Biodiversity and ecology of understory vegetation in plantation forests of northern New Brunswick, with an emphasis on epixylic bryophytes

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University of New Brunswick
Understory flora contribute much to the biodiversity of forest ecosystems, but can be impoverished by intensive management (e.g. plantation forestry). In particular, epixylic (wood-dwelling) species can respond negatively to reductions in coarse woody debris (CWD), and to the altered mesoclimate of the understory. Modified management practices (to reduce these negative impacts) must be experimentally tested and refined based on an understanding of ecological processes. I compared the responses of understory herbaceous and epixylic communities to four treatments (unthinned or commercially thinned with moderate debris, moderate debris with snags, or no debris) in mid-rotation spruce plantations of northwestern New Brunswick. I also tested whether moisture regulation is the primary mechanism by which epixylic flora associate with CWD, by examining the relationships between substrate properties, CWD surface humidity, and cover of epixylic functional groups, and by growing bryophytes on CWD with modified moisture capacity. Both communities showed increased species richness and cover with thinning. Debris removal primarily benefited early seral herbaceous species, but negatively impacted epixylic liverworts and disturbance-sensitive mosses. Epixylic groups with water-conserving structure were most strongly associated with moisture availability, while others showed only weak relationships. The surface humidity of woody debris and the forest floor did not differ. In the growth experiment, CWD type influenced moss growth under a thinned, but not an unthinned, canopy; CWD moisture capacity was negatively related to moss growth, and natural substrates outperformed synthetic ones. Additional monitoring and experimentation is required to determine whether leaving stands unthinned, or thinning stands with moderate debris (with or without snags) is preferable for conserving biodiversity; however, debris removal appears to be detrimental. The moisture capacity of CWD is not the primary mechanism by which epixylic flora achieve high diversity and abundance; the direct effects of log properties and canopy condition are more important than those mediated through microclimate. By the precautionary principle, a diversity of CWD types and canopy conditions should be maintained in managed stands.