SOIL CHARCOAL FROM SHRUBLANDS TO SUB-ALPINE FORESTS IN THE COLORADO FRONT RANGE

Soil charcoal may be sequestered for millennia in temperate forest soils and this is important given that >80% of terrestrial organic carbon (C) is in soils. We measured this super-passive soil C pool, composed of biomass-derived charcoal, which is the legacy of wildland fires. Our objective is to quantify soil charcoal from foothills shrublands upslope to subalpine forests along the Colorado Front Range where natural wildland fire is and has been an important disturbance component. We hypothesize differences in fire regimes over the millennia have resulted in variable soil charcoal amounts. Geospatial data were used to place random sample plots in foothills shrublands (Cercocarpus montanus), and four forest types; ponderosa pine (Pinus ponderosa), Douglas-fir (Pseudotsuga menziesii), lodgepole pine (Pinus contorta) and spruce-fir (Picea engelmannii – Abies lasiocarpa). Sample plots were stratified to an easterly aspect, 10-30% slopes, and a central 200 m elevation band within each vegetation type. Soil samples (0-10 cm depth) were collected and analyzed for total soil C and soil charcoal C via chemical digestion and dry combustion techniques.

Soil charcoal is most abundant in spruce-fir soils (1.9 +/- 0.92 Mg C/ha) with roughly equivalent amounts in lodgepole pine and ponderosa pine soils (1.4 +/- 1.02 Mg C/ha and 1.4 +/- 0.54 Mg C/ha respectively). The pattern of more soil charcoal observed at higher elevations has been identified in other studies. Charcoal is least abundant in Douglas-fir and foothills shrubland soils (1.0 +/- 0.67 and 0.54 +/- 0.44 Mg C/ha respectively). Overall, soil charcoal is four times more abundant in spruce-fir forests than in foothills shrublands. Spruce-fir forests have the most above ground biomass, slower decomposition rates and a less frequent mean fire return interval than the other four types. Foothills shrublands have the least biomass, comparatively rapid decomposition rates and a more frequent mean fire return interval. We propose that high biomass and slow turnover rates in the spruce-fir forests creates conditions for net soil charcoal accumulation.