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The effects of variable-retention harvesting and stand age on fine-root decomposition and fungal communities in coastal temperate rainforests

University of British Columbia

Fine-root litter is the principal source of carbon (C) stored in forest soils and a dominant source of C for fungal decomposers. Differences in decomposer capacity among fungi may be an important determinant of fine-root decomposition. Variable-retention harvesting (VRH) provides refuge for ectomycorrhizal fungi, but its influence on fine-root decomposers is unknown, as are the effects of functional shifts in fungal communities on C cycling. I compared fungal communities decomposing fine-roots (in litterbags) under VRH (aggregate and dispersed retention), clearcut, and uncut stands at two sites (6- and 13-years post-harvest), and two decay stages (43-days and 1 year after burial), in Douglas-fir forests in coastal British Columbia. Fungal species and guilds were identified from fine-roots using high-throughput sequencing. Aggregate-retention maintained fungal communities similar to those found in uncut stands, but only at the 6-year post-harvest site. Ericoid and ectomycorrhizal guilds were not more abundant under VRH, but stand-age treatments (6-, 13-, and 70-years post-harvest) significantly structured species composition. Ectomycorrhizal abundance on decomposing fine roots may partially explain why fine roots typically decompose slower than surface litter. Stand age was a good predictor of vegetation and soil chemistry; grass and forb abundance, and nitrogen availability, were highest in young stands, whereas soil C and N increased in older stands. Despite differences among stand-age treatments, environmental factors were poor predictors of fungal community composition. However, in a 2-year decomposition study, environmental differences among treatments at the 6-year post-harvest site resulted in faster fine-root decomposition rates under clearcut harvesting, whereas 15-19% more C was retained under VRH and in uncut stands. As nitrate availability correlated with decomposition rate, management practices that reduce post-harvest nitrate availability are likely to result in lower C losses from fine-root decomposition. The effect of harvesting was not persistent as fine roots decomposed at similar rates in forests and openings at the 13-year post-harvest site. Altogether, aggregate-retention harvesting preserved fungal communities more commonly found in uncut stands, and slowed fine-root decomposition relative to clearcut harvesting. As such, this is a recommended harvesting practice in coastal British Columbia if fungal conservation and reduced harvesting-related C losses are management objectives.

Text

2018-04-05

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http://hdl.handle.net/2429/65134

Forestry

University of British Columbia

UBCV

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