Agricultural and Forest Meteorology 166–167 (2012) 144–155 Contents lists available at SciVerse ScienceDirect Agricultural and Forest Meteorology jou rn al h om epa ge: www.elsevier.com/locate/agrformet Spatio-temporal effects of forest canopy on understory microclimate in a long-term experiment in Switzerland Georg von Arx ∗ , Matthias Dobbertin, Martine Rebetez Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Switzerland a r t i c l e i n f o Article history: Received 2 February 2012 Received in revised form 8 June 2012 Accepted 28 July 2012 Keywords: Air temperature Climate change Forest microclimate Relative humidity Seedling recruitment a b s t r a c t Forest canopy generally moderates below-canopy air temperature and relative humidity and thus creates a specific microclimate for tree seedling growth. Climate change will alter the moderating capacity, which may render the below-canopy conditions unsuitable for recruitment of the hitherto dominant tree species. We assigned long-term meteorological data (1997–2010) recorded inside and outside of 14 different forest ecosystems in Switzerland to three forest types (broadleaved, non-pine conifer, pine), two altitudinal levels (low, high), the four seasons and general weather situations (normal, hot/dry, cold/wet) to compare moderating capacity of each of these classifiers. Our results confirmed a general moderating effect of canopy on below-canopy microclimate with a decrease of daily maximum air temperature of up to 5.1 ◦ C (overall average: 1.8 ◦ C) and an increase of daily minimum relative humidity of up to 12.4% (overall average: 5.1%) in the long-term average, respectively. Broadleaved and non-pine conifer forests moderated daytime microclimate about twice as much as pine forests, while at nighttime considerably less cooling down and even negative effects on levels of relative humidity compared to the open area were recorded at the pine forest sites. Moderating capacity was stronger at low altitude than at high altitude. It was strongest during the growing season, particularly in summer, and depended in a complex way on the general weather situation. Deviations from the general seasonal and weather condition patterns most likely occurred when soil moisture pools were depleted. Despite the moderating capacity, below-canopy microclimate did not lag behind open area microclimate. Based on our results we conclude that natural recruitment in pine forests and high-altitude forests may respond most sensitively to climate change. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Forest canopy creates a specific understory microclimate that differs from the surrounding local climate. This alteration of local climate is the result of a complex interplay of several stand char- acteristics and physiographic settings. Regardless of complexity, most studies about forest microclimate found a general moderating effect of forest canopy on common meteorological parameters (e.g., Grimmond et al., 2000; Mitscherlich, 1981; Renaud et al., 2011). Tree seedlings, which constitute the most vulnerable life stage within the life cycle of a tree (Niinemets, 2010), are adapted to and depend on the specific microclimatic conditions created by forest Abbreviations: T, air temperature; T mean-d , daytime mean air temperature; Tmean-n, nighttime mean air temperature; Tmax, daily maximum air temperature; T min , daily minimum air temperature; RH, relative humidity; RH mean-d , daytime mean rela- tive humidity; RHmean-n, nighttime mean relative humidity; RHmax, daily maximum relative humidity; RH min , daily minimum relative humidity. ∗ Corresponding author at: Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Zuercherstrasse 1, 8903 Birmensdorf, Switzerland. Tel.: +41 44 7392 316; fax: +41 44 7392 215. E-mail address: georg.vonarx@wsl.ch (G. von Arx). canopy (Bertrand et al., 2011; Jemison, 1934). Increasing global temperature and frequency of extreme weather events will affect below-canopy microclimate with the potential to shape future structures of forest ecosystems, particularly where natural regen- eration is common practice (Bertrand et al., 2011; Cunningham et al., 2006; Dulamsuren et al., 2009; Hunziker and Brang, 2005; Kharuk et al., 2009; Raymond et al., 2006). Forest ecosystems with different characteristics may show different sensitivities to these changes (Lindner et al., 2010). Quantitative data about spatio- temporal relationships between below-canopy microclimate and local open-area climate based on long time frames and a range of different forest ecosystems is scarce, yet important to improve models, optimize forest management, and, ultimately, secure future economical and ecological functioning of forest ecosystem. The influence of forest canopy on understory microclimate is directly and indirectly related to the presence of canopy and stems. Branches, leaves and needles reflect and absorb part of the solar radiation during the day, allowing less energy to reach the ground below the canopy (Geiger et al., 2009). The degree of absorption is largely determined by stand structure and species composition (Geiger et al., 2009; Hunziker and Brang, 2005; Mitscherlich, 1981; Renaud et al., 2011). During the night, infrared heat released from 0168-1923/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.agrformet.2012.07.018