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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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Predicting Douglas-fir Sapling Mortality Following
Prescribed Fire in an Encroached Grassland
Eamon A. Engber1,2and J. Morgan Varner1
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
Key words: crown scorch, grassland restoration, post-fire
tree mortality, tree invasion.
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
©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.
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).
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).
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).
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
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
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.
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
... Douglas-fi r leaves are short and fall individually resulting in compact litter (Engber et al. 2011). Tanoak litter, however, is deeper and less dense promoting good fl ammability when dry (Engber and Varner 2012a). ...
... Resistance to low-intensity surface fi res increases with size because of increased bark thickness (Plumb and McDonald 1981). Dried tanoak leaves are very fl ammable in comparison to most California oaks (Engber and Varner 2012a). ...
... Fires probably spread in cured herbaceous fuels and oak litter. The litter of Oregon oak and California black oak are among the most fl ammable of California oak species (Engber and Varner 2012a). There is no information on fi re sizes in this vegetation type but they probably were diverse depending on annual variation in climate. ...
... The FOFEM RA and FOFEM SP models were not parameterised with small-diameter stems (,10 cm diameter at breast height, DBH; there are two exceptions -the lower diameter limit was 8 cm for Pseudotsuga menziesii, Ryan and Reinhardt 1988; and 6 cm for 'yellow pine', Hood and Lutes 2017), and validation of these models with small stems is rare (but see Engber andVarner 2012 andKane et al. 2017). Some studies have developed logistic mortality models specifically for smalldiameter stems (e.g. ...
... The FOFEM RA and FOFEM SP models were not parameterised with small-diameter stems (,10 cm diameter at breast height, DBH; there are two exceptions -the lower diameter limit was 8 cm for Pseudotsuga menziesii, Ryan and Reinhardt 1988; and 6 cm for 'yellow pine', Hood and Lutes 2017), and validation of these models with small stems is rare (but see Engber andVarner 2012 andKane et al. 2017). Some studies have developed logistic mortality models specifically for smalldiameter stems (e.g. ...
... They influence future fire behaviour and act as ladder fuels, and they are ecologically important as components of the understory (re-sprouters) or advanced regeneration (surviving conifers). Previous studies have found that crown scorch influences mortality differently for small-diameter stems compared with mature trees (Kolb et al. 2007;Engber and Varner 2012), and this interaction may compromise the accuracy with which FOFEM predicts mortality for small-diameter trees. ...
Post-fire tree mortality models are vital tools used by forest land managers to predict fire effects, estimate delayed mortality and develop management prescriptions. We evaluated the performance of mortality models within the First Order Fire Effects Model (FOFEM) software, and compared their performance to locally-parameterised models based on five different forms. We evaluated all models at the individual tree and stand levels with a dataset comprising 34 174 trees from a mixed-conifer forest in the Sierra Nevada, California that burned in the 2013 Rim Fire. We compared stand-level accuracy across a range of spatial scales, and we used point pattern analysis to test the accuracy with which mortality models predict post-fire tree spatial pattern. FOFEM under-predicted mortality for the three conifers, possibly because of the timing of the Rim Fire during a severe multi-year drought. Locally-parameterised models based on crown scorch were most accurate in predicting individual tree mortality, but tree diameter-based models were more accurate at the stand level for Abies concolor and large-diameter Pinus lambertiana, the most abundant trees in this forest. Stand-level accuracy was reduced by spatially correlated error at small spatial scales, but stabilised at scales $1 ha. The predictive error of FOFEM generated inaccurate predictions of post-fire spatial pattern at small scales, and this error could be reduced by improving FOFEM model accuracy for small trees.
... Once maintained by frequent surface fires, these ecosystems have been fire excluded for many decades, allowing encroachment by surrounding conifers, primarily Douglas-fir (Pseudotsuga menziesii), that were previously limited by fire (Thysell and Carey, 2001). Seedlings and small trees now reach a size with thick enough bark that can withstand fire (Engber and Varner, 2012). Over time Douglas-fir are capable of growing up through the oak crowns (i.e. ...
... However, it is important to state that some of these saplings may represent advanced regeneration that were released following the girdling treatment. Regardless of the age of these trees, managers can expect that Douglas-fir saplings may be present 10 years after girdling, a size that is often more difficult to kill with the application of prescribed fire (Engber and Varner, 2012). Thus, managers would want to reintroduce fire well before this growth transition to the sapling stage. ...
The prolonged absence of fire in Oregon white oak (Quercus garryana) woodlands and savannas of the Pacific Northwest has resulted in substantial conifer encroachment over the past century. Use of low intensity prescribed burns often lacks sufficient intensity to kill larger encroached trees, requiring alternative approaches. In the Bald Hills region of Redwood National Park, managers have implemented girdling treatments to kill Douglas-fir (Pseudotsuga menziesii) over the past two decades with the objective of recovering remnant oaks and restoring historical woodland area. We surveyed 258 girdled Douglas-fir to examine the effectiveness of girdling treatments to create snags and the impacts of girdling on tree regeneration and fuel recruitment over a 17 year time since girdling chronosequence. Girdling was successful in killing 91% of the treated Douglas-fir independent of the method used (axe or chainsaw). Larger trees with a low girdle width-to-tree diameter ratio tended to survive girdling. Trees with a girdle width-to-tree diameter ratio ≥1 were most effective at killing trees. Snags generated through girdling decayed over time but did not significantly reduce in height over the time period examined. Bird activity was observed on 19% of snags across the chronosequence but 88% of 10 y old snags had signs of bird foraging. Fine woody fuel loading and fuelbed depths were potentially elevated 7 years after treatment but did not persist through the time period examined. Regeneration of Douglas-fir and oaks were highly variable and did not seem to be clearly linked to girdling activity. Our results indicate that girdling is a highly effective treatment to kill Douglas-fir and aid restoration of Oregon oak woodlands with limited negative impacts on surface fuel hazards or regeneration.
... By some estimates, Oregon white oak woodlands and associated prairies have declined due to encroachment by at least 30% from their historical extent (Crawford and Hall, 1997;Fritschle, 2008). Low intensity prescribed fire is a commonly used method to maintain and improve conditions (Underwood et al., 2003;Hamman et al., 2011), but are sometimes ineffective at killing invading Douglas-fir (Engber and Varner, 2012). The reduced effectiveness of prescribed fire is in part due to reduced herbaceous fuel loads and higher dead surface fuel moisture that lowers fire intensity in encroached stands (Engber et al., 2011), prompting managers to remove Douglas-fir by thinning or girdling treatments to improve oak vigor and restore oak woodlands (Devine and Harrington, 2013;Kane et al., 2019). ...
Full-text available
Fuel moisture exerts considerable influence on fire behavior and effects but varies substantially spatially and temporally. A portion of the spatial variation in dead surface fuel moisture of woodland and forest ecosystems is likely attributed to differences in stand conditions (i.e. stand density and composition). Existing evidence suggests that the role of stand conditions on dead surface fuel moisture is inconsistent across studies and more research examining the effects of stand conditions on seasonal microclimate and fuel moisture is needed. This study examined the effects of different stand conditions within Oregon white oak (Quercus garryana) woodlands and forests, including intact oak-dominated stands without Douglas-fir (Pseudotsuga menziesii), oak stands that have been invaded by Douglas-fir (encroached), and thinned stands with Douglas-fir removed within Oregon white oak ecosystems of northern California on seasonal variation of microclimate and dead surface fuel moisture. Stand condition had a strong effect on microclimate and fuel moisture over time with thinned stands consistently having warmer and drier conditions with lower fuel moisture than encroached stands in most surface fuel components. Differences in fuel moisture among stand conditions were most pronounced in the late spring and early fall. Observed fuel moisture values did not correlate well with estimates from remotely automated weather stations or process-based calculations, especially when observed fuel moisture values exceeded 20%. Collectively, these findings provide strong evidence that stand conditions can modify microclimate that contributes to variation in dead surface fuel moisture within Oregon white oak ecosystems of northern California. These differences are often not considered within commonly used fuel moisture estimates, emphasizing the need to develop better models and tools to allow managers to more accurately anticipate fuel moisture conditions in varying stands to model landscape variation in fire behavior and effects and to meet fuel reduction and restoration objectives.
... These studies and monitoring projects were designed for a range of purposes, listed in the FTM_Fires.csv file, including modelling post-fire tree mortality 6,7,15,17,18,40,42,47,48,50,58,71,74,79,[90][91][92] , understanding the effectiveness of prescribed fire at reducing fuel loading, future fire severity, restoring historical forest structure 38,39,56,57,[59][60][61]66,67,69,82,93 , tracking post-fire successional dynamics 43,45,59,62,63,90,94 , developing remote sensing indices to understand landscape fire effects 46,70,95 , carbon emission modeling 65 , plant physiological research 36,[53][54][55] , and research on interactions between fire and bark beetles 4,7,10,18,34,35,37,41,43,44,49,51,81 . The file Dataset_citations.csv provides the primary citations for each contributed dataset in the FTM database. ...
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Wildland fires have a multitude of ecological effects in forests, woodlands, and savannas across the globe. A major focus of past research has been on tree mortality from fire, as trees provide a vast range of biological services. We assembled a database of individual-tree records from prescribed fires and wildfires in the United States. The Fire and Tree Mortality (FTM) database includes records from 164,293 individual trees with records of fire injury (crown scorch, bole char, etc.), tree diameter, and either mortality or top-kill up to ten years post-fire. Data span 142 species and 62 genera, from 409 fires occurring from 1981-2016. Additional variables such as insect attack are included when available. The FTM database can be used to evaluate individual fire-caused mortality models for pre-fire planning and post-fire decision support, to develop improved models, and to explore general patterns of individual fire-induced tree death. The database can also be used to identify knowledge gaps that could be addressed in future research.
... In addition, burning-induced reductions in pine density (48.7%) were higher than in oak density (33.9%) in this study. Most previous studies focus on species-level or monoculture forests alone, which limits our ability to accurately predict changes in tree mortality across diverse forest types [88][89][90]. In fact, variations in mortality occur in coniferous and broadleaved species, but the survival of broadleaved trees is much higher than the survival of conifers in a coniferous-broadleaf mixed forest in the Mediterranean Basin [91]. ...
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Fire is the predominant natural disturbance that influences the community structure as well as ecosystem function in forests. This study was conducted to examine the soil properties, loss of aboveground biomass, and understory plant community in response to an anthropogenic fire in a coniferous (Pinus massoniana Lamb.) and broadleaf (Quercus acutissima Carruth.) mixed forest in a subtropical–temperate climatic transition zone in Central China. The results showed that soil pH, NO3--N concentration, and microbial biomass carbon (C) increased three months after the fire; however, there were no significant differences in soil organic C, total nitrogen (N), NH4+-N concentration, or microbial biomass N between the burned and unburned observed plots. The total aboveground biomass was 39.0% lower in the burned than unburned plots four weeks after fire. Direct biomass combustion (19.15 t ha−1, including understory shrubs and litters) was lower than dead wood biomass loss (23.69 t ha−1) caused by the fire. The declining trends of tree mortality with increasing diameter at breast height for both pine and oak trees suggest that small trees are more likely to die during and after fires due to the thinner bark of small trees and tree and branch fall. In addition, burning significantly stimulated the density of shrub (160.9%) and herb (88.0%), but it also affected the richness of shrub and herb compared with that in the unburned plots two months after the fire. The rapid recovery of understory plants after fires suggest that the diversity of understory species could benefit from low-severity fires. Our findings highlight that the decomposition of dead wood and understory community recovery should be considered for offsetting C emissions after fires for further research.
... To help restore Oregon white oak ecosystems, managers have increased their use of prescribed fire to control woody species, increased bare ground for native herbaceous plant establishment, and prepared sites for herbicide application to control non-native forbs and grasses (Sugihara and Reed 1987;Agee 1996;Tveten and Fonda 1999;Dennehy et al. 2011;Dunwiddie and Bakker 2011;Engber and Varner 2012;Livingston et al. 2016). Common practice aims for a fire return interval of 3 to 5 years, approaching historical fire regimes (Hamman et al. 2011;Sprenger and Dunwiddie 2011). ...
Full-text available
Pacific Northwest USA oak woodlands and savannas are fire-resilient communities dependent on frequent, low-severity fire to maintain their structure and understory species diversity, and to prevent encroachment by fire-sensitive competitors. The re-introduction of fire into degraded ecosystems is viewed as essential to their restoration, yet can be fraught with unintended negative consequences. We examined the response of mature Oregon white oak (Quercus garryana Douglas ex Hook.; Garry oak) to “first entry” woodland restoration burns following long fire-free periods. Thirteen to twenty-five months post burn, topkill of oaks was minimal (3%) and mortality was rare in three prescribed burns, despite high levels (mean = 92%) of crown scorching, and irrespective of proportional duff consumption around oak bases (mean = 21%). Percentage of crown scorch volume was the strongest predictor of oak crown dieback, but response was highly variable, especially when canopy scorch was ≥80%. Comparison of our results with FOFEM (First Order Fire Effects Model), a common fire effects model, revealed high model inaccuracy, likely due to lack of a species-specific equation for prediction of Oregon white oak mortality. The results of this study indicate that Oregon white oak is highly resistant to mortality in restoration burns, even following long fire-free intervals. Prescribed fire is not contraindicated in areas with extant mature oaks, and may promote oak regeneration via basal sprouting.
... 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). ...
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.
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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The role of fire in the maintenance of oak-dominated ecosystems is widely recognized. Fire exclusion results in structural and compositional shifts that alter fuelbed composition and structure, together influencing fire behavior and effects. To clarify the influence of overstory structure on fuels and fire intensity in oak woodlands and savannas, we examined fuelbeds across a gradient from open grassland to Douglas-fir-(Pseudotsuga menziesii (Mirb.) Franco) invaded Oregon white oak (Quercus garryana Douglas ex Hook.) woodland in the Bald Hills of Redwood National Park, California, USA. Herbaceous mass decreased markedly from a high in grasslands (3.38 Mg ha -1) to a low in invaded woodlands (0.03 Mg ha -1), whereas leaf litter and woody fuel mass increased substantially along this gradient. Mean fire temperatures at 30 cm height ranged from 74.7 °C in invaded woodland up to 207.9 °C in grassland. Highly flammable grassland and savanna communities maintain heavy herbaceous mass, but low woody mass, favoring quick-spreading, relatively high-intensity fires. The encroachment of Douglas-fir into grasslands and oak-dominated communities dampens flammability through changes in fuelbed com-position and structure (e.g., the replacement of herbaceous fuels with woody fuels), underscoring the necessity for ecological restoration efforts that focus on fuelbed structure in addition to other common restoration goals.
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Oregon white oak or Garry oak (Quercus garryana Dougl. ex Hook.) is a shade-intolerant, deciduous species that has been overtopped by conifers during the past century in parts of its range due to an altered disturbance regime. We examined the response of suppressed Oregon white oak trees in western Washington, USA, to three levels of release from overtopping Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco). We treated individual oak trees with either full release from competition, partial (“half”) release from competition, or a stand-level thinning of Douglas-fir not directed toward release (control). Five years after treatment, oak trees had suffered no mortality or windthrow. Stem diameter growth was 194% greater in the full-release treatment relative to the control. Acorn production varied widely by year, but in years of higher production, acorn production was significantly greater in both release treatments than in the control. Frequency of epicormic branch formation was significantly increased for years 1 and 2 by the full release; the greatest response occurred between 2 and 6m above ground level. The greatest number of epicormic branches formed on trees on which the majority of original limbs had died back prior to treatment. Trees with relatively less crown dieback at the time of treatment generally had greater stem growth and acorn production responses to release treatments. Our findings indicate that these released Oregon white oak trees are beginning to recover after an extended period of suppression.
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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.
Before 1800, frequent fires maintained Idaho fescue prairies and Garry oak woodlands on Fort Lewis. Fire exclusion in the 1900s, however, has allowed Scot's broom, Douglas-fir, and numerous herbaceous aliens to invade native prairies and oak woodlands. Since 1978, a management program using prescribed fires on 3-5 yr rotations has been used in an effort to maintain the open communities. We evaluated the role of fire on fescue prairies, oak woodlands, and broom thickets using prescribed fires in fall 1994 and spring 1995, and compared preburn/postburn species frequency to identify fire maintainers, increasers, and decreasers. Fall rims were more effective than spring fires, and best promoted native species and communities. Prescribed rims had no effect on Idaho fescue frequency, which maintained dominance in the postfire prairie. Other native prairie graminoids and forbs, and hairy cats-ear, a prominent alien, were maintained by fire. Prescribed fires also maintained open Garry oak woodlands, reduced Scot's broom cover in broom thickets, and killed small Douglas-firs. These fires, however, tended to favor alien species instead of native species. A large prairie subjected to >50 yr of broadcast burns ignited annually by artillery fire has been converted from fescue prairie to an open meadow dominated by hairy cats-ear and alien grasses, such as sweet vernal grass. Of the three regimes we investigated, fire intervals shorter or longer than the 3-5 yr fire rotation now employed on Fort Lewis are detrimental to fescue prairie and oak woodland. Excessive burning or fire exclusion causes loss of prairie and oak woodland.
Wildfires change plant communities by reducing dominance of some species while enhancing the abundance of others. Detailed habitat-specific models have been developed to predict plant responses to fire, but these models generally ignore the breadth of fire regime characteristics that can influence plant survival such as the degree and duration of exposure to lethal temperatures. We provide a decision framework that integrates fire regime components, plant growth form, and survival attributes to predict how plants will respond to fires and how fires can be prescribed to enhance the likelihood of obtaining desired plant responses. Fires are driven by biotic and abiotic factors that dictate their temporal (seasonality and frequency), spatial (size and patchiness), and magnitude (intensity, severity, and type) components. Plant resistance and resilience to fire can be categorized by a combination of life form, size, and ability to disperse or protect seeds. We use a combination of life form and vital plant attributes along with an understanding of fire regime components to suggest a straightforward way to approach the use of fire to either reduce or enhance particular species. A framework for aiding decisions is organized by life form and plant size. Questions regarding perennating bud and seed characteristics direct restoration practitioners to fire regimes that may achieve their management objectives of either increasing or decreasing plants with specific life form characteristics.
Woody plant encroachment has degraded grassland and savanna ecosystems worldwide by decreasing herbaceous production and diversity, and altering these physiognomies toward woodlands. This study evaluated the long-term efficacy of fire and herbicide restoration strategies used in the southern Great Plains to reduce Honey mesquite (Prosopis glandulosa) dominance, restore a grassland/savanna physiognomy, and increase herbaceous production and diversity. Three treatments were evaluated: high-intensity winter fire, aerial spray of clopyralid + triclopyr (C + T), and aerial spray of clopyralid and were compared to untreated mesquite woodland (control). Post-treatment mesquite stand physiognomy was different between fire (low mortality, high basal sprouting), C + T (high mortality, high basal sprouting of surviving plants), and clopyralid (moderate mortality, low basal sprouting of surviving plants) treatments. From 6 to 8 years post-treatment, herbaceous production was increased in C + T and clopyralid treatments but not in the fire treatment. Mesquite regrowth in the fire treatment exerted a competitive influence that limited herbaceous production. Herbaceous functional group diversity was increased in fire and C + T treatments due to a decrease in C3 perennial grass dominance and an increase in C4 perennial grasses and/or C3 forbs. Treatments that maintained mesquite overstory (control and clopyralid) had lower herbaceous diversity due to C3 perennial grass dominance and lower C4 perennial grass cover. The clopyralid treatment demonstrated greatest potential for long-term restoration of southern Great Plains savanna by reducing mesquite canopy cover to historic levels, limiting mesquite basal regrowth and increasing grass production.