Regional patterns of postfire canopy recovery in the northern boreal forest of Quebec: interactions between surficial deposit, climate, and fire cycle1This article is one of a selection of papers from the 7th International Conference on Disturbance Dynamics in Boreal Forests.

In many northern forest ecosystems, the postfire transition from a closed-crown forest to open woodland is often observed but poorly understood. This paper looks at the effect of interactions between surficial deposit, climate, and fire cycle on postfire forest recovery within a large territory (190 000 km2) of the boreal forest of eastern Canada. Postfire recovery was estimated using the time elapsed to move from the burnt stage to the regenerated stage and the young forest stage. The main objective was to determine if forests situated in dry regions (characterized by a high proportion of dry coarse surficial deposits, low precipitation, and short fire cycle) tend to reestablish more slowly after fire, obtaining a more open stand compared with wetter regions characterized by a longer fire cycle. To identify the best explanatory model for postfire recovery, multinomial logistic regressions with the Akaike information criterion were conducted using a combination of physicoclimatic factors. Our best model suggests that the most significant predictors of postfire recovery are time since fire (χ2 = 1370.06), surficial deposit type (χ2 = 651.95), the Canadian Drought Code (χ2 = 247.75), and the growing season precipitation (χ2 = 102.80). Fast recovery and dense forest regeneration are associated with subhydric till deposits only in the regions characterized by a long fire cycle (>500 years). Conversely, slow regeneration conducive to a sparse young forest was usually associated with regions characterized by a short fire cycle (<200 years) underlain by dry coarse deposits such as juxtaglacial but also mesic deposits in some cases. Our results also show that slow recovery and reduced forest regeneration are most likely to occur following fires that occurred in dry years, regardless of the deposit type and region.

The effects of surficial deposit - drainage combinations on spatial variations of fire cycles in the boreal forest of eastern Canada

Spatial variations in the fire cycle of a large territory (190 000 km2) located in the boreal forest of eastern Canada were assessed using random sampling points. Our main objective was to determine if regions characterised by a large proportion of dry surficial deposit–drainage (SDD) burn more frequently than regions with a smaller proportion. Through a regionalisation of the landscape units, we analysed the effects of SDD on spatial variations of the fire cycle. A discriminant analysis involving the SDD and other physical variables (precipitation, temperature, aridity index, water bodies, elevation and slope) made it possible to identify a combination of variables characterising each region. A considerable variation in fire cycle was observed among the different SDD types (from 144 to 425 years) and between regions (from 90 to 715 years). Through the discriminant analysis, this study suggests that a combination of possible climatic top-down (precipitation R2 = 0.727, aridity index R2 = 0.663 and temperature R2 = 0.574) and bottom-up factors (xeric undifferentiated till R2 = 0.819 and humid undifferentiated till R2 = 0.691) could explain this variation at the regional scale. Implications of those results for forest protection against fire and regional development are briefly discussed.

Age calibration of carbonate rind thickness in late pleistocene soils for surficial deposit age estimation, Southwest USA

Carbonate rinds have been used for cross-correlation of landforms as well as a quantitative indicator of soil age. Using the measured rind thickness of clasts found within a deposit, whose age has been independently determined, allows the construction of a calibrated surface-age proxy. Measurements were taken at sites within the Mojave Desert, the northwestern Sonoran Desert, the southern Great Basin, and the western Colorado Plateau. These sites are all within about 300 km of the intersection of the borders of the states of California, Arizona, and Nevada. In the study area, elevation varied from 200 to 1200 m, MAP was from 95 to 195 mm, and MAT was from 18.4° to 23.3°C. The calibrated proxy, while not accounting for the effects of parent material or climate on rind development, does show a strong correlation (R2 = 0.74, P < 0.05) between carbonate rind thickness and surface age for deposits of late to middle Pleistocene age. The calibrated chronosequence, rind thickness = 0.0889 + 0.0079 [surface age]), is in general valid over a large region of southwestern United States. This statistical relation suggests that parent material, climate, and elevation may not be as strong a control on carbonate accumulation as is age for younger soils.

Toward Seismic Microzonation—2-D Modeling and Ambient Seismic Noise Measurements: The Case of an Embanked, Deep Alpine Valley

In the Sion area of Switzerland, part of a deep, embanked sediment-filled valley, investigations on soil site effects have been conducted using two independent methods. Two-dimensional (2-D) modeling was performed with a program based on the Indirect Boundary Element Method (Pedersen et. al. 1994). Numerical simulations allow taking into account the subsurface geometry of the valley and its peculiar characteristics, such as a variable shape ratio and a high shear-wave velocity contrast. The H/V method has been applied on ambient seismic noise measurements recorded on sites as close as possible to the 2-D modeling. This technique allows capturing the fundamental resonant frequency of the deepest sediments as well as identifying the existence of a surficial deposit. Both approaches agree on the fact that the fundamental resonant frequency of the valley is below 1 Hz. The amplification level of the predominant frequency obtained with numerical simulation is up to two times higher than the one given by the H/V ratio. These results provide the basis for further investigations in order to resolve differences.

ON THE ORIGIN OF FE IN SOME PODZOLIC SOILS FORMED ON CALCAREOUS PARENT MATERIALS IN THE CANADIAN ROCKY MOUNTAINS

The formation of Podzols in the Rocky Mountain Region of Alberta has been related to the presence of an easily weatherable volcanic ash component within the upper solum of these soils. A silty surficial deposit, comprising a mixture of volcanic ash and locally derived aeolian detritus, is the parent material for the upper solum of pedons occurring in stable landscape positions. Source areas for this locally derived aeolian detritus comprise highly calcareous materials. Weathering of the carbonate component of the locally derived aeolian material releases considerable amounts of iron and aluminum, sufficient to produce a Bf horizon given an appropriate environment. The presence of a Bf horizon in this region is insufficient evidence to implicate volcanic ash as a parent material component in the genesis of the pedon. Key words: Genesis, Rocky Mountain region, weathering of CaCO3, volcanic ash, local aeolian materials, Podzolic soils