Forest restoration treatments have subtle long‐term effects on soil C and N cycling in mixed conifer forests

Abstract

Decades of fire suppression following extensive timber harvesting have left much of the forest in the intermountain western United States exceedingly dense, and forest restoration techniques (i.e., thinning and prescribed fire) are increasingly being used in an attempt to mitigate the effects of severe wildfire, to enhance tree growth and regeneration, and to stimulate soil nutrient cycling. While many of the short-term effects of forest restoration have been established, the long-term effects on soil biogeochemical and ecosystem processes are largely unknown. We assessed the effects of commonly used forest restoration treatments (thinning, burning, and thinning + burning) on nutrient cycling and other ecosystem processes 11 yr after restoration treatments were implemented in a ponderosa pine (Pinus ponderosa var. scopulorum)/Douglas fir (Pseudotsuga menziesii var. glauca) forest at the Lubrecht Fire and Fire Surrogates Study (FFS) site in western Montana, USA. Despite short-term (<3 yr) increases in soil inorganic nitrogen (N) pools and N cycling rates following prescribed fire, long-term soil N pools and N mineralization rates showed only subtle differences from untreated control plots. Similarly, despite a persistent positive correlation between fuels consumed in prescribed burns and several metrics of N cycling, variability in inorganic N pools decreased significantly since treatments were implemented, indicating a decline in N spatial heterogeneity through time. However, rates of net nitrification remain significantly higher in a thin + burn treatment relative to other treatments. Short-term declines in forest floor carbon (C) pools have persisted in the thin + burn treatment, but there were no significant long-term differences among treatments in extractable soil phosphorus (P). Finally, despite some short-term differences, long-term foliar nutrient concentrations, litter decomposition rates, and rates of free-living N fixation in the experimental plots were not different from control plots, suggesting nutrient cycles and ecosystem processes in temperate coniferous forests are resilient to disturbance following long periods of fire suppression. Overall, this study provides forest managers and policymakers valuable information showing that the effects of these commonly used restoration prescriptions on soil nutrient cycling are ephemeral and that use of repeated treatments (i.e., frequent fire) will be necessary to ensure continued restoration success.

Soil functional responses to ecological restoration treatments in frequent‐fire forests of the western United States: a systematic review

“…Banning et al () found that thinning affected soil N cycling up to at least 2 years post‐treatment, by increasing the potential for nitrification, particularly when combined with prescription burning for rehabilitated post‐mining forests in Western Australia. Furthermore, our results align well with numerous studies reporting inconclusive or net losses in soil macronutrients following single and multiple prescribed burns in fire‐prone grassy woodlands of Australia (Prober et al ; Burrows et al ), Allepo pine ( Pinus halepensis ) shrublands of Spain (Outeiro et al ), and following tree thinning and prescribed burning in other fire adapted ecosystems of North America (Knoepp et al ; Boerner & Brinkman ; Trammell et al ; Grady & Hart ; Ganzlin et al ).…”

Temporal Effects of Thinning on the Leaf C:N:P Stoichiometry of Regenerated Broadleaved Trees in Larch Plantations

“…This is different from the findings of Hu et al [54], who reported that soil P in reforested spruce forests was not affected by thinning in the first four years, but this is consistent with previous studies that found that the time period since the application of thinning seemed to significantly affect soil P [55] and that the extractable P content declined over 30 years post-thinning [52]. The possible reason for such a difference is that soil nutrients are related to the composition and growth of aboveground species [22,37], and thinning could increase the uptake of P by plants [52,56,57]. Larch is a fast-growing tree species, especially during the middle-age period (21-30 years).…”

“…In the larch plantations of this study, the temporal effects of thinning on the soil C concentrations were not significant (Figure 4), which was inconsistent with the first hypothesis and previous studies. Several studies have found that the short-term (1-3 years after thinning) effect increased the total mineral soil C concentrations [50], but the medium-term (5 and 7 years) or long-term (11 years) effects had no significant influence on soil C [22,37]. These results in part suggest that the thinning effects may persist only for a short term.…”

“…Most studies have reported the changes of light availability in the short term (1-3 years) after thinning [31], but the variations in the medium-and long-term after thinning remain unknown [32,33]. On the other hand, a few studies have investigated the response of soil C and N to the length of time after thinning, but there are contradictory results [34][35][36][37]. For example, Hwang and Son reported that soil C and N concentrations in a pitch pine plantation and the available P concentration in a Japanese larch (Larix keampferi) plantation significantly increased over 1 year after thinning [38], but Bai et al found that thinning had no effects on soil total C and N concentrations for both Corymbia variegata and C. citriodora plantations in 5 and 7 years post-thinning [22].…”

Microbial and organic matter patterns in a prescribed burned and thinned managed forest ecosystem

“…), reduce fuel loads, increase general forest productivity and to enhance plant and animal diversity (Gaines and Creed , Ganzlin et al. ). Using fire as a management strategy has the propensity to alter soil organic matter (SOM) and microbial populations.…”