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Model of the range expansion of the pine processionary moth (thaumetopoea pityocampa) in complex alpine terrain

The winter pine processionary moth, Thaumetopoea pityocampa, has a geographic range in southern Europe, northern Africa and Middle East but in the last three decades a substantial expansion of the outbreak area has taken place, both northward and upward in elevation. Modelling and mapping of the range of PPM using GIS is therefore a useful tool for estimating the actual and future impact of the range of PPM on forest management and their socio-economic implications. The influence of weather conditions on nest temperatures and on the feeding activity of the nocturnally active larvae was studied in laboratory and outdoor experiments. Feeding activity was estimated under different temperature regimes. Activity was induced only when day-temperatures were above 9°C and night-temperatures above 0°C. Lower lethal temperatures were estimated by freezing experiments and super-cooling point measurements. For a precise estimation of the relation between nest temperature, air temperature and solar radiation, garden settings for simultaneous measurements were done. Minimum temperatures of the nest correspond well with air temperature minimum; nest maximum can be predicted by air temperature maximum and daily sum of solar irradiation. Climatic conditions, actual position of extreme nests and survival of larvae/colonies transplanted outside the actual range were observed in two alpine valleys (Durance Valley; Venosta Valley). Specific alpine conditions for the occurrence of PPM were studied on a local scale model at Venosta Valley, based on fine scale field surveys (air and nest temperatures, solar radiation) on north and south exposed slopes. Differences in feeding activity between the northern and southern slope can mainly be explained by low potential radiation due to shading of the northern slope during midwinter. Regional models were developed to evaluate the potential range of PPM for the alpine regions Trentino-Alto Adige-Tyrol (Italy-Austria) and Durance Valley (France) using modelling procedures similar to those applied on local scale, but with data from meteorological recording stations. Due to limited availability of solar irradiation data, a sub-model for estimation of global solar irradiation was developed using MeteoSat 7 image-analysis and potential solar irradiation derived from a digital elevation model (DEM). In order to reduce model errors caused by macroclimatic effects and climate zones, the modelling area was divided into adjacent sub-regions.

Daily multiple regressions were calculated for modelling daily air temperature maxima and minima. Validation of the estimated bioclimatic variables was done with independent sampled data (translocation experiments in 2002/03 and 2003/04). Instead of sharp temperature thresholds of nest temperature for feeding induction (>9°C) and night feeding (>0°C), fuzzy functions were used to calculate the relative potential feeding activity during the winter period. The spatial distribution of the relative potential feeding activity was compared with the geographic range of PPM in Southern Tyrol. The class limit for "very likely occurrence" of PPM was defined as the minimum of the relative potential feeding activity for locations at the edge of the range during the "normal" winter 2003/2004. The climatic conditions for feeding activity would also permit the occurrence of PPM in the northern alpine range, but compared with the Southern Alps the higher frequency of extreme frost events are limiting the occurrence of PPM. Concerning French Alps, the actual range of PPM is for some areas far behind the limit of potential occurrence. Scenarios for climate warming and range expansion at Venosta Valley are based on the climatic conditions during the winter 2003/2004 (deviation of the mean winter temperature from the long-term mean winter at Schlanders: +0.27°C). The scenarios of climate change predict that every increase of 1°C in winter minimum could induce an altitudinal range expansion of ~200 m. A deviation of only +1°C dramatically increases the proportion of infested black pine stands at Venosta Valley. Almost all conifer stands in this area could be affected by PPM-infestations under +4°C-conditions.

The model is available at
with an user-friendly approach and the possibility to test various climate change scenarios on the range expansion of the species. We think that forest managers, land planning staff and the general public of the Alpine area will be interested in the model. We plan also to extend the model to other geographic areas beside the South Tyrol and the Briancon district. For this, we are willing to start collaboration with other institutes and services.

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