Does CWD mediate microclimate for epixylic vegetation in boreal forest understories? A test of the moisture-capacitor hypothesis

Coarse woody debris (CWD) is an important microhabitat in forests, particularly for bryophytes and lichens. Many have suggested that this is because CWD provides a uniquely stable and humid surface microclimate, which facilitates epixylic flora, although few have adequately tested this hypothesis. Consequently, it is uncertain whether the apparent relationship between CWD and epixylic vegetation is because CWD regulates moisture, or because moisture is simply correlated with other important CWD traits. This study tested the moisture-capacitor hypothesis of epixylic flora-CWD association by measuring the surface moisture availability directly on CWD under thinned and unthinned forest canopies, and (1) comparing (a) cumulative moisture levels, and (b) variation in moisture levels with ambient and adjacent forest floor microclimates, (2) testing whether associations with log and canopy properties are stronger through direct or indirect pathways, and (3) testing whether desiccation-sensitive functional groups respond more strongly to surface moisture than other groups. Path models were built using canopy cover and log physical traits (log covering, size, acidity, and hardness) as predictors, surface moisture indices as mediators, and epixylic functional group cover as outcome variables. Forest floor and CWD microclimates were equally stable and humid, regardless of canopy cover. Log and canopy properties were more strongly related to epixylic group cover directly than indirectly (through surface moisture). Synthesis: The CWD moisture-capacitor hypothesis is rejected: CWD traits can influence surface moisture availability, but (1) log surface moisture does not differ from the forest floor, and (2) log and canopy properties are more strongly associated with epixylic taxa directly than as mediated through microclimate.