Snag longevity and availability for cavity-nesters in post-harvest landscapes in western Newfoundland

Smith, Caryn Y. (2008) Snag longevity and availability for cavity-nesters in post-harvest landscapes in western Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Standing dead trees (hereafter snags) are a valuable component of forest systems, comprising habitat for wildlife, nursery sites for forest regeneration, and as stores of biomass, nutrients, and carbon. I examined populations of snags present following harvest in a chronosequence of balsam fir (Abies balsamea) dominated boreal forests in western Newfoundland to assess: (1) snag density, longevity and biomass in a post-harvest landscape, and (2) the availability and quality of snags for cavity-nesting birds and other wildlife. Average snag longevity was indicated by a sharp decline in snag numbers 10 to 15 years after harvest, followed by low snag densities until the regenerating forest reached senescence. A smaller but more immediate snag density decline was observed 1 to 4 years after harvest when residual snags and live trees were prone to windfall and domestic harvesting. Snag longevity was positively correlated with stem diameter at breast height (dbh). A large portion (55%) of 1260 snags encountered were balsam fir which have a small average dbh. These small-diameter snags were short-lived, contributing low biomass and a long period of low snag abundance. White birch (Betula papyrifera) produced the largest diameter and longest standing snags. -- Cavity nesters used snag species based on availability with 41% of the 81 cavities identified being in balsam fir. Cavity presence was most strongly positively correlated to snag diameter at breast height (dbh), followed by decay class, time since harvest and height. Less than 40% of snags available throughout the chronosequence had large enough dbh for cavity nesters. Downy Woodpeckers (Picoides pubescens) were responsible for 47% of all cavities identified, excavating balsam fir ~50% of the time. Two larger cavity nesters present, Northern Flickers (Colaptes auratus) and Three-toed Woodpeckers (P. tridactylus), were preferentially using large-diameter white birch snags. Northern Flickers excavated the largest cavities, potentially providing habitat for the greatest variety of secondary cavity-nesters. Managing for snags with >30 cm dbh, which flickers target for excavation, has the greatest potential to enhance the broader snag cavity-based community in western Newfoundland.

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