Does seeding after severe forest fires in western USA mitigate negative impacts on soils and plant communities? (systematic review)
Background
Broadcast seeding is one of the most widely used post-wildfire emergency response treatments intended to reduce soil erosion, increase vegetative ground cover, and minimize establishment and spread of non-native plant species. However, seeding treatments can also have negative effects such as competition with recovering native plant communities and inadvertent introduction of invasive species. Despite ongoing debates over the efficacy of post-fire seeding and potential negative impacts on natural plant community recovery, seeding remains a widely used stabilization treatment in forested ecosystems throughout the western U.S. In 2000, Robichaud et al. reviewed the effectiveness and impacts of the entire suite of burned area rehabilitation treatments used on U.S. Forest Service land, including post-fire seeding. Beyers (2004) published a review specific to post-wildfire seeding, but a good part of the conclusions were drawn from studies occurring in chaparral. Since publication of Robichaud et al. (2000) and Beyers (2004), several developments have altered the context of post-fire seeding. These include: 1) increasing size and severity of wildfires across the western U.S., 2) increased research and quantitative monitoring on post-fire seeding and plant community interactions, 3) increased use, availability, and allocation of funds for native seed mixes, and 4) stronger policy direction for the use of locally-adapted and genetically-appropriate seed sources (seed sources adapted to local site conditions and genetically compatible with existing plant populations). With the last review occurring in 2004 there is a need to re-examine what is known about the effectiveness and ecological impacts of post-fire seeding specific to forested ecosystems across the western U.S.
Objectives
Primary objective: To systematically collect and synthesize the available published and unpublished evidence in order to answer the question “Does seeding after severe forest fires mitigate negative impacts on soils and plant communities?”
Secondary objective(s): Summarize the evidence available to address three questions pertaining to post-wildfire seeding treatment effectiveness and effects: (1) Does seeding after severe forest fires reduce soil erosion? (2) Is seeding effective at reducing non-native plant invasion into burned areas? and (3) Does post-wildfire seeding affect native plant community recovery?
Methods
To identify studies relevant to our review, we searched databases supported by Northern Arizona University during July-November 2008, using a defined combination of search terms. We then eliminated papers, first based on title, then abstract, then full text, based on a set of criteria that specified the review subjects (seeding in western USA forests burned by severe wildfire), intervention (seeding herbaceous plant or shrub seed alone or in combination with other post-fire rehabilitation activities), and outcome (soil stabilization attributes and changes to plant community attributes). We assessed study quality based on study design and statistical robustness, and applied a weight ((highest, high, medium, low, lowest) to each study design category (replicated randomized experiment, observational (multiple location case study), observational (single location case study), monitoring report with quantitative data, monitoring report with qualitative data, BACI, review paper, and expert opinion)) with the greatest weight given to replicated randomized experiments and less to observational and opinion studies. We evaluated post-fire seeding effectiveness based on seeding treatment effectiveness in reducing erosion, non-native species invasions, and effects on native plant community recovery. When available, quantitative data from seeded and unseeded treatments were compared. Each study or individual study unit was given an effectiveness rating (effective, minimal effectiveness, ineffective, negative effect). We used descriptive statistics to explore relationships between post-fire seeding treatments and associated variables.
Main Results
Our review produced 94 relevant studies. Considering the entire dataset (n = 94), replicated and randomized experiments made up the largest study design category. Using quality of evidence criteria, the number of studies of quantitative experimental nature increased from the time period 2000-2009 compared to those studies in 1970-1999. Twenty-three 1studies provided evidence regarding post-fire seeding effects on soil erosion. As sampling designs have become more rigorous in recent years, evidence that seeding is effective in reducing erosion has decreased. Of highest and high quality studies evaluating soil erosion, 89% (8 of 9) were published since 2000, only one of which showed an effective result as a result of additional treatments. Before 2000, the majority of the studies (70%) fell into the lowest quality categories, of which, 71% showed seeding to be effective. A main goal of post-wildfire stabilization treatments is to reduce soil erosion in the year immediately following a fire; however, the majority of studies (7 of 11, 64%2) evaluating soil erosion in seeded versus unseeded controls showed that seeding did not reduce erosion relative to unseeded controls. Comparing cover measurements between seeded and unseeded plots from 20 studies containing a total of 29 study sites, we found that even when study results showed that seeding significantly increased vegetative cover, seeded sites rarely supported sufficient plant cover to stabilize soils within the first and second year post-fire. Of the 11 papers providing direct evidence regarding the role of seeding in reducing non-native species abundance, an almost equal percentage found seeding treatments to be effective (54%, 6 studies) or having a negative effect (45%, 5 studies). However, the majority of effective treatments and those which had a negative effect (83% and 80%, respectively) used non-native species. A majority of studies reported that seeding suppressed recovery of native plants (16 studies, 62%). However, data on long-term impacts of this reduction are limited. Cover data from 15 studies containing 57 different study sites showed decreased seeded cover relative to control plot cover with increasing time since fire. Based on cover data from all 57 sites, total plant cover in seeded sites and controls was nearly identical by years 4 and 5 post-wildfire. A seeding treatment‟s ability to reduce soil erosion and/or affect native plant community recovery appears to be strongly driven by amount and timing of precipitation.
Conclusions
This review suggests that post-fire seeding does little to protect soil in the short-term, has equivocal effect on invasion of non-native species, and can have negative effects on native vegetation recovery with possible long-term ecological consequences. Erosion may be better reduced by mulching, but care must be taken to ensure that mulch is free of non-native seed. Seeding has proven to be equivocal at best for reducing non-native species spread after fire. Early detection of new undesirable species invasions through monitoring post-fire environments, in combination with rapid response methods to quickly contain, deny reproduction, and eliminate these invasions, may allow better control of non-native species establishment than is typically obtained through seeding. Plant community recovery may be improved with the use of locally-adapted, genetically appropriate plant materials, although more research regarding the effects and effectiveness of these species is critical.
Background
Rehabilitation efforts following severe forest fires, the largest and mostly damaging of which tend to burn on public lands of the western United States, commonly focus on re- establishment of a growing plant community to stabilize soils, minimize erosion, and reduce colonization by undesirable non-native species. Substantial resources are invested in these treatments but relatively little is known about their ecological effects or cost:benefit ratios. The U.S. General Accounting Office (GAO) identified a need for better information on the effectiveness of post-fire emergency stabilization and rehabilitation methods used by the U.S. Department of Agriculture (USDA) Forest Service and Department of Interior (DOI) agencies (GAO 2003), based in large part on reviews by Robichaud et al. (2000) and Pyke and McArthur (2002). The most widely used post-fire treatment is seeding, primarily with native or non-native grasses, on which there has been considerable research and continued debate over effectiveness and ecosystem impacts (Beyers 2004). The success of seeding depends on many factors, including species selection, climate, terrain, competition with other species, seedbed preparation, and post-seeding management (Monsen et al. 2004). Beyers (2004) pointed out a lack of data on then-emerging seeding practices, especially increased use of native species and sterile cereal grains for erosion control as well as seeding to prevent spread of invasive non-native plants. The GAO report and the published reviews have sparked an upsurge in research on seeding effectiveness and agency monitoring of post-fire treatments. This new information is scattered and largely unavailable to burned area assessment teams on tight timelines to recommend stabilization measures.
Since publication of the recent reviews (2000-2004), several important developments have altered the context of post-wildfire seeding:
- Areas of high-severity forest fires have increased by as much as an order of magnitude in the Intermountain West, including mega-fires such as the Hayman (CO) and Rodeo-Chediski (AZ). Climate projections consistently indicate that trends in increasing size and severity of wildfires will continue (McKenzie et al. 2004).
- Severe burning up to and within urban areas has presented new challenges for post- fire rehabilitation, for instance in the Cerro Grande fire, Los Alamos (NM).
- Scientific studies of post-wildfire seeding and plant community interactions have increased dramatically since 2000, with an extensive body of developing knowledge on the use of native seeds as well as the ecological consequences of non-native seeding.
- Signing of Executive Order 13112 establishing the National Invasive Species Council to prevent the introduction of invasive species and provide for their control and to minimize the economic, ecological, and human health impacts that invasive species cause.
- Burned Area Emergency Rehabilitation (BAER) teams and agency staff have shifted from relying on non-native species to native species, allocating substantial sums for native seed mixes in the hopes of fostering post-fire environments characterized by native plant communities that are well-adapted to these habitats and resilient to further disturbance (Wolfson and Sieg, in review).
Because of these striking trends, it is clear that the opening decade of the 21st century has been characterized by a sea change in the scale of severe fires, the scientific information available to support management decisions, and the choices made by managers for post-fire seeding. Our goal in the present review is to develop a practical, complete, and up-to-date synthesis that puts the latest information on burn rehabilitation seeding in the hands of managers and scientists.