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Co-Occurrence of Hydrophobicity and Allelopathy in Sand Pits under Burned Slash

USDA Forest Service, Pacific Northwest Res. Stn., Forestry Sciences Lab., 1133 N. Western Ave., Wenatchee, WA 98801

F. Martin Wilt

Agriculture and Environmental Chemistry Graduate Group, Univ. of California, Berkeley, CA 94804

Roger D. Ottmar

USDA Forest Service, Pacific Northwest Res. Stn., Forestry Sciences Lab., 4043 Roosevelt Way NE, Seattle, WA 98105

*Corresponding author.

ABSTRACT

Prescribed burning of slash following tree harvest is a standard practice to reduce fire hazard and prepare seed beds for planting conifers. This study examined the co-occurrence and intensity of hydrophobic and allelopathic layers created in sand pits under burned slash piles and broadcast slash. Following slash burning, 24 sand cores were taken from the sand pits and analyzed for water repellency and allelopathic effects on germination, emergence, and height growth of a bioassay species, blue wildrye (Elymus glaucus Buckley). Water drop penetration that was instantaneous in untreated sand cores was delayed in sand cores from burn treatments for 5 to 90 s in seven cores and for >270 s in five cores. Seedling emergence and height of the bioassay species declined in both the created hydrophobic sand layer and in an allelopathic zone of wettable sand immediately below the hydrophobic layer. Lack of allelopathic response when free water was present suggests that allelopathic effects are exhibited only under unsaturated soil conditions. Results indicate that nonwettable soils and allelopathy effects will act in unison to reduce water penetration and slow plant establishment following wildfires or prescribed burns.

Received for publication June 27, 1994.