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Seeing the forest for the bryophytes : the effects of forest fragmentation on the bryophyte community in coastal temperate rainforests of British Columbia Baldwin, Lyn Kelly

Abstract

In the coastal temperate rainforests of British Columbia, forest fragmentation from logging has created three different landscape elements that vary in habitat quality for bryophytes: unlogged old-growth remnant forest patches, edge habitats and younger-aged forest stands. Bryophytes are an important component of these forest ecosystems, yet little is known about the impact of habitat fragmentation on bryophyte patterns of richness and abundance. In this thesis I examined the effects of three separate components of fragmentation on patterns of bryophyte species richness, abundance and community composition: (1) effects of proximity to the edge within remaining forest patches, (2) effects of stand age in younger-aged forests adjacent to oldgrowth patches and (3) effects of remnant patch size. To achieve greater generality in my conclusions, I examine each of these issues through comparative analysis using different sampling scales and both taxonomic and ecological functional groupings of bryophytes. Edge effects: Edge habitats (sampled from 0-45 m from the patch edge) showed an increase in the richness of clearing-affiliated bryophytes without a loss of old-growth associated bryophytes. In contrast, interior habitats (sampled primarily >100 m from the forest edge) had a greater abundance of old-growth associated bryophyte functional groups (as measured by finescale sampling and percent cover estimates). Old-growth associated species exhibited no consistent changes with increasing distance up to 45 m from the edge, implying that the influence of the forest edge on these species extends at least this far into unlogged remnants. Effect of stand age: Bryophyte species richness peaked in regeneration stands (stand ages 5-30 years), declined to its lowest levels in immature stands (stand ages 25-49 years) and increased in old-growth stands (stand ages >300 years). In comparison with old-growth, both regenerating clear-cuts and immature forests had a greater richness of clearing affiliated bryophytes. There was no loss of old-growth associated species in regeneration plots; however, old-growth associated species in immature plots showed significant declines in richness which suggests that immature forests represent a propagation bottleneck for bryophytes in harvested landscapes. The pattern of bryophyte species occurrence in younger-aged stands supports an initial floristics model of succession. Effects of patch size: The richness and abundance of dispersal-limited and microclimate sensistive groups showed significant declines as patch size decreased. In contrast, groups expected to increase in richness or abundance as patch size decreased (colonist and open canopy species) showed little association with patch size indicating that center of small remnant patches had no greater richness of clearing affiliated species than large remnant patches. The absence of clearing affiliated species in the center of small remnant patches may well reflect the lack of available microhabitats. The results of this thesis imply that the bryophyte community in the study area has been impacted by human disturbance caused by large-scale forest fragmentation. The bryophyte community in disturbed habitats (i.e., edge and younger-aged stands) showed an increase in the number of clearing affiliated species and a decreased abundance of old-growth associated species. This has important management implications. Although variable retention will introduce greater structural heterogeneity into regenerating forests, it will do little to alleviate the effects of fragmentation on bryophytes. The maintenance of bryophyte diversity in coastal temperate rainforests will depend not only on adequate old-growth reserves but also on longer forestry rotations that will help mitigate the adverse environmental effects associated with introduced edges, younger-aged forests and decreasing patch size.

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