Fuel reduction and coarse woody debris dynamics with early season and late season prescribed fire in a Sierra Nevada mixed conifer forest

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Abstract

Fire exclusion has led to an unnatural accumulation and greater spatial continuity of organic material on the ground in many forests. This material serves both as potential fuel for forest fires and habitat for a large array of forest species. Managers must balance fuel reduction to reduce wildfire hazard with fuel retention targets to maintain other forest functions. This study reports fuel consumption and changes to coarse woody debris attributes with prescribed burns ignited under different fuel moisture conditions. Replicated early season burn, late season burn, and unburned control plots were established in old-growth mixed conifer forest in Sequoia National Park that had not experienced fire for more than 120 years. Early season burns were ignited during June 2002 when fuels were relatively moist, and late season burns were ignited during September/October 2001 when fuels were dry. Fuel loading and coarse woody debris abundance, cover, volume, and mass were evaluated prior to and after the burns. While both types of burns reduced fuel loading, early season burns consumed significantly less of the total dead and down organic matter than late season burns (67% versus 88%). This difference in fuel consumption between burning treatments was significant for most all woody fuel components evaluated, plus the litter and duff layers. Many logs were not entirely consumed – therefore the number of logs was not significantly changed by fire – but burning did reduce log length, cover, volume, and mass. Log cover, volume, and mass were reduced to a lesser extent by early season burns than late season burns, as a result of higher wood moisture levels. Early season burns also spread over less of the ground surface within the burn perimeter (73%) than late season burns (88%), and were significantly patchier. Organic material remaining after a fire can dam sediments and reduce erosion, while unburned patches may help mitigate the impact of fire on fire-sensitive species by creating refugia from which these species can recolonize burned areas. Early season burns may be an effective means of moderating potential ecosystem damage when treating heavy and/or continuous fuels resulting from long periods of fire exclusion, if burning during this season is not detrimental to other forest functions.

Introduction

Fire exclusion in mixed conifer forests throughout western North America has led to an unnatural accumulation of twigs, branches, logs, litter, and duff, on the forest floor (Parsons and DeBenedetti, 1979, van Wagtendonk, 1985). Due to the lack of fire and increasing tree densities, the spatial continuity of these surface fuels is now also greater (Miller and Urban, 2000). In addition, more of the large downed logs are in a highly decayed state (Skinner, 2002). When ignited, heavy fuels can contribute to extreme wildfire behavior (Arno, 2000, Brown et al., 2003) with potentially detrimental ecosystem consequences (van Wagtendonk, 1985, Stephens, 1998). The heat released by consumption of heavy fuels may cause torching of nearby trees and the embers released by the torching of trees and burning of decayed snags can lead to long-distance spot fires. Rotten logs are readily ignited by embers and are therefore also important in propagating spot fires.

Besides acting as fuel and potentially influencing fire behavior, organic material on the forest floor provides habitat for a large number of forest species, including small mammals (Tallmon and Mills, 1994, Carey and Johnson, 1995, Ucitel et al., 2003, McCay and Komoroski, 2004), reptiles (James and M’Closkey, 2003), amphibians (Bunnell, 1995), and invertebrates (Harmon et al., 1986, Torgersen and Bull, 1995). The presence of organic matter also influences geomorphic processes. Litter and duff aids in water infiltration and reduces the potential for erosion (Agee, 1973). A strong correlation has been found between post-burn watershed sediment yield and the percentage of forest floor exposed by burning (Benevides-Solorio and MacDonald, 2001, Johansen et al., 2001). Logs and other woody debris can dam and retain sediments on slopes and plays an important role in stream channel dynamics (Harmon et al., 1986, Naiman et al., 2002).

With organic matter on the forest floor acting as fuel, habitat, and providing structural integrity to the forest ecosystem, managers are often faced with conflicting considerations (Brown and See, 1981, Brown et al., 2003, Ucitel et al., 2003). Prescription burning is a commonly used method to treat fuels, but fuel reduction targets to reduce wildfire hazard must be balanced with fuel retention targets to maintain habitat and other forest functions. If too much fuel is removed, the heat released may damage trees excessively and the loss of organic matter may lead to erosion and reduced abundance and diversity of fire-sensitive species (Kauffman and Martin, 1989). Conversely, prescribed fires that consume little of the available fuel may not adequately reduce fire hazard. Achieving such a balance can be particularly challenging when fuel loading is high.

The net ecosystem effect of burning, whether by wildfire or prescribed fire, is often closely tied to the amount of heat released. Heat released is in turn proportional to the amount of available fuel (Alexander, 1982, Johnson and Miyanishi, 1995, Whelan, 1995), but fuel moisture, the physical structure of the fuel bed, weather conditions, and a myriad of other factors lead to a high degree of variability in patterns of consumption and subsequent fire effects (Alexander, 1982, Martin and Sapsis, 1992). The excessive litter, duff, and woody debris found in many areas of the Sierra Nevada where fire has been actively suppressed can result in long-duration heating when fire is returned to the system. In the mixed conifer forest, a significant proportion of the “fine fuel” – litter and smaller twigs and stems – is consumed at the flaming front (flaming combustion), leading to a pulse of heat release that has the greatest impact above ground (i.e. canopy scorch on affected trees). The duff layer is typically consumed through smoldering combustion after the flaming front has passed (Kauffman and Martin, 1989). In areas where the duff layer is thick, this smoldering combustion may be of long duration and generate substantially more heat than flaming combustion (Kauffman and Martin, 1989). Because a significant portion of the heat generated by smoldering combustion is transferred downward (Frandsen and Ryan, 1986, Hartford and Frandsen, 1992), soil and below ground processes are often most strongly impacted. Fire can also persist for long periods in large logs. Decayed logs are more likely to be completely consumed by fire than freshly fallen logs (Brown et al., 1985, Kauffman and Martin, 1989, Skinner, 2002), potentially producing a large amount of heat energy.

Even if extensive crown scorch is avoided with the first burn after a period of fire suppression, the heat produced can injure the cambium, kill roots and lead to the death of even large overstory trees (Ryan and Frandsen, 1991, Swezy and Agee, 1991, Stephens and Finney, 2002). In addition, the greater spatial continuity of fuels may cause fire to burn over a greater proportion of the ground surface. Historically, frequent fires are believed to have kept fuel loads relatively low and the lack of fuel continuity contributed to a highly patchy pattern of fire spread (Swetnam, 1993). The patchiness of fire spread under historical conditions may have been important in reducing the impact of fire on fire-sensitive species by creating abundant refugia from which these species could rapidly recolonize burned areas.

The amount of fuel consumed and percentage of the area burned can be controlled to some extent by varying the fuel moisture and weather conditions that prescription burns are conducted under. In similar mixed conifer forests, Kauffman and Martin (1989) reported that early season burns ignited one month after the last spring precipitation event consumed only 15% of the total available fuel, while early fall burns when fuel moisture was much lower consumed 92% of the total available fuel. Percentage consumption of the litter and duff in early and late season burns was significantly correlated with the moisture content of the lower duff layer. Fuel consumption can also vary by the tree species contributing most of the fuel. Agee et al. (1978) noted that pine litter could be effectively reduced by burning in spring, summer, or fall, but drier summer or fall conditions were required to reduce the more compact white fir (Abies concolor) and giant sequoia (Sequoiadendron giganteum) litter.

Prior to the policy of fire suppression, fires in the mixed conifer zone of the Sierra Nevada burned a given area approximately every 4–40 years (Kilgore and Taylor, 1979, Swetnam, 1993, Caprio and Swetnam, 1995, Skinner and Chang, 1996). In Sequoia and Kings Canyon National Parks, prescription burning has been used to reduce fuels and restore natural ecosystem processes since the late 1960s (Kilgore, 1973). Most of this burning has been done during the fall months, which is within or after the period when the majority of land area is likely to have burned prior to European settlement (mid-summer to early fall) (Caprio and Swetnam, 1995). Early season (late spring/early summer) burns were historically uncommon and usually associated with dry years. Fires in the fall are desirable from a fire management perspective because they are typically followed by the onset of seasonal rain and snow and therefore require less monitoring. However, fall fires potentially have more impact on air quality in the adjacent Central Valley (Cahill et al., 1996), due to stable atmospheric patterns common at this time of year. A greater proportion of the prescription burning in Sequoia and Kings Canyon National Parks has, in the past few years, been conducted earlier in the season under more favorable smoke dispersal conditions.

The purpose of this study was to evaluate differences in surface fuel consumption, fire coverage (proportion of area burned), and coarse woody debris dynamics with early season and late season prescribed fires, to help managers refine burning prescriptions for this vegetation type. The findings are especially relevant to the first restoration burn after a long period of fire suppression.

Section snippets

Study site description

Three replicate early season prescribed burn, late season prescribed burn, and unburned control units were established in a completely randomized design in Sequoia National Park (Fig. 1). The study site was located on a northwest-facing bench above the Marble Fork of the Kaweah River, adjacent to the Giant Forest sequoia grove, at elevations ranging from 1900 m to 2150 m above sea level. Each unit was 15–20 ha in size. Tree species in this old-growth mixed conifer forest were, in order of

Fuel moisture

Fuels within all size categories were significantly wetter during the early season burns than during the late season burns (Table 1). The difference in moisture was especially pronounced for large woody fuels and duff. Early season fuel moisture was for most woody fuel categories somewhat higher than the range within which Sequoia and Kings Canyon National Parks usually conducts prescribed burns in this vegetation type (Table 1). While woody fuels in the late season were within the prescription

Discussion

Fuel moisture was likely the main cause of differences in fuel consumption with early and late season burns. Because energy is necessary to drive off water before combustion is possible, more energy is required to propagate flaming combustion in moist fuels than dry fuels (Frandsen, 1987, Nelson, 2001). Consumption of large woody fuel is often quite high at moisture levels equal to or less than 10–15%, but less than half of these fuels are typically consumed when moisture levels exceed 25–30% (

Acknowledgements

We wish to thank Sequoia National Park Fire Management for conducting the prescribed burns, members of the field crews (Anni Ala, Clara Arndt, Todd Erdody, Eric Fabio, Eric Groth, Shauna Hee, Jeffrey Kane, Dan Lieberman, Hanna Mershman, Katie Panichelle, Dorothy Wallace-Senft, and Kelly Wengronowitz) who assisted with data field data collection, Julie Yee and Jim Baldwin for statistical advice, and Tony Caprio for fire history information. The paper benefited from comments by Karen Phillips,

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    This article was written and prepared by U.S. Government employees on official time and is therefore in the public domain and not subject to copyright.

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    Present address: National Park Service, Southeast Utah Group, 2282 South Resource Blvd., Moab, UT 84532, USA.

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