Impacts of Fire and Fire Surrogate treatments on forest soil properties: a meta‐analytical approach

Abstract

Abstract. The soils underlying the 12 Fire and Fire Surrogates Network include six soil orders and .50 named soil series. Across the network, pretreatment soils varied from 3.7 to 7.1 in pH, and exhibited ranges of twofold in bulk density, fourfold in soil organic C (SOC) content, 10-fold in total inorganic N (TIN), and 200-1000-fold in extractable Ca and K. Nonmetric multidimensional (NMS) ordination of pretreatment soil conditions arrayed the FFS sites along gradients of pH/base cation status, net N transformation rates, bulk density, and SOC. At the network scale, mineral soil exposure was significantly greater in fire-only (mean of 9.2%) and mechanical þ fire (5.0%) treatments than in the controls (1.5%) during the first posttreatment year, and this persisted through the later sampling year (second through fourth year, depending on site) in the fire-only treatment (fire 4.1%, control 1.1%). Bulk density was not affected significantly at the network scale. TIN concentrations during the first posttreatment year increased after all three manipulative treatments, but this effect did not persist to the later sampling year. Neither SOC content nor soil C:N ratio was affected by any of the treatments at the network scale. At the individual site scale, the combined mechanical þ fire treatment produced more significant site 3 treatment 3 year effects than did the fire-only or mechanical-only treatments, though in most cases even the statistically significant differences produced by the manipulative treatments were modest in magnitude. Ordination of first-year standardized effect sizes produced no discernable separation of the three manipulative treatments but did separate the three sites with the greatest fire severity (based on proportional fuel consumption) from the majority of the network sites, with changes in pH, TIN, SOC content, and soil C:N ratio correlating most strongly with this separation. Ordination of the effect sizes from the later sampling year produced somewhat clearer separation of treatments than did the first-year ordination, though fewer sites were represented in this second ordination. Overall, the network-wide effects of the FFS treatments on soil properties appear to have been modest and transient.

Short-Term Belowground Responses to Thinning and Burning Treatments in Southwestern Ponderosa Pine Forests of the USA

“…At SWP, results were consistent with the network scale FFS analysis [79] where C storage, pH, and extractable base cations were not affected by treatments, but C and base cations were highly variable between annual samplings, and within and among sites [15,80]. There were two specific sites in the network-scale FFS analysis that showed increases in pH the first year after thinning and burning, yet pH returned to pre-treatment levels by the 2-year sampling [79]. The pH increases at these sites were attributed to higher burn severity.…”

“…The pH increases at these sites were attributed to higher burn severity. Neither the FFS network, nor the SWP study site specifically, found significant differences in mineral soil cations [79], and this result is most likely due to the low intensity often found with prescription fires. Due to high temperature thresholds, cations are not easily lost in the gaseous form, but often are the major constituents of ash deposited on the mineral soil surface following fire [18].…”

“…Due to high temperature thresholds, cations are not easily lost in the gaseous form, but often are the major constituents of ash deposited on the mineral soil surface following fire [18]. The spatially variable nature of prescribed fire can often confound treatment effects; indeed, fire generally showed small and idiosyncratic effects on soil properties and processes at the FFS network scale [79]. The FFS results support the conclusion that total C and N in the mineral soil is relatively insensitive to fuel reduction treatments.…”

“…A meta-analysis by Johnson and Curtis [80] concluded that changes in total mineral soil C and N contents in coniferous stands following thinning were the result of harvest method and treatment of residues. Total C and N contents in the mineral soil have initially been unaffected following restoration treatments that included thinning and prescribed fire in other ponderosa pine stands in northern Arizona [6,79], but long-term increases in total soil N have also been reported [31,74]. The limited effects of the FFS thinning treatments are due in part to modest stand density reductions from a single application.…”

How wildfires affect soil properties. A brief review

“…Volatilization of N during combustion is directly related to the temperatures reached in soil and the amount of organic matter consumed, but nitrification conditions usually are improved after burning (Mataix-Solera and Guerrero, 2007). Inorganic N concentrations tend to increase in burned areas more than in unburned control areas in the first years after fire (Smithwick et al, 2005;Turner et al, 2007;Boerner et al, 2009). Fire-induced changes in soil inorganic N content can be attributed to a combination of direct and indirect effects of fire, N release from dead roots and compounds where it was previously immobilized (Smithwick et al, 2005;Rivas et al, 2012).…”