Well-combusted duff (<3 cm depth) is generally considered the best seedbed for small-seeded species on upland sites, but we ask here, What is the optimal, postfire residual duff thickness? We hypothesize that a duff thickness equal to (but not greater than) the length of the germinant will offer the best conditions, because at this thickness, the duff layer will not prohibit radicle penetration into the mineral soil, and yet it will serve as a water-conserving mulch. Data from a recent fire in the Rocky mountains of British Columbia were used to show that for three species of Pinus and Picea, (1) duff depths <3 cm were far more clement substrates than thicker duff, and (2) there was a peak in relative survivorship at about 1–2 cm, somewhat shallower than the typical hypocotyl length for these species. Additional data sets from studies previously conducted at boreal and northern cordilleran sites in Alberta, Saskatchewan, Yukon, and Quebec (a combined 21 fires) bolstered these results.
We studied the density of ascocarps (mushrooms) of morels (Morchella) and pixie cups (Geo-pyxis carbonaria) as a function of postfire duff (forest floor organic layer) depth in the first 4 y after a wildfire. The great majority of ascocarps of both species appeared in the first summer (2004) after an Aug 2003 fire in predominantly pine-spruce montane stands in Kootenay National Park, British Columbia. The spatial distribution of the ascocarps of both species was strongly biased toward (i) microsites with thin postfire duff and (ii) proximity to standing burned tree trunks. The bases of ascocarps of both species invariably were found just below the surface of the mineral soil. A field experiment in nearby intact forest showed that complete or partial duff removal in the absence of damage to the roots or crown did not lead to ascocarps of either species. We conclude that for both fungal species an unusually large abundance of ascocarps simultaneously requires damage to the associated trees and major duff reduction.
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