Log moisture and moss growth under thinned and unthinned forest canopies

Sean R. Haughian and Katherine A. Frego

Water availability is important for mosses, because they do not have roots; they simply absorb liquid water (directly through their leaves) from rainfall, dew, and surface flow. Many different species of moss seem to prefer living on rotting logs, and scientists have often attributed this tendency to the logs acting as moisture reservoirs: the logs absorb water when it is raining, and release water when it is not raining, thereby ensuring that the mosses on the surface stay moist for much of the time. However, nobody has ever tested whether rotting logs actually supply water to the mosses that grow upon them.

We tested how well a log-dwelling species of moss grows on a series of artificial logs in which we had changed the volume of water they could hold, and compared them to natural rotting logs. We expected the artificial logs with the largest water capacity would produce the most moss growth. This test was done in a forest in Atlantic Canada, in two forest types: a dense spruce forest and a more open spruce forest. We also measured the surface humidity of these logs before and after watering, to see if water capacity was related to surface humidity.

We found that the mosses grew best on natural logs, in the more open forest. Among artificial logs, we found that mosses grew better on logs with small water capacities than those with large capacities. This was despite the fact that logs with a larger water capacity also had a more humid surface.

We concluded that logs probably do not supply much moisture to the surface, at least not enough to support moss growth. Instead, we think that precipitation is the most important source of moisture for log-dwelling mosses. Nevertheless, the fact remains that many moss species prefer to live on rotting wood; we propose that rotting wood may actually help mosses remain moist by allowing small puddles to form on its surface. Beyond that, other aspects of rotting wood, such as nutrient content or longevity, may explain this pattern, but require further tests.

Image caption: Image provided by authors.
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