Comparaison des caractéristiques climatiques sous couvert forestier et hors couvert

We compared open-site and below-canopy climatic conditions from 14 different sites in Switzerland based on LWF data (Long-term Forest Ecosystem Research). The 14 sites represent different locations, orientations and elevations, from the Jura Mountains to the southern side of the Alps, composed of deciduous, coniferous and mixed forests. Meteorological measurements were carried out under the canopy at the observation plots, and in open areas outside the forest plots. We analysed air temperature during summer 2003, in connection with the exceptionally high temperatures measured during that summer and during the 11-day August heat wave. From April to October 2003, there was a significant correlation between the absolute value of temperature and the difference between open-site and below-canopy temperature: the warmer the temperature, the stronger the impact of the forest. For maximum temperature, the difference was higher in deciduous and mixed forests, especially those with beech as the dominant tree species, compared to conifer forests. For minimum temperature, the discrepancy was higher in conifer forests but, generally, the north-oriented slopes showed greater differences than the other slopes. The results are similar for a decade period of temperature analysis between below-canopy and open-field, from 1998 to 2007. We also compared below-canopy and open-field data for minimum, maximum and daily mean temperature, relative humidity (RH), maximum and daily mean photosynthetically active radiation (PAR) and wind speed (m/s) from 1998 to 2007. We analysed the forest influence on local summer and winter climate according to the forest type (coniferous, mixed, deciduous), soil type, basal area (m2/ha) and tree height (m). In summer, deciduous and mixed forests generally have the strongest impact on daytime temperatures, from 3 to 5 K cooler under the canopy, with a higher RH, up to 17%. Coniferous forests have a moderating effect on night temperatures, from 1 to 4 K warmer below-canopy, with a lower HR, from 4 to 15% less humid under the canopy. In winter, the forest lowered the day-time temperatures, from 0.2 to 5K, accordingly to the orientation and the dominant specie, and generally increased night-time temperatures, up to 6.2 K in north-oriented conifer sites. The day-time RH was enhanced in all forest types compared to open-field, from 5 to 22%, except in an open Scots pine forest, and the night-time RH was generally reduced below-canopy, especially in coniferous stands, being up to 22% less humid below-canopy in pine forests. We applied the Heat Index (HI) and the NET (Net effective temperature) index to the open-field and below-canopy meteorological data (maximum temperature, relative humidity and wind speed) in order to determine the bioclimatic potential of the different forest stands in summer from 1998 to 2007, and more specifically during the summer 2003 heat wave. We applied the NET index and the Windchill index to determine the forest bioclimatic potential for human wellbeing during winter from 1998 to 2007. In summer, generally, the bioclimatic conditions were clearly more comfortable for humans in the forest than in open-field. When the conditions came out as very hot or requiring extreme caution in open-field, they were in the more comfortable zone in the deciduous and mixed forest at hill and mountain level. This was particularly evident during the exceptionally hot summer of 2003. In winter, bioclimatic conditions were also more comfortable below-canopy, mainly due to the forest effectively reducing wind speed. In order to cope with the likely increasing frequency and intensity of heat waves, our results show and quantify the role of forests in providing a cool shelter. The results are of particular value to urban areas, where forested parks could provide an important source of relief during heatwaves. It is therefore necessary to maintain and develop the accessibility of forested areas to city dwellers. Accordingly to our sample, the most efficient ecosystems for this purpose appear to be generally the deciduous and mixed forests and more specifically the beech and beech–silver fir stands. The opposite effect was seen in mugo pine and Scots pine forests, with warmer temperatures under the canopy compared to open-site.