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Fuel load and tree density changes following prescribed fire in the giant sequoia-mixed conifer forest: the first 14 years of fire effects monitoring
Abstract
Understanding both short- and long-term effects of prescribed fire is critical for successful fire management. Fire effects monitoring results are used to assess fire management objective achievement for Sequoia and Kings Canyon National Parks. Forest floor fuel load and tree density in a network of 0.1 hectare permanent plots are monitored prior to and following prescribed fires in the parks. In the giant sequoia-mixed conifer forest, total fuel load was reduced by 71% immediately following prescribed fire; the duff component was reduced by 93% while woody fuels were reduced by 56%. Woody fuel load nearly doubled in the 10 years following prescribed fire, slightly exceeding prefire woody fuel load. This increase was a result of fire burning in dense, prefire stands, killing branches and small understory trees that later fell to the forest floor. Duff accumulated at a slower rate, reaching only 28% of prefire levels 10 years after fire. After 10 years, total fuel load reached 75% of prefire levels, indicating subsequent fires may be needed as soon as 10-years postfire to further reduce fuels. After much of the woody debris downed after the initial fires is consumed, successive fires for fuel reduction may then need to be less frequent, as reduced stand density should result in less tree mortality to contribute to forest floor fuels. Prefire tree species composition was generally dominated by white fir (Abies concolor (Gordon & Glend.) Lindley), especially in the smaller diameter classes, demonstrating white fir's relative success where fire has been excluded for at least the last 80 years. White fir, red fir, and total tree densities (three dominant species combined) were reduced following prescribed fire. Giant sequoia (Sequoiadendron giganteum (Lindley) Buchholz) relative density increased, with a marked increase in young giant sequoias 10 years following prescribed fire.
... Fire has been an integral part of Sierra Nevada ecosystems for thousands of years; it reduces flammable surface fuels, thins forest trees, stimulates sprouting of shrubs and other hardwoods, releases seeds, prepares seedbeds favorable for germination of serotinous cones, recycles nutrients, and influences insect and disease populations (Kilgore 1972;Vankat and Major 1978;Haase and Sackett 1998;Keifer 1998). The "natural" fire regime of Sequoia and Kings Canyon National Park (SEKI) in the past was one of variable severity across multiple vegetation types and topography. ...
... Sequoia and Kings Canyon National Parks are located in Tulare and Fresno Counties, California in the southern Sierra Nevada (Keifer 1998 Keifer et. al. 1999). ...
... al. 1999;Caprio & Graber 2000). Soils are generally coarse textured, acidic and soil depth ranges from shallow to very deep (Keifer 1998;Keifer et. al. 2000). ...
... Monitoring is a critically important part of the restoration process and provides the input necessary to evaluate ecosystem functioning (Keifer and Stanzler 1995;Keifer 1998;Keifer and others 2000a;Mutch and Parsons 1998;Haase and Sackett 1998). Many ecological, sociological, and political considerations will influence the decision regarding the acceptability of ecosystem function. ...
... Knowledge of intra-stand variation in natural fire regimes will add to our ability to manage forests with the appropriate level of gap structure. Gaps are critical to the regeneration of certain species in Sierran forests, for example, Pinus ponderosa and Sequoiadendron giganteum (Kilgore and Biswell 1971;Mutch and Swetnam 1995;Keifer 1998;Stephens and others 1999). Gaps play two critical roles in the regeneration of these species -they provide a suitable site for seedling recruitment and, because of the absence of mature trees, fuels accumulate more slowly (fig. ...
A conceptual model of fire and forest restoration and maintenance is presented. The process must begin with clearly articulated goals and depends upon derivation of science-driven models that describe the natural or desired conditions. Evaluating the extent to which contemporary landscapes depart from the model is a prerequisite to determining the need for restoration. Model landscapes that include the historical range of variability are commonly used as target conditions in setting restoration objec- tives. Restoration is a corrective step that ultimately must be replaced by a maintenance process. In a world of changing climate, structural targets of historical conditions will become progressively less meaningful to ecosystem maintenance. Future fire manage- ment needs to focus more on fire as a process, in particular as it pertains to proper ecosystem functioning. One area in need of much further research is the critical role of gap formation in forest regeneration. Forests of the Sierra Nevada in California, like other western coniferous forests, have had ecosystem processes greatly disturbed by fire management practices of the 20 th century. This impact has been repeatedly documented through historical studies of fire frequencies revealed in the annual growth rings of fire-scarred trees. These dendrochro- nology studies show a high frequency of fire prior to Euroamerican settlement, with fires in many mid-elevation forest stands occurring at intervals of roughly every 5-25 years (fig. 1). The fact that these estimates are based upon trees that have persisted through repeated fires demon- strates that the pre-Euroamerican fire regime was one of low intensity/severity fires over a significant portion of the landscape. Beginning in the latter half of the 19 th century, fire frequency declined and throughout the 20 th century,
... 14, No. 5 FIG. 4. Elasticities for unburned sugar pine populations over different observation periods. For each size class, each cluster of bars, reading left to right, represents the following years of observation: for Suwanee Creek, 1983 Creek, –1988 Creek, , 1988 Creek, – 1994 Creek, , 1994 Creek, –1999 for Crystal Road, 1992 for Hodgedon Meadows, 1991–2001 for Crane Creek, 1993–1998 0.09, df 1, P 0.76, n 968). Instead, stem size appeared to be positively related to the presence of blister rust attack as shown by logistic regression relating the most recent live tree diameter to the presence of blister rust symptoms in the 2001 survey (likelihood ratio test statistic 20.2, df 1, P 0.0001, n 1193). ...
... Prescribed fire is a widely used forest-thinning tool, although protecting sugar pine populations while reintroducing fire after more than 100 years of fuels accumulation could present challenges. Post-fire mortality rates for sugar pine were high, but this response to fire is not unusual compared to other species in the Sierra Nevada (Keifer 1998, van Mantgem et al. 2003). Large individuals were resistant to damage caused by fire, in agreement with earlier work on sugar pine and other conifer species (Ryan and Reinhardt 1988, Stephens and Finney 2002). ...
An introduced pathogen, white pine blister rust (Cronartium ribicola), has caused declines in five-needled pines throughout North America. Simultaneously, fire ex- clusion has resulted in dense stands in many forest types, which may create additional stress for these generally shade-intolerant pines. Fire exclusion also allows fuels to accu- mulate, and it is unclear how affected populations will respond to the reintroduction of fire. Although white pine blister rust and fire exclusion are widely recognized threats, long- term demographic data that document the effects of these stressors are rare. We present population trends from 2168 individuals over 5-15 years for an affected species, sugar pine (Pinus lambertiana), at several burned and unburned sites in the Sierra Nevada of California. Size-based matrix models indicate that most unburned populations have negative growth rates (l range: 0.82-1.04). The growth rate of most populations was, however, indistinguishable from replacement levels ( l5 1.0), implying that, if populations are indeed declining, the progression of any such decline is slow, and longer observations are needed to clearly determine population trends. We found significant differences among population growth rates, primarily due to variation in recruitment rates. Deaths associated with blister rust and stress (i.e., resource competition) were common, suggesting significant roles for both blister rust and fire exclusion in determining population trajectories. Data from 15 prescribed fires showed that the immediate effect of burning was the death of many small trees, with the frequency of mortality returning to pre-fire levels within five years. In spite of a poor prognosis for sugar pine, our results suggest that we have time to apply and refine management strategies to protect this species.
... Wagtendonk,J.W. and Buhler,M. Fire Ecology Vol.2,No.1 accumulation results are also similar to earlier results with a smaller subset of the same giant sequoia-mixed conifer forest monitoring plot data which showed the ten-year post-fire mean total fuel load reached 75 percent of pre-fire levels for 7 plots (Keifer 1998) and 63 percent of pre-fire levels for 12 plots (Keifer and Manley 2002). The relatively high woody fuel component ten years post-fire is likely due to the large number of understory trees killed by fire in the white-fir and giant sequoia-mixed conifer forests (64 percent and 62 percent, respectively; Table 2) that fell to the forest floor during the ten year period following fire. ...
... These stand structure changes are also consistent with earlier results reported with smaller subsets of the giant sequoia-mixed conifer f orest monitoring plot data that showed most of the 40-50 percent density reduction resulting from prescribed fire generally occurred in trees less than 40 cm in diameter (Keifer 1998, Keifer et al. 2000. Twenty years after a second prescribed fire treatment, i n three giant sequoia-mixed conifer plots, woody fuels, as well as litter and duff, were somewhat less than that in unburned areas. ...
After nearly a century of fire exclusion in many central and southern Sierra Nevada mixed- conifer forests, dead and down surface fuels have reached high levels without the recurring fires that consume the accumulated organic matter. The effects of prescribed fires used to reduce fuel loads and restore fire have been monitored in Yosemite, Sequoia, and Kings Canyon National Parks for over 30 years. Ten years following prescribed fire treatments in Sequoia and Kings Canyon, mean total fuel loads accumulated to 84 percent of pre -fire levels in ponderosa pine forests, 83 percent in white fir-mixed conifer, and 66 percent in giant sequoia-mixed conifer forests. Thirty-one years after burning, mean fuel load of fine and sound woody fuels increased in ponderosa pine research plots in Yosemite. Most fuel accumulation appeared to occur within the first decade after fire, however the post-fire fuel complex was different than that pre-fire. In areas that have remained unburned, surface fuel accumulation appears to be relatively slow and may indicate that decay rates have approached accumulation rates for the mixed-conifer forest types. This long-term information has important implications for fire management planning, including scheduling fuel hazard reduction and subsequent maintenance treatments.
... bly higher than what might have been present without fire suppression. The average of 191.6 Mg/ha of fuel found prior to the prescribed burns in this study was greater than fuel loadings reported for second growth and old-growth mixed conifer forests in northern portions of the Sierra Nevada by Kauffman and Martin (1989) (range, 74.8– 163.9 Mg/ha). Keifer (1998) estimated the amount of pre-burn fuel to be 143.5 Mg/ha in several plots of mixed conifer/giant sequoia forest in Sequoia National Park that hadn't burned in over 40 years. A nearby mixed conifer that had also not experienced fire since pre-settlement times contained 210 Mg/ha of fuel (Mutch and Parsons, 1998), which is comparable to le ...
... Slopes of the litter and duff depth to weight regression relationships developed for this study were very similar to the estimates reported by van Wagtendonk et al. (1998) for white fir (litter: 9.88 versus 10.05 for this study and van Wagtendonk et al. (1998), respectively; duff: 14.85 versus 15.18 for this study and van Wagtendonk et al. (1998), respectively), helping to validate the accuracy of both sets of numbers. The 88% reduction in fuel mass recorded in the late season burn treatment was comparable to levels of consumption seen in other fires in mixed conifer forests conducted under dry fall conditions (Kauffman and Martin, 1989; Kilgore, 1972; Mutch and Parsons, 1998), slightly lower than the 91% fuel reduction reported for a dry early fall prescribed fire on a nearby southeast-facing slope in the same watershed (Stephens and Finney, 2002), and somewhat greater than an average consumption of 71% for multiple prescribed fires conducted under a range of fuel moisture conditions in Sequoia National Park (Keifer, 1998). Fuel reduction in the early season burns (67%), while still substantial, was within the range of values reported by Kauffman and Martin (1989) for late spring burns in Sierran mixed conifer forest (61–83%). ...
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.
... However, the outcomes of the monitoring and evaluation activities carried out by management agencies are rarely reported to a wider audience. Literature on the subject (Czuhai and Cushwa, 1968;Burrows, 1987;Keifer, 1998;James, 1999) is, consequently, very scarce and often sketchy, in spite of the decisive contribution that such information could give to the contemporary debates focused on the relevance of prescribed burning as a land management tool (Morrison et al., 1996;Bradstock et al., 1998;Baeza et al., 2002;Keeley, 2002). ...
The ignition of low-intensity fires in the dormant season in the pine stands of north-western Portugal seeks to reduce the existing fuel hazard without compromising site quality. The purpose of this study is to characterise this practice and assess its effectiveness, based on information resulting from the normal monitoring process at the management level, and using operational guidelines, fire behaviour models and a newly developed method to classify prescribed fire severity.
Although the region’s humid climate strongly constrains the activity of prescribed fire, 87% of the fires analysed were undertaken under acceptable meteorological and fuel moisture conditions. In fact, most operations achieved satisfactory results. On average, prescribed fire reduces by 96% the potential intensity of a wildfire occurring under extreme weather conditions, but 36% of the treated sites would still require heavy fire fighting resources to suppress such fire, and 17% would still carry it in the tree canopy. Only 10% of the prescribed burns have an excessive impact on trees or the forest floor, while 89% (normal fire weather) or 59% (extreme fire weather) comply with both ecological integrity maintenance and wildfire protection needs. Improved planning and monitoring procedures are recommended in order to overcome the current deficiencies.
... [3] In the Sierra Nevada, a century of fire suppression has resulted in fuel loads that exceed historical levels [Knapp et al., 2005], has increased the potential for more severe crown fires [Keeley and Stephenson, 2000;Stephenson et al., 1991], and has reduced opportunities for seedling recruit-ment by some of the forest dominants [Keifer, 1998;Stephenson, 1996]. Alterations in fire regime are implicated in the decline of one of the more majestic Sierran trees, sugar pine (Pinus lambertiana Dougl.) ...
The effects of prescription burning on watershed balances of major ions
in mixed conifer forest were examined in a 16-year paired catchment
study in Sequoia National Park, California. The objective was to
determine whether fire-related changes in watershed balances persist as
long as estimated low-end natural fire-return intervals (≤10 years),
and whether cumulative net export caused by fire could deplete nutrient
stocks between successive fires. Inputs (wet + dry deposition) and
outputs (stream export) of N, S, Cl-,
HCO3-, Ca2+, Mg2+,
Na+, K+, H+, and SiO2 were
measured for 7 years preceding, and 9 years following, a prescribed burn
of one of the catchments. After fire, runoff coefficients increased by
7% (in dry years) to 35% (in wet years). Inorganic N was elevated in
stream water for 3 years after fire. Increased export of water,
SO42-, Cl-, SiO2, and base
cations continued through the end of the study. Pools and processes
attributed to fire led to the cumulative loss, per hectare, of 1.2 kg N,
16 kg S, 25 kg Cl-, 130 kg Ca2+, 19 kg
Mg2+, 71 kg Na+, 29 kg K+ and 192 kg
Si, above that predicted by prefire regression equations relating export
in the paired catchments. This additional export equaled <1% of the
N, up to one-third of the Ca and Mg, and up to three-fourths of the K,
contained in the forest floor prior to combustion. Changes in watershed
balances indicated that low-end natural fire-return intervals may
prevent complete reaccumulation of several elements between fires.
... In the American Southwest researchers have found low-intensity prescribed fire to be relatively ineffective for selectively removing small trees (Sackett et al. 1996; Fulé et al. 2002), which has translated into recommendations for forest restoration that emphasize mechanical thinning (Covington 2000 ). In contrast, prescription burning in the Sierra Nevada has been successful at reducing small-tree densities (Keifer 1998), and prescribed fire without mechanical treatments remains a feasible forest-thinning tool (Stephenson 1999 ). Our results suggest that systematic differences in stem damage, perhaps mediated by differences in fuels characteristics, could provide a partial explanation of the variable mortality responses between these regions. ...
We subjected 159 small ponderosa pine (Pinus ponderosa Dougl. ex P. & C. Laws.) to treatments designed to test the relative importance of stem damage as a predictor of postfire mortality. The treatments consisted of a group with the basal bark artificially thinned, a second group with fuels removed from the base of the stem, and an untreated control. Following prescribed burning, crown scorch severity was equivalent among the groups. Postfire mortality was significantly less frequent in the fuels removal group than in the bark removal and control groups. No model of mortality for the fuels removal group was possible, because dead trees constituted <4% of subject trees. Mortality in the bark removal group was best predicted by crown scorch and stem scorch severity, whereas death in the control group was predicted by crown scorch severity and bark thickness. The relative lack of mortality in the fuels removal group and the increased sensitivity to stem damage in the bark removal group suggest that stem damage is a critical determinant of postfire mortality for small ponderosa pine.
... measures fuels and forest structure within small-scale plots (≤0.1 ha), usually with only a single plot established within each burned area (Paul Reeberg, National Park Service, personal communication). Besides its potential to support satellite-based observations of fire effects (Key 2006), these data might be best used when individual plot data are assembled together across a particular vegetation type to offer a broad picture of fire effects (e.g., Keifer 1998). ...
Fire is an important feature of many forest ecosystems, although the quantification of its effects is compromised by the large scale at which fire occurs and its inherent unpredictability. A recurring problem is the use of subsamples collected within individual burns, potentially resulting in spatially autocorrelated data. Using subsamples from six different fires (and three unburned control areas) we show little evidence for strong spatial autocorrelation either before or after burning for eight measures of forest conditions (both fuels and vegetation). Additionally, including a term for spatially autocorrelated errors provided little improvement for simple linear models contrasting the effects of early versus late season burning. While the effects of spatial autocorrelation should always be examined, it may not always greatly influence assessments of fire effects. If high patch scale variability is common in Sierra Nevada mixed conifer forests, even following more than a century of fire exclusion, treatments designed to encourage further heterogeneity in forest conditions prior to the reintroduction of fire will likely be unnecessary.
... Proponents for mechanical thinning as a precursor to the reintroduction of fire have noted that it may be difficult for fire alone to reduce the abnormally dense cohort of firs that have grown since fire exclusion has occurred ( Stone, 1978, 1982). However, several other authors have reported successes reducing trees in this cohort using fire as their only management tool (Kilgore, 1973; van Wagtendonk, 1983; Keifer, 1998). Our results suggest that it will become increasingly difficult to remove these trees using fire alone, although presently there has probably been insufficient time for firs to become resistant to cambial damage from most prescribed fires. ...
An essential component to models of fire-caused tree mortality is an assessment of cambial damage. Cambial heat resistance has been traditionally measured in large overstory trees with thick bark, although small trees have thinner bark and thus are more sensitive to fire. We undertook this study to determine if current models of bark heat transfer are applicable to small trees (<20 cm diameter at breast height (dbh)). We performed this work in situ on four common species in the mixed conifer forests of the Sierra Nevada, California.The allometric relationship between bole diameter and bark thickness for each species was linear, even for very small trees (5 cm dbh). Heating experiments demonstrated that bark thickness was the primary determinant of cambial heat resistance. We found only slight, but statistically significant, among species differences in bark thermal properties. Our most significant finding was that small trees were more resistant to heating than expected from commonly used models of bark heat transfer. Our results may differ from those of existing models because we found smaller trees to have a greater proportion of inner bark, which appears to have superior insulating properties compared to outer bark. From a management perspective, growth projections suggest that a 50-year fire-free interval may allow some fire intolerant species to achieve at least some degree of cambial heat resistance in the Sierra Nevada.
... Most of the reduction is in firs 30 cm diameter, and especially in firs 10 cm diameter (those trees that occur in abnormally dense thickets resulting from fire exclusion). The relative density of sequoias 1.4 m tall has more than tripled in 10 yr following prescribed fire, mostly at the expense of white fir (Keifer 1998), thus pushing grove structure toward pre-Euro-American reference conditions (Stephenson 1996). Additionally, a recent computer model of Sierran mixed-conifer forest dynamics (excluding giant sequoias) suggests that, following a century of fire exclusion, reintroduction of fire alone will immediately begin to restore forest basal area, spatial structure, and species composition (Miller 1998, Miller and Urban 2000). ...
National Park Service policy directs that more natural conditions be restored to giant sequoia groves, which have been altered by a century of fire exclusion. Efforts to find a reasonable and practical definition of "natural" have helped drive scientists and land managers to use past grove conditions as reference conditions for restoration. Extensive research aimed at determining reference conditions has demonstrated that past fire regimes can be characterized with greater precision than past grove structures. Difficulty and imprecision in determining past grove structure has helped fuel a debate between "structural restorationists," who believe that forest structure should be restored mechanically before fire is reintroduced, and "process restorationists," who believe that simple reintroduction of fire is appropriate. I evaluate old and new studies from sequoia groves to show that some of the arguments of both groups have been flawed. Importantly, it appears that restoration of fire without a preceding mechanical restoration may restore the pre-Euro-American structure of sequoia groves, at least within the bounds of our imprecise knowledge of past grove structure. However, the same may not be true for all forest types that have experienced lengthy fire exclusion. Our ability to draw robust generalizations about fire's role in forest restoration will depend heavily on a thorough understanding of past and present interactions among climate, fire, and forest structure. Use of reference conditions will be central to developing this understanding.
Twenty-one vegetation types occurring in Sequoia National Park are discribed and their changes under western man's influence are documented. Age-population structure of the trees, repeated old photographs, and historical descriptions were the primary evidence for recording and interpreting vegetation changes. These changes are strongly correlated with past land uses. Nineteenth-century livestock grazing is considered to be the primary factor in changes in the blue oak foothill woodland, lodgepole pine and subalpine forests, and various grassland vegetation types. Twentieth-century fire suppression is the primary factor in most changes in the various shrublands and other woodlands and forests. Vegetation changes include increases of introduced species in herb-dominated systems, and increases in cover and density of certain woody plants in tree and shrub-dominated vegetation types.