Responses of forest insects to climate change. Herbivory and plant quality along. European elevational gradients

This thesis describes the effect of elevation on the abundance of forest insects along steep mountain slopes, to be used as a spatial analogue of climate change. The elevational gradients in different places in Europe were used as a proxy for the temperature change, using the lapse rate of 0.6 °C every 100 m. Elevational gradients provide an optimal experimental setting to test the relationship between temperature variation and herbivory. Direct and indirect (through the host plant and by the natural enemies) effects of the temperature were checked. The tree quality was measured by leaf toughness, leaf size, C/N ratio, tree diameter and height and, for cone and seed insects, by the length of cones. Insect herbivores were taken into account because they are amongst the most important primary consumers of forest ecosystem, even when they are studied at endemic density as in the present study. The response to the changing temperature was checked for insects guilds (leaf sap feeder, chewers, gall makers, miners) in the Chapter 2, for selected insect species like pine processionary moth (Thaumetopoea pityocampa) in Chapter 3, and also for cone and seed insects of Norway spruce (Cydia strobilella, Dioryctria abietella, Ernobius abietis, Strobilomyia anthracina) in Chapter 4. We concentrated on four main European tree species (Fagus sylvatica, Larix decidua, Picea abies, Pinus sylvestris) because they are the most important trees regarding their abundance and economy value. In the Chapter 1 we used a large data set where we measured leaf herbivory and leaf traits along steep elevational gradients in different biogeographical regions of Europe. We found either neutral or positive effect of increasing temperatures on herbivory while no clear effect of biotic interactions emerged. Among guilds, chewers and leaf miners responded more often than sap feeders and gall makers to temperature. The response to temperature of different host species varied strongly, with conifers more sensitive than broadleaves (Fagus sylvatica). There was a clear elevational pattern of toughness and leaf size (except for Larix decidua) but leaf traits were almost never related with changes in herbivory. There was not interaction between the herbivory guilds except for the miners and chewers of Larix decidua. This community approach (like dividing the insect species into the guilds) is, however, crucial to help understanding the ecosystem level response of insect herbivory to temperature. In the Chapter 3 the abundance of the nests of pine processionary moth (Thaumetopoea pityocampa) along two elevational gradients in southern Spain (Sierra Nevada mountains) was considered. The number of nests increased with elevation, which is opposite to what was expected. Possible explanatory factors were searched in stand conditions (tree density, host percentage), tree quality (tree height, needle toughness, C/N ratio, needle length), egg parasitoids, and possible competitors (leaf sap feeders and chewers). Winter temperature did not explain moth abundance, which was in turn explained by tree height, leaf C/N, but mainly by a higher realized fecundity at high elevation, likely linked to better growth conditions, and by higher egg parasitism at lower elevation. In Chapter 4 the colonization of Norway spruce mature cones by cone and seed insects was analyzed along four alpine elevational gradients (three in Italy, one in France). Specialised insects feeding on seeds (Cydia strobilella and Strobilomyia antracina) did not respond to temperature whereas generalits species feeding on cone tissues did (Dioryctria abietella and Ernobius abietis). There was also a significant interaction, as the seed feeder Cydia strobilella affected positively the generalists Dioryctria abietella and Ernobius abietis. Generally, all chapters based on the elevational gradient approach provided results useful to better understand the effect of future climate change on insect herbivores, although many of the interactions among elevation, host plant, and insects are still to unveil