Log moisture capacity does not predict epixylic bryophyte growth under thinned and unthinned forest canopies

Abstract

Summary Coarse woody debris (CWD) serves as habitat for diverse and rare taxa in forest systems. Because the abundance of mosses appears to be correlated with log size and stage of decay, many have suggested that CWD serves as a moisture reservoir, ensuring a humid microclimate and facilitating moss growth, but no one has tested this connection. Intact forest canopies are also thought to maintain humid conditions that benefit moss growth. If microclimatic regulation is the primary mechanism contributing to high moss abundance on CWD, then epixylic moss growth should increase with the capacity of the log moisture reservoir, and the importance of the reservoir size should increase with canopy opening. Three types of synthetic logs, identical in size and shape but differing in moisture capacity, and two natural substrates, well‐decayed birch and cedar logs, were used to test the effects of log moisture capacity on growth of Dicranum flagellare under thinned and intact canopies of a spruce plantation over 20 months. Surface humidity was positively associated with moisture capacity, but did not reach the water compensation point outside of precipitation events. Under a closed forest canopy, moss growth was negligible across all log types. Under an open forest canopy, moss growth was greater on natural substrates than synthetic ones, and negatively related to moisture capacity. Coarse woody debris facilitates a sufficiently humid surface for only a short time after precipitation, presumably when a film of liquid water is maintained near the surface. For D. flagellare, canopy condition is a more accurate predictor of growth than CWD‐moisture capacity; any beneficial properties of CWD appear to be lessened by a dense forest canopy (as in silvicultural plantations), probably because it reduces access to liquid water from precipitation. We propose that the surface moisture availability of CWD depends on optimal depth of a ‘resisting layer’ below the log surface, representing a tradeoff between water retention and loss (via percolation or runoff at extremely deep vs. shallow resisting layer depth). Coarse woody debris may possess microclimate‐regulating traits that benefit moss growth, but it does not appear to act as a moisture capacitor. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12864/suppinfo is available for this article.

The relationship between epixylic bryophyte communities and microclimate

“…In these conditions, bryophyte communities are enriched with desiccation‐intolerant epixylic species, mainly with liverworts. This pattern confirms the findings of Haughian and Frego (2017a, b) who stated that the water‐holding capacity and microclimate buffering ability of the logs is limited and the occurrence of the sensitive species is mainly determined by microclimate per se. Similar patterns have been demonstrated for fungi (Pouska et al, 2016) and lichens (Marini et al, 2011), where microclimate also can modify successional pathways on otherwise similar logs.…”

“…However, forest site and stand structural characteristics considerably modify the regional climate creating specific below‐canopy microclimates (Kovács et al, 2017). The main drivers significantly influencing bryophyte communities in the forest are topographical characteristics such as elevation, slope and aspect (Holland and Steyn, 1975; Åström et al, 2007; Oldén et al, 2019), the microtopography of the site (Beatty, 1984; Ulanova, 2000), local water regime (Hylander et al, 2002; Stewart and Mallik, 2006; Staniaszek‐Kik et al, 2016) and canopy openness (CO) (Tinya et al, 2009; Haughian and Frego, 2017b).…”

“…A closed canopy buffers extremes of maximum temperature and vapor pressure deficit on the forest floor (Kovács et al, 2017; Davis et al, 2019), creating optimal environments for these sensitive species. On the contrary, some species favors spots under sparser canopy with easier access to rainfall and light (Haughian and Frego, 2017b; Górski et al, 2019).…”

“…In the past, many authors presumed that due to their high water‐holding capacity, the logs themselves strongly modify their surface microclimate by providing more water to the epixylic flora living there (Rambo and Muir, 1998; Botting and DeLong, 2009; Stokland et al, 2012). However recent studies from Haughian and Frego (2017a, b) showed that in logs, water is not likely to be supplied to the surface from their internal parts and there might be other physical and biological factors shaping bryophyte assemblages which have not been described yet.…”

“…The authors mostly define microclimatic conditions based on temperature and air humidity measured on the surface or in the sub‐surface of the logs. These methods have been used to answer various questions involving a diversity of colonizing species of bryophytes and fungi (Király et al, 2013; Dahlberg et al, 2014; Pouska et al, 2016), bryophyte growth (Haughian and Frego, 2017b) or factors determining the occurrence of rare species (Górski et al, 2019).…”