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Manipulating soil heating patterns to optimize barrens restoration

Sturtevant, B. R., M. B. Dickinson, R. K. Kolka, C. C. Kern, D. M. Donner, K. M. Quigley and M. M. Bushman. 2020.


Prescribed burning is frequently applied to restore and maintain open-canopy ecosystems such as pine woodlands and barrens. Critical uncertainties on the effectiveness of these burns exist in regard to soil heating and its effects on soil nutrient pools and other biological processes such as hardwood regeneration and seedbank viability. The First Order Fire Effects Model (FOFEM) is widely used for fire planning in such situations, but the embedded soil heating model has not been validated across a wide range of soil and fuel conditions. Understanding how soil heating influences below-ground processes such as soil nutrients, seed germination, and hardwood regeneration is needed to inform effective restoration strategies. We partnered with the Chequamegon-Nicolet National Forest in northern Wisconsin to investigate 1) fire behavior, 2) first-order soil heating, and 3) second-order effects of soil heating within a long-term barrens restoration area, the Moquah Barrens, to evaluate the FOFEM soil heating predictions and the restoration success of initial-entry and repeated prescribed burns. We established 112 plots and 23 unburned reference plots across a range of current and historic vegetation types with contrasting fuel manipulation treatments. Late dormant season prescribed burns were conducted in 2016 and 2018. We collected prefire, postfire, and 1-year postfire measurements of fuel loads, vegetation, soil nutrients, and seed bank. To capture metrics of fire behavior and soil heating responses, we instrumented a subset of plots to evaluate soil heating model performance against soil properties. We evaluated a suite of burn severity methods, analyzed carbon and nutrient content of the forest floor and mineral soil, evaluated soil nutrient exchange rates, and quantified seedbank and hardwood regeneration responses. We found FOFEM substantially overestimated the consumption of litter, duff, and woody fuels indicating modeled combustion rates overestimated heat transfer to the soil. Within the Moquah Barrens, even relatively thin duff layers insulated the soil from heat, although biologically relevant belowground heating occurred where duff was especially thin or absent. Fire effects on soil nutrient stocks was most evident in the forest floor horizon that is subject to first-order fire effects. Changes in soil nutrient stocks and exchange rates were ephemeral, reflecting transient nutrient inputs from the ash layer that linked with surface fuel consumption. Brush cut-and-leave and fuel addition treatments lessened brush density 1-year postfire relative to other fuel treatments supporting the hypothesis that increased soil heating with brush fuel load, rather than brush-cutting itself, drive effects on belowground root systems. We found a viable seedbank in duff and mineral soil layers consisting of targeted restoration barren species and related to vegetation condition. Pre- and postfire germinant densities were similar, suggesting fire intensity and severity did not influence seed abundance. Our study suggests soil heating is not a major factor under late dormant season conditions. While such burns remain effective for maintenance of barrens, our results support expansion of the burn window when soil heating may be more likely. FOFEM evaluation suggested the current model may not be appropriate when applied to sandy soils. Shortfalls identified herein are leading to key improvements, including parameterization of new sandy soil options. A researchmanagement partnership allowed us to accomplish large-scale fire research and answer complex restoration questions through knowledge co-production. This study provides information that will strengthen the success of Lake States burn programs and provides results widely applicable to other eastern US ecosystems under prescribed fire management.

Key Words

ash, burn severity, char, dormant season burns, duff, FIREMON, FOFEM, fuel consumption, fuel loading, mechanical brush cutting, mesophication, nutrient exchange rates, paint tag, pine barrens, pine woodlands, prescribed burn, pyrogenic carbon, restoration, sandy loam, seed bank, soil fertility, soil heating, soil hydraulic connectivity, thermocouple probe, top kill