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

1. 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. 2.
Three types of synthetic logs, identical in size and shape but differing
in moisture capacity, and two natural substrates, well-decayed birch and
Thuja 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. 3. 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. CWD
facilitates a sufficiently humid surface for only a short time after
precipitation, presumably when a film of liquid water is maintained near
the surface. 4. For Dicranum 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. 5. 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). CWD may possess microclimate-regulating traits that benefit moss
growth, but it does not appear to act as a moisture capacitor.