Canadian Forest Service Publications

Short-term effects of mastication on fuel moisture and thermal regime of boreal fuel beds. 2015. Schiks, T.J.; Thompson, D.K.; Wotton, B.M. Canadian Journal of Forest Research 45(7):867-876.

Year: 2015

Issued by: Northern Forestry Centre

Catalog ID: 36082

Language: English

Availability: PDF (request by e-mail)

Available from the Journal's Web site.
DOI: 10.1139/cjfr-2014-0431

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Mechanical mastication is becoming a common fuel management treatment to reduce vertical fuel connectivity as well as crown fire initiation and potential fireline intensity, but the moisture dynamics of these novel fuel types have been largely unstudied. We recorded concurrent in situ meteorological observations with moisture and temperature profiles (5 and 13 cm) at depth for masticated fuel bed in three treatments in a lodgepole pine-black spruce boreal stand located in the Upper Foothills of west-central Alberta. Mulch at 5 cm depth remained at or near 100% moisture content for a majority of the observation period within the treatment that lacked any residual canopy cover. Only during a 10 day rain-free period was substantial drying observed at 5 cm depth. At the interface with the underlying duff layer, mulch remained at upwards of 150% moisture content. Drying was typically <10% of potential evaporation rates, except on days after rain, when drying equal to 25–100% of potential evaporation was observed. Thermal properties of the mulch showed approximately five times the thermal diffusivity of soils, but less than a fuel crib, suggesting that masticated fuel beds have a thermal and moisture regime more similar to soils than fuel cribs, with a diffusion-dominated regime enhanced by minor advection. Low moisture movement observed from depth to the surface promoted a highly variable surface moisture that created a surface mulch with temperatures far in excess of air temperatures during periods of full sun. Such excess temperatures are shown to be an efficient indicator of surface dryness and high ignition probability.

Plain Language Summary

A new method of preventing forest fires or stopping their growth and spread is to mulch some of the forest’s trees and shrubs mechanically, creating a compacted layer of woody particles over the forest floor. This common FireSmart treatment has been studied in Mediterranean and subtropical forests, but not in boreal forests. This research was to examine whether mulching would pose a risk of fire smouldering in the mulch. The moisture content and temperature of boreal mulch beds were investigated. The researchers used a continuous monitoring technique to examine moisture content and temperature at the surface and at depth in three different areas where trees had been mulched in different patterns. While surface mulch dried quickly, mulch at 5 and 13 cm below the surface did not dry to the point that would allow smouldering. Only during a two-week rain-free interval was substantial drying observed, at the site where all trees were mulched, leaving the mulch fully exposed to the sun. They concluded that thick mulch beds in a boreal or foothills climate do not pose a risk of deep smouldering in the mulch as long as rain-free periods are less than approximately two weeks. This study provides a new and low-cost method of monitoring mulch bed moisture continuously, as well as providing guidelines on the smouldering risk in mulch beds.