Microclimatic conditions mediate the effect of deadwood and forest characteristics on a threatened beetle species, Tragosoma depsarium

Abstract

While climate change has increased the interest in the influence of microclimate on many organisms, species inhabiting deadwood have rarely been studied. Here, we explore how characteristics of forest stands and deadwood affect microclimate inside deadwood, and analyse how this affects wood-living organisms, exemplified by the red-listed beetle Tragosoma depsarium. Deadwood and forest variables explained much of the variation in temperature, but less of the variation in moisture within deadwood. Several variables known to influence habitat quality for deadwood-dependent species were found to correlate with microclimate. Standing deadwood and an open canopy generates warmer conditions in comparison to downed logs and a closed canopy, and shaded, downed and large-diameter wood have higher moisture and more stable daily temperatures than sun-exposed, standing, and small-diameter wood. T. depsarium occupancy and abundance increased with colder and more stable winter temperatures, and with higher spring temperatures. Consistently, the species occurred more frequently in deadwood items with characteristics associated with these conditions, i.e. downed large-diameter logs occurring in open conditions. Conclusively, microclimatic conditions were found to be important for a deadwood-dependent insect, and related to characteristics of both forest stands and deadwood items. Since microclimate is also affected by macroclimatic conditions, we expect species’ habitat requirements to vary locally and regionally, and to change due to climate warming. Although many saproxylic species preferring sun-exposed conditions would benefit from a warmer climate per se, changes in species interactions and land use may still result in negative net effects of climate warming.

Microclimate in hollow trees and how it affects an inhabiting beetle species,Osmoderma eremita

“…Thus, our study together with some others (Paclik & Weidinger, 2007;Sedgeley, 2001;Wiebe, 2001) provide evidence that tree diameter is an important governor of the internal thermal conditions of hollows, but there are also studies that have found no relationship between tree diameter and microclimate (Clement & Castleberry, 2013;Coombs et al, 2010;Rhodes et al, 2009). That a larger diameter buffers temperature fluctuations has also been found for standing and downed deadwood (Lindman et al, 2022), probably because wood buffers the changes in temperature and moisture (Walczy nska & Kapusta, 2017). Many beetles that specialise on hollow trees occur more frequently in trees with larger diameters (Ranius & Jansson, 2000).…”

“…O. eremita occurred more frequently in hollows with a warm and stable microclimate, that is, where the average temperature was higher in summer, and the temperature fluctuations in summer and moisture variation were lower. High temperatures and narrow temperature fluctuations have also been found to be important for other insects, for example, a moth (Moore et al, 2021), and a beetle: (Lindman et al, 2022) but we have not seen this documented for any insect species associated with tree hollows.…”

“…Chiari et al, 2012). O. eremita is still confined to living hollow trees all over its distribution range, which may be due to the fact that these hollows consistently have a more stable microclimate compared with both the ambient microclimate and the microclimate in downed and standing deadwood (Lindman et al, 2022).…”

“…The explanation may be that during the warmer part of the year, most of the solar energy is absorbed within the top canopy, so the temperature is higher at the height of the tree crown compared to further down the tree trunk (De Frenne et al, 2021). In turn, during the colder part of the year, after the seasonal reduction in canopy cover, the stronger wind speed higher up (De Frenne et al, 2021) cools the microclimate when solar radiation has little warming effect on it (Lindman et al, 2022).…”

“…While the microclimatic conditions in tree hollows have been shown to be important for nesting birds and mammals (e.g., Maziarz & Wesołowski, 2013;Sedgeley, 2001;Wiebe, 2001), this has been poorly studied for invertebrates (see, however, Gouix et al (2012) and Chiari et al (2012), who both related a qualitative categorisation of moisture of hollow trees to the occurrence of certain beetle species). For beetles associated with other types of deadwood, only recently have microclimatic variables and species' data been analysed together (Lettenmaier et al, 2022;Lindman et al, 2022;Müller et al, 2020).…”