PARENT SESSION
Poster Session #39: Disturbance Ecology.
Thursday, August 9, 2001. Presentation from 3:30 PM to 5:00 PM. Exhibition Hall

Rates of windthrow-caused soil disturbance inferred from soil charcoal radiocarbon dates.

GAVIN, DANIEL¹, LERTZMAN, KENNETH², ^{1 2}

ABSTRACT- Tree-falls disturb soil profiles by lifting a root mat to a vertical position which eventually decays, causing organic and mineral soil horizons to mix. Uprooting by windthrow recurs locally on scales of centuries to millennia, and thus is difficult to quantify with plot-based studies. We estimated the rate of windthrow-caused soil turnover by using a set of radiocarbon dates of charcoal from different soil horizons. Our approach assumes that following fire charcoal is deposited within surficial organic horizons or immediately above mineral horizons and is eventually mixed into deeper mineral horizons by tree-falls, and that the probability of a tree-fall is spatially homogenous and affected only by the time since the last fire or the last tree-fall. Therefore, the soil turnover rate can be inferred from the proportion of sample sites with charcoal remaining in the organic horizon, calculated for each time-since-fire age class (based on charcoal ages). 141 radiocarbon dates were obtained from 81 sites in an old-growth forest on the west coast of Vancouver Island. A Weibull curve fit to these dates suggests the mean soil turnover interval was 1550 years. The soil turnover interval on hillslopes (2030 years) was significantly greater than on terraces (920 years). On hillslopes, the soil turnover interval was greater than previous estimates of canopy tree turnover, suggesting that trees on hillslopes are windfirm and frequently die standing without causing soil disturbance.

KEY WORDS: forest windthrow, soil disturbance, forest dynamics, canopy gaps