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Predicting Douglas-fir Sapling Mortality Following Prescribed Fire in an Encroached Grassland

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Tree encroachment in fire-maintained woodlands and grasslands is a major management concern, yet little infor-mation exists regarding the mechanisms of small tree mortality following prescribed burns. We sought to clarify the relative importance of tree size and fire-induced injury in the post-fire mortality of encroaching Douglas-fir trees and to compare results with an existing mortality model for larger Douglas-fir trees. Crown injury to small Douglas-fir trees was a significant explanatory variable in post-fire mortality models, with results suggesting a 20% thresh-old in crown scorch. Crown injury was strongly related to bole injury, and delayed mortality was important as we documented new mortality 20 months post-burn. Mortality models for large Douglas-fir tend to over-predict small tree mortality, underscoring the need to better understand the mechanisms of fire-caused mortality for small, encroaching trees.
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SHORT COMMUNICATION
Predicting Douglas-fir Sapling Mortality Following
Prescribed Fire in an Encroached Grassland
Eamon A. Engber1,2and J. Morgan Varner1
Abstract
Tree encroachment in fire-maintained woodlands and
grasslands is a major management concern, yet little infor-
mation exists regarding the mechanisms of small tree mor-
tality following prescribed burns. We sought to clarify the
relative importance of tree size and fire-induced injury in
the post-fire mortality of encroaching Douglas-fir trees and
to compare results with an existing mortality model for
larger Douglas-fir trees. Crown injury to small Douglas-
fir trees was a significant explanatory variable in post-fire
mortality models, with results suggesting a 20% thresh-
old in crown scorch. Crown injury was strongly related
to bole injury, and delayed mortality was important as we
documented new mortality 20 months post-burn. Mortality
models for large Douglas-fir tend to over-predict small tree
mortality, underscoring the need to better understand the
mechanisms of fire-caused mortality for small, encroaching
trees.
Key words: crown scorch, grassland restoration, post-fire
tree mortality, tree invasion.
Introduction
Fire influences the structure and composition of savanna and
woodland ecosystems worldwide (Bond & Keeley 2005), and
the encroachment of woody species in these ecosystems in
the absence of fire can proceed rapidly (Staver et al. 2011).
Encroachment results in reduced herbaceous diversity and fuel
mass (Devine et al. 2007; Engber et al. 2011), and crown
retreat and mortality of remnant savanna trees (Devine &
Harrington 2006; Spector & Putz 2006). Recent work in
North America has addressed mowing, burning, and herbicide
applications to restore plant community composition (Ansley
& Castellano 2006; Dunwiddie & Bakker 2011), yet gaps exist
in the understanding of treatment effects on small trees.
Prescribed fire is a common restoration treatment in Pacific
Northwest, U.S. grassland ecosystems encroached by Douglas-
fir (Pseudotsuga menziesii ; Hamman et al. 2011), yet mecha-
nisms of fire-caused mortality of small (<3 m tall, hereafter
saplings) saplings are poorly understood. Sapling size has been
the only factor evaluated in past Douglas-fir encroachment
studies (Gruell et al. 1986; Sugihara & Reed 1987; Tveten &
Fonda 1999), with larger saplings surviving more than smaller
saplings. In contrast, mortality models for tree-size Douglas-
fir have incorporated fire-caused injury to the crown and bole
(e.g. Ryan & Reinhardt 1988; Hood 2010), thereby addressing
the underlying mechanisms of mortality. Further models for
1Department of Forestry, Box 9681, Mississippi State, MS 39762-9681, U.S.A.
2Address correspondence to E. A. Engber, e-mail eamon_engber@nps.gov
©2012 Society for Ecological Restoration
doi: 10.1111/j.1526-100X.2012.00900.x
saplings are needed given the frequency with which managers
use prescribed fire in their treatment.
To address these needs, we evaluated the importance of
Douglas-fir size and fire-induced injury, patterns of delayed
mortality, and relationships between crown and bole injury fol-
lowing fire. Our results were intended to increase understand-
ing of Douglas-fir sapling mortality and provide a framework
for related restoration efforts.
Methods
The study site was a 40 ha grassland/woodland (hereafter
Eastside) within the Bald Hills of Redwood National Park.
Eastside is on an upper, NE-facing aspect (20.3% average
slope) at an elevation of circa 865 m. Surface fuel loading
averaged 5.6 (±2.2) Mg/ha and consisted primarily of
herbaceous fuels (81.3%), the remainder shrubs and ferns.
Restoration treatments were initiated by conifer harvest in
2002 (hand cutting, piling, and burning), yet subsequent
encroachment was substantial, with circa 250 saplings/ha
established by fall 2009. In response, managers ignited a
prescribed burn on 1 October 2009 using strip and spot
ignition patterns, with 0.32 m flame lengths.
Three months preceding the prescribed burn, we tagged and
measured height and diameter at ground level (DAG) on 100
Douglas-fir saplings (0.53 m tall) in a heavily encroached
portion of the burn unit. Bark thickness was measured
on 19 saplings adjacent to tagged saplings to establish a
relationship between bark thickness and DAG (r2=0.80)
for use in a mortality model for mature Douglas-fir (Ryan
& Reinhardt 1988). Three weeks post-burn, percent crown
volume scorched (PCVS; discolored foliage and buds) and
Restoration Ecology 1
Post-Fire Mortality of Small Douglas-fir
Figure 1. Differences in crown injury (PCVS and PCVC) and tree size (diameter at ground level and height) across living and dead Douglas-fir saplings
5.5, 9, and 20 months following a prescribed burn.
percent crown volume consumed (PCVC; charred foliage
and buds) were estimated on all saplings. Twenty saplings
(from the original 100) were randomly selected for cambium
assessment; bark was peeled to reveal underlying cambium
injury [living or dead, from Wagener (1961)]. Maximum
height of cambium injury was recorded (CImax). Tree mortality
was assessed 5.5, 9, and 20 months post-burn.
Regression analyses were conducted to assess effects of
sapling size on crown injury and crown injury on bole
injury. Two-sample t-tests were used to compare sapling
size and crown injury across dead and living saplings 20
months post-burn. Variables differing significantly between
dead and living trees were investigated further in a binary
logistic regression model. Appropriate transformations were
employed if assumptions of statistical tests were not met using
untransformed data (Zar 1999).
Binary logistic regression was conducted to assess the
effects of crown injury on mortality probability for the final
mortality assessment. Logistic models were developed for each
of the three dates based on PCVS; probability curves for our
saplings were compared with a logistic mortality model for
mature Douglas-fir (Ryan & Reinhardt 1988).
Results
The prescribed fire resulted in relatively high PCVS, averaging
81.7 (±23.9)% on the 100 saplings. Values for PCVC were
much lower, averaging 39.3%, with considerable variation
(coefficient of variation =75%). Regression analyses revealed
weak, nonsignificant associations between tree size and crown
injury (all r0.20). CImax was moderately associated with
PCVS (r=0.67, p=0.001) but strongly associated with
PCVC (r=0.94, p<0.001).
Cumulative post-fire mortality was 47, 74, and 94% at
5.5, 9, and 20 months post-burn, respectively. No significant
differences in tree size were found between dead and living
trees 20 months post-burn (p>0.92), though small trees
died earlier (Fig. 1); therefore, sapling size was excluded from
logistic regression models. Crown injury differed significantly
between dead and living saplings (p<0.002), with dead
saplings suffering circa 40– 50% (absolute) greater crown
injury. PCVS and PCVC were both strong mortality predictors
(Fig. 2), with the change in deviance slightly larger for PCVC
(Table 1).
Discussion
Although fire is a common restoration treatment in tree-
invaded grasslands and savannas (Pyke et al. 2010), small trees
have been under-represented in post-fire mortality studies. This
study highlights the efficacy of prescribed fire in halting the
encroachment of woody species in grasslands and points to
important variables that can increase accuracy of predictive
mortality models for small trees.
2Restoration Ecology
Post-Fire Mortality of Small Douglas-fir
Figure 2. Post-burn mortality probabilities for Douglas-fir saplings, by
PCVS and months post-burn. Mortality probability predictions from the
Ryan and Reinhardt (1988) model (developed for larger Douglas-fir) are
also shown.
Table 1. Logistic regression mortality parameters (β), change in
deviance, and p-values for term significance (p) for Douglas-fir saplings
based on fire-caused crown scorch (PCVS; model 1) and consumption
(PCVC; model 2).
Parameter
Model β0β1X1Deviance p
12.3419 5.1938 PCVS 12.49 <0.001
2 0.5960 6.9354 PCVC 14.23 <0.001
Both PCVS and PCVC were arcsine-square root transformed prior to analysis.
Crown injury is a well-recognized post-fire mortality predic-
tor for many species (Hood 2010), and our work supports its
importance for small Douglas-fir. If we consider 50% mortality
probability a cutoff for tree death, results suggest PCVS above
20% is sufficient to kill trees, while any measurable crown
consumption results in mortality probabilities exceeding 50%.
Interestingly, Ryan and Reinhardt’s (1988) mortality model
predicts a much lower PCVS threshold for our trees (circa
10%). Future work could investigate whether this discrepancy
relates to differential injury tolerance between saplings and
mature Douglas-fir trees. The weak correlation between crown
injury and sapling size was unexpected but may be explained
by limited sapling sizes and heterogeneity in fire behavior.
Bole injury is a well-recognized predictor of post-fire
Douglas-fir large tree mortality (Hood et al. 2008), and the
substantial cambium injury observed on destructively sampled
saplings in this study may help explain fire-caused mortality.
The strong correlation between crown and cambium injury
suggests that a single measurement may suffice to estimate
post-burn mortality in these scenarios. Battaglia et al. (2009)
found that tree size, crown injury, and bole char all improve
mortality predictions for ponderosa pine (Pinus ponderosa)
saplings. These differences suggest that we may need to
reassess traditional tree mortality models and focus research on
small trees where their eradication is a common management
objective.
Implications for Practice
Fire-induced crown injury to Douglas-fir saplings is a
significant mortality predictor and should be incorpo-
rated into models used in the planning of restoration
treatments.
Small tree mortality monitoring should occur for at least
20 months post-burn to capture delayed mortality and
increase accuracy of predictive models.
Commonly used mortality models for large Douglas-fir
trees tend to over-predict sapling mortality, so managers
should consider using alternative models in the planning
of restoration treatments.
Acknowledgments
Managers at Redwood National Park (J. McClelland and R.
Young) provided site history and access. We are grateful to
M. Battaglia, L. Arguello, C. Edgar, N. Sugihara, D. Sarr, and
two Restoration Ecology reviewers for comments on an earlier
version of the manuscript. Field assistance was provided by
L. Quinn-Davidson, M. Cocking, E. Banwell, and P. Cigan.
Funding was provided by the National Park Service Klamath
Inventory & Monitoring Network and USDA McIntire-Stennis
Cooperative Forestry Research Program.
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4Restoration Ecology
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... Mixtures of pine and black oak within a stand are thought to increase forest resilience to fire (Skinner, 1995). Black oak litter is highly flammable and promotes rapidly spreading, low-intensity, surface fire (Engber and Varner, 2018), and black oak in mixed stands breaks up crown fuel continuity thereby reducing crown fire spread (Skinner et al., 2018a). The tree group and gap structure is also thought to be important for regeneration and persistence of black oak, particularly large diameter trees, as establishment and growth of this species benefit from high-light environments (Long et al., 2016). ...
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Knowledge of how tree groups and gaps are formed and maintained in frequent-fire forests is key to managing for heterogeneous and resilient forest conditions. This research quantifies changes in tree group and gap spatial structure and abundance of ponderosa pine (Pinus ponderosa) and California black oak (Quercus kelloggii) with stand development after wildfires in 1990 and 1994 in an old-growth forest in the Ishi Wilderness, southern Cascades, California. Forest demography and tree group and gap structure were quantified by measuring, mapping, and aging trees in six 1-ha permanent plots in 2000 and 2016. Tree recruitment, mortality, and growth were estimated using demographic models and spatial characteristics including gap structure were identified using an inter-tree distance algorithm and the empty space function. Potential fire behavior and effects in 2016 were estimated to determine if the current forest would be resilient to a wildfire in the near future. Stand density and basal area in both 2000 and 2016 resembled reference conditions for pre-fire suppression frequent-fire forests in the western United States. Wildfires initially promoted California black oak regeneration via sprouting, but oak regeneration from seed declined relative to ponderosa pine over the post-fire period. In 2000, ∼15% of trees were classified as single tree groups and an average tree group had 6 trees (range 2-38) which increased to 9 trees (range 2-240) in 2016. Small groups (2-4 trees) had similar-aged trees while larger groups were multi-aged. By 2016, single tree groups decreased by ∼30%, and the size, density, and intensity of clustering within tree groups increased, with an average tree group size of 9 (range 2-240) in 2016. Rates of post-fire regeneration, particularly the high rate for ponderosa pine, drove spatial dynamics in tree group and gap structure. Although the size and frequency of canopy gaps were similar in 2000 and 2016, the density of seedlings and saplings in gaps was higher in 2016, and large gaps were being fragmented by gap filling. Potential fire behavior predicted surface fire and low overstory tree mortality, suggesting the current forest would be resilient to a wildfire. However, burning will be necessary in the future to reduce the demographic pressure of ponderosa pine, promote black oak, and to maintain and create future spatial heterogeneity. Cumulatively, this research demonstrates that wildland fires under certain conditions can maintain and restore fire resilience in ponderosa pine forests reducing the negative ecological consequences related to past fire exclusion.
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The invasion, or “encroachment”, of native conifers commonly occurs in the absence of frequent fire in deciduous woodlands and grasslands of the Pacific Northwest, USA. To effectively target restoration activities, managers require a better understanding of the outcomes of prescribed fire and the spatial patterns of conifer invasions. We examined the duration of prescribed fire effectiveness for controlling conifer invasions, as well as multiple site characteristics (including distance to potential seed trees, prescribed fire history, and topographic variables) that influenced conifer invasions following fire in grassland and oak woodland communities in the Bald Hills of Redwood National Park, California. Prescribed fire substantially reduced counts of small conifers (< 0.91 m in height), but reinvasion was rapid for sites ≤75 m from the forest edge, returning to pre‐fire levels by 2 years post‐fire. Following prescribed fires the presence of conifers was largely determined by proximity of overstory trees, with more than 95% of conifer seedlings (stems <1.37 m in height) found within 44 m of an overstory conifer. Number of fires and years since the most recent fire were not strongly related to counts of conifer seedlings and density of conifer saplings (stems from 0.1 to 10 cm diameter at breast height, 1.37 m). Our results suggest that in the Bald Hills vulnerability to conifer invasion is principally a function of proximity to seed sources, and the frequent application of prescribed fire or surrogate treatments are needed to prevent conifer seedlings from attaining fire‐resistant sizes. This article is protected by copyright. All rights reserved.
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Each year wildland fires kill and injure trees on millions of forested hectares globally, affecting plant and animal biodiversity, carbon storage, hydrologic processes, and ecosystem services. The underlying mechanisms of fire-caused tree mortality remain poorly understood, however, limiting the ability to accurately predict mortality and develop robust modeling applications, especially under novel future climates. Virtually all post-fire tree mortality prediction systems are based on the same underlying empirical model described in Ryan and Reinhardt (1988 Can. J. For. Res. 18 1291–7), which was developed from a limited number of species, stretching model assumptions beyond intended limits. We review the current understanding of the mechanisms of fire-induced tree mortality, provide recommended standardized terminology, describe model applications and limitations, and conclude with key knowledge gaps and future directions for research. We suggest a two-pronged approach to future research: (1) continued improvements and evaluations of empirical models to quantify uncertainty and incorporate new regions and species and (2) acceleration of basic, physiological research on the proximate and ultimate causes of fire-induced tree mortality to incorporate processes of tree death into models. Advances in both empirical and process fire-induced tree modeling will allow creation of hybrid models that could advance understanding of how fire injures and kills trees, while improving prediction accuracy of fire-driven feedbacks on ecosystems and landscapes, particularly under novel future conditions.
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This report synthesizes the literature and current state of knowledge pertaining to reintroducing fire in stands where it has been excluded for long periods and the impact of these introductory fires on overstory tree injury and mortality. Only forested ecosystems in the United States that are adapted to survive frequent fire are included. Treatment options that minimize large-diameter and old tree injury and mortality in areas with deep duff and methods to manage and reduce duff accumulations are discussed. Pertinent background information on tree physiology, properties of duff, and historical versus current disturbance regimes are also discussed.
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Oregon white oak (or Carry oak, Quercusgarryana) woodlands and savannas of the coastal Pacific Northwest are legacies of an anthropogenic fire regime that ended with European settlement in the mid-1 800s. Historically, these oak stands had a sparse overstory and an understory dominated by fire-tolerant grasses and forbs. Post-settlement fire suppression resulted in widespread invasion and subsequent overstory dominance by conifers, causing mortality of shade-intolerant oak trees and shifting understory plant communities to shade-tolerant species. In a study on four southwestern Wash- ington sites, our objective was to determine the effects of overstory conifer removal, primarily Douglas-fir (Pseudotsuga rnenziesii), on microclimate, native and non-native understory cover, and sapling growth. Overstory conifer removal created a warmer, drier understory microclimate during summer months. Conifer removal had little effect on native understory cover during five years post-treatment; however, cover of non-native plants, primarily grasses and woody understory species, increased significantly during the same period. Height growth of Oregon white oak and Douglas-fir saplings exhibited a delayed, but positive, response to overstory conifer removal, although the treatment response of Douglas-fir was 133% greater than that of oak. Increases in non-native understory cover and the rapid growth of young Douglas-fir indicate the importance of pre- and post-treatment understory management to control undesirable plants and promote native species such as Oregon white oak.
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In Pacific Northwest prairies and oak woodlands, cessation of anthropogenic burning in the mid-1800s resulted in large-scale degradation and loss of habitat due to tree and shrub encroachment. Widespread invasive species, deep thatch accumulations, and extensive moss cover now limit the ability of native plants to germinate and thrive. These changes in habitat structure and function have contributed to the decline of several plant and animal species. Over the past decade, prescribed fire has been increasingly applied throughout the Willamette Valley-Puget Trough-Georgia Basin Ecoregion and used in conjunction with other techniques (herbicide, seeding native species) to restore native habitat with variable results. This variability likely is a result of differential fire intensity, dictated by fuels, weather and application technique, all of which can be controlled for by altering fire season, fire frequency, pre-fire treatments and fire extent. In order to burn at the spatial and temporal scales necessary for effective habitat restoration, however, prescribed burn programs must overcome several socio-political, programmatic and economic challenges. This requires a collaborative approach to prescribed fire training, implementation and research. Future research on fire season, fire frequency, species-specific responses to fire and effects of fire surrogates on ecosystem structure and functioning will help to refine prescribed fire management for maximum effectiveness in prairie and oak woodland restoration.
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