ORIGINAL PAPER Thomas Holst Æ Helmut Mayer Æ Dirk Schindler Microclimate within beech stands—part II: thermal conditions Received: 3 December 2003 / Accepted: 25 February 2004 / Published online: 22 April 2004 Ó Springer-Verlag 2004 Abstract Within the framework of an interdisciplinary project on the effects of climate and forest management on beech-dominated deciduous forests (Swabian Jura, south-western Germany), forest meteorological investi- gations are carried out to analyse the influences of exposure and canopy density on the microclimate within various beech stands (Fagus sylvatica L.). This second paper of the series ‘‘Microclimate within beech stands’’ focuses on the thermal conditions that exist mainly in the near-surface layer at different test plots. They were analysed by air temperature, T a , (at 1.5 m a.g.l) and soil temperature, T s , at six depths measured continuously in the period 1999–2003. The main results can be summa- rized as follows. (1) The thermal situation within the beech stands described by T a depended primarily on the turbulent air-mass exchange conditions which were based on the slope-specific energy balance of the forest floor and advective heat fluxes. (2) The thermal situation within the soil described by T s at 3 and 20 cm depth was governed by the molecular heat transport. Therefore, the heating and cooling rates of T s were always lower than for T a . Higher T a and T s values for the test plots on the SW slope showed that the thermal conditions within the beech stands depended primarily on the exposure. (3) Based on slope-specific differences of daily extremes of T a and T s at 3 cm depth between the silviculturally treated and control plots, the influence of the more pronounced height growth of the understorey vegetation under the near-surface thermal conditions could be clearly verified for the NE slope. Keywords Fagus sylvatica L. Æ Air temperature Æ Soil temperature Æ Exposure Æ Canopy density Æ South- western Germany Abbreviations PAR: Photosynthetically active radiation Æ PAI: Plant area index Æ LAI: Leaf area index Introduction The climatic conditions of a forest site are determined by its radiation, heat and water balance as well as by the balance of nutritional compounds. The main influence is governed by the synoptic situation and its long-term development. In complex terrain, the main quantities of the balance equations are additionally influenced by exposure and elevation, which leads to a specific climate at single sites. Silvicultural treatment, such as thinning, modifies the climatic conditions, and stand-specific microclimates arise below the canopy (e.g. Geiger 1961; Lee 1978; Aussenac 2000). Due to their importance for different forest processes and structures (e.g. stand regeneration, development of understorey vegetation, bio-geochemical processes), the microclimatic conditions below the canopy have been analysed already at various sites (e.g. Mayer 1979; Chen et al. 1993, 1999; Chen and Franklin 1997; Carlson and Groot 1997; Friedland et al. 2003), but these experimental investigations rarely exceeded 1 year. It is known that ecological processes in forests react on long-term microclimatic conditions, which, however, can be modified by extreme weather conditions. Hence investigations on the effects of microclimate on stand development and biodiversity need to last significantly longer than one year (More- croft et al. 1998; Grimmond et al. 2000). Beech (Fagus sylvatica L.) is the stand-forming tree species in many natural forest stands throughout cen- tral Europe, and forest administrations have demanded a stronger concentration on beech during the past few years (Hanewinkel 2001; Kenk and Guehne 2001). Nevertheless, there is a lack of systematic studies on the microclimate within beech stands, including the influence of regional climatic change as well as the consequences of forest management and topographic effects. T. Holst (&) Æ H. Mayer Æ D. Schindler Meteorological Institute, University of Freiburg, 79085 Freiburg, Germany E-mail: thomas.holst@meteo.uni-freiburg.de Tel.: +49-761-2036929 Fax: +49-761-2033586 Eur J Forest Res (2004) 123: 13–28 DOI 10.1007/s10342-004-0019-5