and intensity.