Forest management changes microclimate and bryophyte communities in the Cascade Mountains of western Washington

Background/Question/Methods Overstory trees moderate understory microclimate through direct and indirect effects of shading.  Canopy shading reduces exposure to solar radiation, extremes in temperature, and changes in substrate quality, factors that may be critical to the persistence of forest-dependent species such as bryophytes (mosses and liverworts).  We examined effects of increasing levels of forest canopy removal on understory microclimate and bryophyte community structure in an overstory retention experiment in mature coniferous forests of the Cascade Mountains in western Washington.  We sampled understory microclimate (solar radiation and air temperature) and bryophyte abundance and diversity in three harvest treatments representing progressively lower levels of retention (100, 40, and 15%  of original basal area) in each of three experimental blocks, 7-8 years after treatment.   We tested (a) whether changes in bryophyte cover and richness were correlated with level of retention, (b) whether bryophyte responses to retention differed among substrates (forest floor, decayed logs, and tree trunks), and (c) whether these patterns were consistent with microclimatic changes.

Results/Conclusions Bryophyte cover and richness declined with decreasing retention, consistent with increases in solar radiation and air temperature.  Declines were large on decayed logs and on south-facing (vs. north-facing) sides of retained trees, but small on the forest floor.  Our results indicate that canopy removal can have negative effects on forest bryophyte communities—particularly those associated with woody substrates.  Declines may be driven by changes in microclimate or indirectly, by changes in substrate quality.  Understanding the effects of timber harvest on organisms sensitive to changes in microclimate are critical in managed forests; these effects are likely to be exacerbated by climate warming.