Singular and Combined Effects of Blowdown, Salvage Logging, and Wildfire on Forest Floor and Soil Mercury Pools

Abstract

A number of factors influence the amount of mercury (Hg) in forest floors and soils, including deposition, volatile emission, leaching, and disturbances such as fire. Currently the impact on soil Hg pools from other widespread forest disturbances such as blowdown and management practices like salvage logging are unknown. Moreover, ecological and biogeochemical responses to disturbances are generally investigated within a single-disturbance context, with little currently known about the impact of multiple disturbances occurring in rapid succession. In this study we capitalize on a combination of blowdown, salvage logging and fire events in the sub-boreal region of northern Minnesota to assess both the singular and combined effects of these disturbances on forest floor and soil total Hg concentrations and pools. Although none of the disturbance combinations affected Hg in mineral soil, we did observe significant effects on both Hg concentrations and pools in the forest floor. Blowdown increased the mean Hg pool in the forest floor by 0.76 mg Hg m(-2) (223%). Salvage logging following blowdown created conditions leading to a significantly more severe forest floor burn during wildfire, which significantly enhanced Hg emission. This sequence of combined events resulted in a mean loss of approximately 0.42 mg Hg m(-2) (68% of pool) from the forest floor, after conservatively accounting for potential losses via enhanced soil leaching and volatile emissions between the disturbance and sampling dates. Fire alone or blowdown followed by fire did not significantly affect the total Hg concentrations or pools in the forest floor. Overall, unexpected consequences for soil Hg accumulation and by extension, atmospheric Hg emission and risk to aquatic biota, may result when combined impacts are considered in addition to singular forest floor and soil disturbances.

Post‐Fire Comparisons of Forest Floor and Soil Carbon, Nitrogen, and Mercury Pools with Fire Severity Indices

“…In our study, 72% of the Hg pool was lost from the forest floor, 7% from the 0-to 10-cm soil depth and 10% from the 10-to 20-cm soil depth. Similar to our results, Mitchell et al (2012) conducted a study in the same region and found that 84% of the Hg pool in the forest floor was emitted back to the atmosphere after a large wind event that blew down trees, after which the site was salvage logged and then had fire, but found no differences in the mineral soil pool after any fire treatments. Woodruff et al (2009) also measured Hg loss as a result of fire in northern Minnesota and found 100% Hg pool losses in the forest floor and 87% from the A horizon of severely burned areas.…”

“…Others have found little or no change in accumulation in aquatic biota following fire Carignan, 1999, 2000;Allen et al, 2005). Several studies have measured soil Hg concentrations or pools before and after fire (Harden et al, 2004) or compared burned to nearby unburned areas after fire (Amirbahman et al, 2004;DiCosty et al, 2006;Engle et al, 2006;Biswas et al, 2008;Navratil et al, 2009;Mitchell et al, 2012) and generally found that fire has the largest impact on forest floor and surface mineral soil Hg concentrations and pools, with the effect of fire decreasing with soil depth.…”

Yellow Perch (Perca flavescens) Mercury Unaffected by Wildland Fires in Northern Minnesota

“…Fires are known to decrease the pools of C and Hg in upland soils (Amirbahman et al, 2004; Woodruff and Cannon, 2010; Miesel et al, 2012) due to the combustion of these materials during fire and atmospheric release of CO 2 and gaseous and particulate Hg species (Friedli et al, 2003), as well as the subsequent erosion and runoff from fire‐disturbed soils (Eklöf et al, 2016). In addition to fire occurrence, the severity of the fire affects the size of C and Hg stocks remaining in upland soils: more severe fires release more previously stored C and Hg to the atmosphere, leading to less remaining soil C and Hg, compared with less severe fires (Certini, 2005; Mitchell et al, 2012). The impact of the Tuscarora fire on the Everett lake watershed was minimal, as it burned only 11% of the lake watershed area and was predominantly a low‐severity burn.…”

“…As discussed above, the impact of fire on upland soil chemical, physical, and biological properties varies with fire severity. For example, compared with low‐ to moderate‐severity burns, watersheds that burn more severely have more potential for post‐fire runoff and erosion (Certini, 2005; Mitchell et al, 2012) and, consequently, altered lake nutrient and Hg loads (although in cases of very severe fire and complete organic matter volatilization, Hg export can decrease and fire may ultimately reduce Hg loads in biota; Bank et al, 2005). We may not have observed fire impacts on fish Hg due to the low‐ and moderate‐severity impact of the Tuscarora and Ham Lake fires, respectively, on our experimental watershed.…”

Ecological consequences of compound disturbances in forest ecosystems: a systematic review

“…Enhanced R. copallinum establishment paralleled other compound interactions where clonal regrowth of sprouting plants was observed. Nonetheless, despite increased fire severity on wind-and-fire impacted sites , forest floor and soil mercury levels were comparable to fire-only sites (Mitchell et al 2012). Outside of the southeastern United States, two other well-studied wind-and-fire combinations occurred in the Minnesota subboreal and Colorado Rocky Mountains (Fig.…”