Elsevier

Forest Ecology and Management

Volume 260, Issue 10, 15 October 2010, Pages 1816-1823
Forest Ecology and Management

Mulching effects on vegetation recovery following high severity wildfire in north-central Washington State, USA

https://doi.org/10.1016/j.foreco.2010.08.026Get rights and content

Abstract

Straw mulch application after high severity wildfire has gained favor in recent years due to its efficacy in reducing soil erosion hazards. However, possible collateral effects of mulching on post-fire vegetation recovery have received relatively little study. We assessed mulching effects on plant cover and species richness, tree seedling establishment, and exotic species densities in the second year following the 2006 Tripod Wildfire in north-central Washington State, USA, by observing vegetation responses to spatial variability in mulch cover and depth. Mulch cover averaged about 35%, with a median depth of 0.5 cm. Vegetation recovery was generally slow, with median plant cover of only 10%. Tree seedling densities were low and spatially variable. Vegetative cover, species richness, and seedling densities all declined with increasing elevation. Mulch cover was positively associated with plant cover, plant species richness, and conifer seedling densities when second year mulch cover did not exceed 40%. Only when mulch cover exceeded 70% did mulching begin to negatively affect vegetation recovery relative to areas with no mulch. Vegetation responses to mulch depth were minimal at depths under 3 cm, but quite strong when mulch depth exceeded 5 cm. Exotic plant frequency and density were positively associated with mulch cover, but exotic plant cover was low on average (<1%). In this study, mulch added significant cover to sites with slow natural recovery of vegetation, thereby likely reducing erosion hazard. Mulching also appears to have facilitated native plant recovery and conifer seedling establishment except at very high application levels, easing management concerns about longer-term impacts of mulching treatments on post-fire vegetation recovery.

Research highlights

▶ Post-fire straw mulch application had little negative impact on vegetation recovery. ▶ Moderate mulch cover facilitated vegetation recovery including seedling establishment. ▶ Mulch did introduce exotic species, but cover remained low. ▶ Vegetation recovery was strongly negatively correlated with elevation.

Introduction

Monitoring and assessment of the effectiveness and ecological impacts of post-fire rehabilitation and restoration treatments is a critical step for implementing adaptive management in post-fire forest and range management. High severity wildfires can increase erosion and flooding hazards by reducing ground cover, exposing bare mineral soil, and altering soil physical properties (DeBano et al., 1998, Benavides-Solorio and MacDonald, 2001, Wondzell and King, 2003). Hillslope stabilization treatments such as seeding or mulching are often used following high severity wildfires to reduce these threats and protect important resources and habitats (Robichaud et al., 2000, Beyers, 2004). Hillslope stabilization treatments should ideally act quickly to increase soil cover and reduce erosion and flooding hazards, without impeding natural vegetation recovery (USDA, 1995). Unfortunately, monitoring and reporting of treatment effectiveness and collateral ecological impacts is often lacking (U.S. General Accounting Office, 2003).

Mulching is a hillslope stabilization treatment designed to directly provide organic cover for burned soils, thereby reducing surface erosion caused by raindrop impacts, surface runoff, and transport of eroded sediment (Bautista et al., 1996, Robichaud, 2005, Wagenbrenner et al., 2006, Napper, 2006, Groen and Woods, 2008). As a treatment that is not dependent on biological processes for success, it is most effective in the first year after wildfire, when vegetation recovery is just beginning and soil erosion and runoff hazards are highest (Robichaud, 2005), with diminishing effectiveness in later years as mulch decomposes. The rapid establishment of predictable cover, combined with uncertain efficacy of alternative treatments (Robichaud et al., 2000, Robichaud et al., 2008) has led to increased mulch usage in recent years, despite high costs, especially in high risk areas (Robichaud, 2005, Robichaud et al., 2009).

The ecological effects of mulch treatments have been variable and may depend on the application rate and site factors. Previous work suggests mulch can inhibit plant establishment, reduce tree regeneration, and introduce exotic species (Beyers, 2004, Kruse et al., 2004, Robichaud, 2005). Mulch may inhibit vegetation establishment and growth by covering bare mineral soil seedbeds, shading emerging plants, reducing nitrogen availability, or producing alleopathic effects (Wu et al., 2001, Kruse et al., 2004). Reductions in plant establishment and growth are more likely when deep piles of mulch develop (Robichaud et al., 2000, Robichaud et al., 2009). Alternatively, mulch may promote plant establishment and growth by retaining soil moisture, which could be particularly beneficial on dry sites (Robichaud et al., 2000). Indeed, previous studies have shown that post-fire mulching was associated with increased plant growth or cover (Bautista et al., 1996, Bautista et al., 2009, Badía and Martí, 2000, Peterson et al., 2009).

Introduction of exotic plant species is another potential collateral effect of mulching. Invasion by exotic species is well recognized as a threat to ecosystem recovery following wildfire (Crawford et al., 2001, Freeman et al., 2007). Mulching has the potential to facilitate exotic plant introduction and establishment by transporting seeds on contaminated mulch and improving establishment conditions for locally present exotic plant species (Kruse et al., 2004, Robichaud, 2005). Mulch applied on public lands is frequently required to be “weed-seed free” (Beyers, 2004); however, weed-seed free mulch may still carry seeds of exotic species (Robichaud et al., 2003) or such seeds can be introduced at the staging area (Faust, 2008). Kruse et al. (2004) found increased exotic species abundance following post-fire mulching in California, but overall the potential for mulching to introduce exotic species has received little study.

The purpose of this study was to assess the effects of straw mulch cover and depth on live plant cover, plant species richness, tree seedling densities, and exotic plant abundance (presence and density) following the 2006 Tripod Fire. We employed an observational approach, using variability in mulch cover and depth following operational mulching treatments to evaluate mulching intensity effects on vegetation recovery. We sought to answer three questions:

  • (1)

    Does mulching intensity (cover or depth) significantly influence live plant cover, plant species richness, tree regeneration densities, or exotic species densities after wildfire?

  • (2)

    Are there thresholds of mulching intensity, below which treatment effects on vegetation recovery are neutral to positive?

  • (3)

    How does topographic position (elevation, slope, aspect) affect native vegetation recovery rates and the need for land surface treatments to reduce erosion?

Section snippets

Study site

The Tripod Wildfire began on July 24, 2006 and burned about 80,000 ha in north-central Washington State, USA. About 24% of the burned area was classified as high burn severity and 27% at moderate burn severity (USDA Forest Service, 2006). The fire mostly burned coniferous forest, but forest types varied with elevation. Ponderosa pine (Pinus ponderosa C. Lawson) and Douglas-fir (Pseudotsuga menzesii (Mirb.) Franco) dominated low elevation forests, while higher elevation forests were dominated by

Mulch cover and depth

Mulch cover in the second growing season following fire averaged 35.3% across the 18 sampling areas. Point estimates of mulch cover ranged from 0% to 100%, with a median value of 25%. Mulch cover was less than 5% on 80 quadrats, which provided a baseline for vegetation responses. At the transect-level, mean mulch cover ranged from 3.1% to 59.7%. However, variance partitioning with the unconditional means model indicated that over 85% of the variability in mulch cover was due to variability

Discussion

We found little evidence that mulching inhibited vegetation recovery following wildfire. Plant cover and species richness were largely unaffected by mulching even with relatively high second year mulch cover. Furthermore, mulching appeared to provide a small benefit to recovering plants at moderate mulch cover and low mulch depths. Previous studies of mulch effects on vegetation recovery have been variable, with different studies suggesting that mulch facilitated (Bautista et al., 1996, Badía

Conclusion

Post-fire slope stabilization treatments should ideally provide protection against flooding and erosion without compromising long-term ecosystem recovery and function (USDA, 1995). In this study, we found that mulching had few negative effects on vegetation recovery, except in the relatively rare cases where mulch cover was very high or mulching resulted in deep piles. In contrast, moderate amounts of mulch appeared to facilitate growth and establishment of some species, including conifer

Acknowledgements

We thank J. West, T. McGinley, B. Lo, and B. Wilson for their efforts in field data collection. We thank Pat Cunnigham and Ashley Steel for statistical reviews of the manuscript and two anonymous reviewers for comments that improved the manuscript. This study was funded by the National Fire Plan through the Pacific Northwest Research Station of the U.S. Forest Service.

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