Foliar Moisture Content Research Articles

Evaluating Predictive Models of Tree Foliar Moisture Content for Application to Multispectral UAS Data: A Laboratory Study

Water supply is a critical component of tree physiological health, influencing a tree’s photosynthetic activity and resilience to disturbances. The climatic regions of the western United States are particularly at risk from increasing drought, fire, and pest interactions. Existing methods for quantifying drought stress and a tree’s relative resilience against disturbances mostly use moderate-scale (20–30 m) multispectral satellite sensor data. However, tree water status (i.e., water stress) quantification using sensors like Landsat and Sentinel are error-prone given that the spectral reflectance of pixels are a mixture of the dominant tree canopy, surface vegetation, and soil. Uncrewed aerial systems (UAS) equipped with multispectral sensors could potentially provide individual tree water status. In this study, we assess whether the simulated band equivalent reflectance (BER) of a common UAS optical multispectral sensor can accurately quantify the foliar moisture content and water stress status of individual trees. To achieve this, water was withheld from groups of Douglas-fir and western white pine saplings. Then, measurements of each sapling’s foliar moisture content (FMC) and spectral reflectance were converted to BER of a consumer-grade multispectral camera commonly used on UAS. These bands were used in two classification models and three regression models to develop a best-performing FMC model for predicting either the water status (i.e., drought-stressed or healthy) or the foliar moisture content of each sapling, respectively. Our top-performing models were a logistic regression classification and a multiple linear regression which achieved a classification accuracy of 96.55% and an r2 of 82.62, respectively. These FMC models could provide an important tool for investigating tree crown level water stress, as well as drought interactions with other disturbances, and provide land managers with a vital indicator of tree resilience.

Conifer encroachment increases foliar moisture content in a northwestern California oak woodland

Background Foliar moisture content influences crown fire ignition and behaviour. Some spatial variation in foliar moisture is attributable to differences in stand conditions but evidence describing this role is lacking or contradicting. Aims To examine the role of stand conditions on tree physiology and foliar moisture content in Oregon white oak (Quercus garryana) and Douglas-fir (Pseudotsuga menziesii). Methods We monitored foliar moisture content in both species, and tree physiology (stomatal conductance and leaf water potential) in oak, across three stand conditions, including intact stands (unencroached), stands invaded by Douglas-fir (encroached), and thinned stands with Douglas-fir removed. Key results Encroached stands had higher foliar moisture content than intact or thinned stands. Higher stand density was associated with higher foliar moisture content in both species and foliage ages. Encroached stands also had higher midday leaf water potential compared with intact or thinned stands. Conclusions These findings provide strong evidence that stand conditions and thinning treatments can influence foliar moisture content in Oregon white oak ecosystems, with likely implications for other ecosystems. Implications Better understanding of the role of stand conditions on foliar moisture content may contribute to improved spatial and temporal prediction of foliar moisture content and modelling of potential crown fire behaviour.

Decoupling between soil moisture and biomass drives seasonal variations in live fuel moisture across co-occurring plant functional types

BackgroundWildfires are important global disturbances influencing ecosystem structure and composition. The moisture content of living and senescent plant components are key determinants of wildfire activity, yet our understanding of how seasonal fluctuations in water availability and biomass control live foliar moisture content (LFMC) across co-occurring plant functional types is limited in diverse forested landscapes.ResultsWe recorded root-zone volumetric water content (VWC) and sampled leaf mass area (LMA) and LFMC of three co-occurring plant functional types across six field sites. We used a linear mixed effects model to quantify the drivers of LFMC and understand whether LFMC dynamics were coupled or decoupled from site conditions. Both LMA and VWC were significant predictors of LFMC variability (p < 0.001), although the strength and direction of these relationships varied across functional types. LFMC dynamics of understorey plant functional types were strongly coupled to site conditions, where the site random effect explained 44.2% (shrub) and 74.8% (herb) of the variability in LFMC across the season respectively. In contrast, overstorey plants were decoupled from site conditions, which explained only 8.1% of the variability in LFMC.ConclusionsLFMC of understorey plants responded to changes in soil water availability (VWC), while overstorey trees responded to biomass fluctuations (LMA). We present a conceptual model describing the influence of these factors on LFMC, which aligns with our findings and draws on the broader literature. This knowledge and conceptual approach can be used to improve our ability to characterize seasonal LFMC variation across different plant functional types, in turn improving our capacity to predict wildfire risk.

Crown fuel consumption in Canadian boreal forest fires

Predictive crown fuel consumption models were developed using empirical data from experimental burning projects. Crown fuel load for foliage, bark, branchwood and stemwood were calculated for live overstorey and understorey trees in each plot using nationally derived tree biomass algorithms. Standing dead tree branchwood and stemwood biomass were similarly calculated. Crown bulk density values were calculated for all non-stemwood fuel components. Factors that affect the initiation and spread of crown fires (live crown base height, foliar moisture content, surface fuel consumption, critical surface fire spread rate, critical surface fire intensity) and components of the Canadian Forest Fire Weather Index System were not statistically significant variables. Crown bulk density was moderately correlated with crown fuel consumption but was not an influential factor. A new crown fuel consumption model was developed by regression analysis using fuel load of overstorey tree foliage and standing dead tree branchwood, and fire rate of spread through crown fraction burned. A simpler model was developed using only overstorey tree foliage fuel load and fire rate of spread. Both models provide forest and fire management agencies with enhanced ability to determine crown fuel consumption, fire behaviour and carbon emissions in boreal fires using basic forest inventory or biomass/carbon datasets.

Changes in Telfaria occidentalis leaf morphology, quality and phytochemical composition under different local preservation regimes in Nigeria

Leafy vegetables have short shelf life due to their succulent nature. This study assesses local preservation methods to infer the most suitable and prevalent approach for increasing the shelf life of Telfaria occidentalis. Common post-harvest preservation of T. occidentalis includes refrigeration, wrapping in banana leaves, exposure on bare floor, drying on rooftops, and mixing with leaves of Vernonia amygdalina. Daily physical observations and moisture content determination were done for ten days after exposure to the local preservation conditions. Results suggest that wrapping T. occidentalis leaves with plantain leaves was the most prevalent preservation method in the study area. However, this method had the highest rate of T. occidentalis leaf degradation (which began after 3 days) compared to the other methods employed in the study. In terms of moisture content and ferulic acid composition, refrigeration was observed to be the most suitable preservation method. It maintained foliar moisture content of > 60% for eight days, whereas moisture content was < 25.5% on the fourth day in the other preservation method assessed. Similarly ferulic acid content in the refrigerated samples (147.44 mg/ml) was highest at 10 days after, compared to the baseline value (42.87 mg/ml) and those of leave samples on bare floor (56.95 mg/ml). In conclusion, local preservation methods like drying and exposure on rooftops and bare floor, respectively, should be supported to increase the availability of dried T. occidentalis leaves while electric power supply required for refrigeration need to be improved.

Foliar Moisture Content from the Spectral Signature for Wildfire Risk Assessments in Valparaíso-Chile.

Fuel moisture content (FMC) proved to be one of the most relevant parameters for controlling fire behavior and risk, particularly at the wildland-urban interface (WUI). Data relating FMC to spectral indexes for different species are an important requirement identified by the wildfire safety community. In Valparaíso, the WUI is mainly composed of Eucalyptus Globulus and Pinus Radiata—commonly found in Mediterranean WUI areas—which represent the 97.51% of the forests plantation inventory. In this work we study the spectral signature of these species under different levels of FMC. In particular, we analyze the behavior of the spectral reflectance per each species at five dehydration stages, obtaining eighteen spectral indexes related to water content and, for Eucalyptus Globulus, the area of each leave—associated with the water content—is also computed. As the main outcome of this research, we provide a validated linear regression model associated with each spectral index and the fuel moisture content and moisture loss, per each species studied.

Post-fire fuel succession in a rare California, USA, closed-cone conifer

BackgroundIncreasing frequency and size of wildfires over the past few decades have prompted concerns that populations of obligate seeding species may be vulnerable to repeat, short-interval fires that occur prior to these species reaching maturity. The susceptibility of populations to this risk is partially dependent on the amount and characteristics of fuel loading over time and their influence on fire behavior and effects. This study characterized fuel dynamics and modeled fire behavior across a time-since-fire chronosequence in stands of the rare, serotinous conifer, Baker cypress (Hesperocyparis bakeri [Jeps.] Bartel), ranging in age between 3 and 147 years post fire.ResultsLitter and fine woody fuel loading (1- to 100-hour) were highest in the 10-year-old and 147-year-old stands, while coarse fuel loading (1000-hour) peaked in the 10-year-old stand and subsequently decreased with time since fire. Duff loading consistently increased with time since fire. Cone production had not occurred in the first 10 yr of stand development. Foliar moisture content in Baker cypress was inversely correlated with stand age, and older foliage had lower moisture content than younger foliage. Modeled surface fire behavior was highest in the 10-year-old and 107-year-old stands in accordance with higher litter, fine woody fuel, or shrub fuel accumulation. While foliar moisture content was higher in younger stands and influenced the critical fireline intensity, we did not observe changes in fire type.ConclusionsFine-fuel loading in Baker cypress stands followed a U-shaped pattern over time (first decreasing, then stable, then increasing), consistent with findings in other forests with stand-replacing fire regimes. Our results indicated that early-successional stages of Baker cypress forests have sufficient fuels to allow for the spread of wildfire and 10-year-old stands could burn with substantive fire behavior prior to cone production. Whenever possible, we recommend suppressing wildfire in stands less than 20 yr old to avoid substantial decreases or local extirpation of these rare Baker cypress populations. Our results highlight the importance of knowing the cone production patterns, fuel dynamics, and corresponding fire behavior over the development of obligate-seeder species to assess the risk of population loss due to short-interval fires.

Experimental investigation of flammability and numerical study of combustion of shrub of rockrose under severe drought conditions

Structure of vegetation significantly influences its flammability and resulting fire spread. Despite considerable amount of laboratory studies, experimental works carried out with full plant specimens, representative of field conditions, are still limited. Present study aims to collect meaningful experimental data on structure and flammability of shrub of rockrose and evaluate the predictions of a fire model (WFDS) against this dataset. Spatial distribution of fuel elements, sorted according to their characteristic thickness, was established from destructive measurements. 28 fire tests were conducted with full plants under a calorimeter. Foliar moisture content was in the range of 4–18% on dry basis. Radiant panels were used as source of ignition. Flammability was investigated using ignitability, sustainability, combustibility and consumability. Comparison to previous studies highlighted the necessity of standardization among test procedures. Principal component analysis revealed four flammability regimes depending on proportion of thin fuel elements within the crown, position of ignition and duration of preheating. Finally, combustion dynamics of a shrub was numerically investigated with WFDS. A bulk density model was developed from the characterization study and used as input data for the numerical code. Predicted HRR was in good agreement with experiments, although simulation results need improvement in initiation phase of burning.

Relative importance of climate and mountain pine beetle outbreaks on the occurrence of large wildfires in the western USA.

Extensive outbreaks of bark beetles have killed trees across millions of hectares of forests and woodlands in western North America. These outbreaks have led to spirited scientific, public, and policy debates about consequential increases in fire risk, especially in the wildland-urban interface (WUI), where homes and communities are at particular risk from wildfires. At the same time, large wildfires have become more frequent across this region. Widespread expectations that outbreaks increase extent, severity, and/or frequency of wildfires are based partly on visible and dramatic changes in foliar moisture content and other fuel properties following outbreaks, as well as associated modeling projections. A competing explanation is that increasing wildfires are driven primarily by climatic extremes, which are becoming more common with climate change. However, the relative importance of bark beetle outbreaks vs. climate on fire occurrence has not been empirically examined across very large areas and remains poorly understood. The most extensive outbreaks of tree-killing insects across the western United States have been of mountain pine beetle (MPB; Dendroctonus ponderosae), which have killed trees over >650,000km2 , mostly in forests dominated by lodgepole pine (Pinus contorta). We show that outbreaks of MPB in lodgepole pine forests of the western United States have been less important than climatic variability for the occurrence of large fires over the past 29years. In lodgepole pine forests in general, as well as those in the WUI, occurrence of large fires was determined primarily by current and antecedent high temperatures and low precipitation but was unaffected by preceding outbreaks. Trends of increasing co-occurrence of wildfires and outbreaks are due to a common climatic driver rather than interactions between these disturbances. Reducing wildfire risk hinges on addressing the underlying climatic drivers rather than treating beetle-affected forests.

Seasonal variations in red pine (Pinus resinosa) and jack pine (Pinus banksiana) foliar physio-chemistry and their potential influence on stand-scale wildland fire behavior

The ‘Spring Dip’ in conifer live foliar moisture content (LFMC) has been well documented but the actual drivers of these variations have not been fully investigated. Here we span this knowledge gap by measuring LFMC, foliar chemistry, foliar density and foliar flammability on new and old foliage for an entire year from both Pinus resinosa (red pine) and Pinus banksiana (jack pine) at a site in Central Wisconsin. We found that needle dry mass increased by up to 70% in just three weeks and these increases were manifested as strong seasonal variations in foliar moisture content and foliar density. These needle dry mass changes were driven by an accumulation of starch in old foliage, likely resulting from springtime photosynthesis onset, and also by accumulations of sugar and crude fat in new needles as they fully matured. Foliar starch, sugar and crude fat content accounted for 84% of the variation in foliar density across both species. Flammability differences were also strongly related to changes in foliar density, where density accounted for 39% and 25% of the variations in foliar time-to-ignition of jack pine and red pine respectively. Finally, we use the computational fluid dynamics-based wildland fire model FIRETEC to examine how these foliar physio-chemical changes may influence wildland fire behavior. Under the lowest canopy density and windspeed, simulated fires in dormant condition stands did not propagate as crown fires while spring dip stands successfully spread as crown fires as a result of the higher potential energy content of the canopy. Simulated wildland fire spread rates increased by as much as 63%, nominal fireline width increased by as much as 89% and active fire area more than doubled relative to dormant season fuel conditions and the most significant changes occurred in areas with low canopy cover and low within-tree bulk density. Our results challenge the assumption that live conifer foliage flammability is limited only by its water content; this study suggests a new theory and an expanded view of the factors that dominate live fuel flammability and that subsequently influence larger scale wildland fire behavior.

Laboratory experiments to estimate interception of infrared radiation by tree canopies

Estimates of biomass-burning in wildfires or prescribed fires are needed to account for the production of trace gases and aerosols that enter the atmosphere during combustion. Research has demonstrated that the biomass consumption rate is linearly related to fire radiative power (FRP), and that total biomass consumed is linearly related to fire radiative energy (FRE). Measurement of these is biased by certain characteristics of a forest canopy, such as foliar moisture content and tree canopy cover. Laboratory experiments were conducted to assess the influence of canopy cover on the FRP observed from an overhead sensor (e.g. an aircraft or satellite). A range of canopy cover from 0 to 90% and two classes of canopy (non-transpiring living and desiccated branches) were used in the experiments. Experiments suggest that in cases of complete or nearly complete canopy closure, fires obscured by the canopy may be below the detection threshold of above-canopy FRP sensors. Results from this research will reduce uncertainties in estimates of biomass consumption in surface fires burning under forest canopies.

Seasonal relationships between foliar moisture content, heat content and biochemistry of lodgepole line and big sagebrush foliage

Wildland fires propagate by liberating energy contained within living and senescent plant biomass. The maximum amount of energy that can be generated by burning a given plant part can be quantified and is generally referred to as its heat content (HC). Many studies have examined heat content of wildland fuels but studies examining the seasonal variation in foliar HC among vegetation types are severely lacking. We collected foliage samples bi-weekly for five months from two common species in the western USA: lodgepole pine (Pinus contorta Douglas ex Loudon) and big sagebrush (Artemisia tridentata Nutt). We measured HC, live fuel moisture content (LFMC) and biochemical components in the leaf dry mass. Our results showed that HC increased for both species, coinciding with LFMC decrease during the growing season. Measured HC values were higher than the constant value in standard fuel models. Lasso regression models identified biochemical components for explaining temporal HC and LFMC variation in lodgepole pine (HC: R2adj = 0.55, root mean square error (RMSE) = 0.35; LFMC: R2adj = 0.84, RMSE = 10.79), sagebrush (HC: R2adj = 0.90, RMSE = 0.13; LFMC: R2adj = 0.96, RMSE = 7.66) and combined data from both species (HC: R2adj = 0.77, RMSE = 0.33; LFMC: R2adj = 0.61, RMSE = 19.75). These results demonstrated the seasonal change in HC and LFMC resulted from temporal biochemical composition variation in dry mass. This new knowledge about HC seasonal change will ultimately lead to improved predictions of wildland fire spread and intensity.

Measuring foliar moisture content with a moisture analyzer

Near-instantaneous estimation of the moisture content of live fuels is complicated because of the large control exerted by physiological mechanisms. The commonly accepted reference method for measuring fuel moisture content is oven drying, which is time consuming. This study evaluates the use of a moisture analyzer (ML-50, A&amp;D Company, Limited, Tokyo, Japan) for measuring the foliar moisture content of two common European species. The moisture of live leaves of Arbutus unedo L. (strawberry tree) and Quercus robur L. (pedunculate oak) was measured within a period of 15 min using two drying temperatures and compared with the oven-dried value. Correction factors were determined for estimating the oven-dried moisture content based on the measurement by the moisture analyzer. The power delivered during the drying process plays an important role in the moisture measured by the analyzer in relation to the oven-dried value. Increasing the drying time beyond the minimum period necessary for obtaining a reliable prediction of the oven-dried moisture does not significantly change the moisture measured at lower temperatures. The moisture analyzer is able to estimate the live foliage moisture content with high accuracy.

Modeling spatial and temporal dynamics of wind flow and potential fire behavior following a mountain pine beetle outbreak in a lodgepole pine forest

Patches of live, dead, and dying trees resulting from bark beetle-caused mortality alter spatial and temporal variability in the canopy and surface fuel complex through changes in the foliar moisture content of attacked trees and through the redistribution of canopy fuels. The resulting heterogeneous fuels complexes alter within-canopy wind flow, wind fluctuations, and rate of fire spread. However, there is currently little information about the potential influence of different rates and patterns of mortality on wind flow and fire behavior following bark beetle outbreaks. In this study, we contrasted within-canopy wind flow and fire rate-of-spread (ROS) at two different ambient wind speeds using FIRETEC for two differing bark beetle attack trajectories for a lodgepole pine (Pinus contorta) forest. These two attack trajectories represent different realizations of a bark beetle outbreak and result in different amounts and patterns of mortality through time. Our simulations suggested that the mean within-canopy wind velocities increased through time following the progression of mortality. In addition, we found that for a given level of mortality, a bark beetle outbreak that resulted in a higher degree of aggregation of canopy fuels had greater mean within-canopy wind velocities due to the channeling of wind flow. These findings suggest that bark beetle mortality can influence the mean within-canopy wind flow in two ways: first, by reducing the amount of vegetation present in the canopy acting as a source of drag; and second, by altering spatial patterns of vegetation that can lead to channeling of wind flow. Changes in the fire rate-of-spread were positively related to the level and continuity of bark beetle mortality. Peak rates of spread were between 1.2 and 2.7 times greater than the pre-outbreak scenario and coincided with a high level of mortality and minimal loss of canopy fuels. Following the loss of canopy fuels the rate of fire spread declined to levels below the initial phases of the outbreak in low wind speed cases but remained above pre-outbreak levels in high wind speed cases. These findings suggest that the rate and pattern of mortality arising from a bark beetle outbreak exerts significant influence on the magnitude and timing of alterations to the within-canopy wind flow and rate of fire spread. Our findings help clarify existing knowledge gaps related to the effect of bark beetle outbreaks on fire behavior and could explain potential differences in the reported effects of bark beetle outbreaks on fire behavior through time.

De-coupling seasonal changes in water content and dry matter to predict live conifer foliar moisture content

Live foliar moisture content (LFMC) significantly influences wildland fire behaviour. However, characterising variations in LFMC is difficult because both foliar mass and dry mass can change throughout the season. Here we quantify the seasonal changes in both plant water status and dry matter partitioning. We collected new and old foliar samples from Pinus contorta for two growing seasons and quantified their LFMC, relative water content (RWC) and dry matter chemistry. LFMC quantifies the amount of water per unit fuel dry weight whereas RWC quantifies the amount of water in the fuel relative to how much water the fuel can hold at saturation. RWC is generally a better indicator of water stress than is LFMC. We separated water mass from dry mass for each sample and we attempted to best explain the seasonal variations in each using our measured physiochemical variables. We found that RWC explained 59% of variation in foliar water mass. Additionally, foliar starch, sugar and crude fat content explained 87% of the variation in seasonal dry mass changes. These two models combined explained 85% of the seasonal variations in LFMC. These results demonstrate that changes to dry matter exert a stronger control on seasonal LFMC dynamics than actual changes in water content, and they challenge the assumption that LFMC variations are strongly related to water stress. This methodology could be applied across a range of plant functional types to better understand the factors that drive seasonal changes in LFMC and live fuel flammability.

Foliar moisture content variations in lodgepole pine over the diurnal cycle during the red stage of mountain pine beetle attack

Widespread outbreaks of the mountain pine beetle (Dendroctonus ponderosae Hopkins) in the lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) forests of North America have produced stands with significant levels of recent tree mortality. The needle foliage from recently attacked trees typically turns red within one to two years of attack indicating successful colonization by the beetle and tree death. Attempts to model crown fire potential in these stands have assumed that the moisture content of dead foliage responds similarly to changes in air temperature and relative humidity as other fine, dead surface fuels. However, this assumption has not been verified. In this exploratory study we sampled the moisture content of dead foliage on an hourly basis through two different diurnal cycles during the fire season and compared the results to measurements of 10-h fuel moisture indicator sticks and predictions made from models used to estimate dead fuel moisture in the USA, Canada, and Australia. The observed degree of variation in dead foliar moisture content was small (6.9–14.5%) with a mean value of ∼10%. All existing models performed poorly, but measurements of 10-h fuel moisture and a modified version of an existing model where timelags were extended to ∼20-h had the best fit to the data. The results from our study suggest that the dead foliage on attacked trees does not respond similarly to changing environmental conditions as other fine, dead surface fuels as has been assumed. This in turn has important implications for wildland fire suppression operations, including firefighter safety, and in modeling fire behavior, and solicits the need for further research.

Wildfire Heterogeneity: Empirical vs. Simulated Observations—The Carmel 2010 Wildfire as a Case Study

Three general factors affect wildfire behavior and outcome: vegetation properties, weather patterns during the wildfire and topography. A heterogeneous landscape emerges from the interactions among these factors in open and natural areas following wildfires. Herein, an attempt is made to analyze the spatial heterogeneity of the Carmel 2010 fire using aerial photographs and fire spread simulation models. Additionally, simulation models were used to assess the sensitivity of fire properties to variations in foliar moisture content and wind speed. Image analysis results indicate that broadleaved communities in the Carmel region are less flammable compared to pine stands, resulting in larger areas of undamaged vegetation within the extent of the burned area. Once a vegetation patch is burned, however, the proportion of the area severely affected by the fire is not fundamentally different, with 64.6% of the trees severely damaged in the pine stands, compared to 60.5% in the broadleaved stands. Simulation resul...

Corrigendum to: Assessing the effect of foliar moisture on the spread rate of crown fires

This paper constitutes a digest and critique of the currently available information pertaining to the influence of live fuel or foliar moisture content (FMC) on the spread rate of crown fires in conifer forests and shrublands. We review and discuss the findings from laboratory experiments and field-based fire behaviour studies. Laboratory experimentation with single needles or leaves and small conifer trees has shown an unequivocal effect of FMC on flammability metrics. A much less discernible effect of FMC on crown fire rate of spread was found in the existing set of experimental crown fires carried out in conifer forests and similarly with the far more robust database of experimental fires conducted in shrubland fuel complexes. The high convective and radiant heat fluxes associated with these fires and the lack of appropriate experimental design may have served to mask any effect of FMC or live fuel moisture on the resulting spread rate. Four theoretical functions and one empirical function used to adjust rate of fire spread for the effect of foliar or live fuel moisture were also concurrently examined for their validity over a wide range of FMC conditions with varying outcomes and relevancy. None of these model functions was found suitable for use with respect to dead canopy foliage.

Assessing the effect of foliar moisture on the spread rate of crown fires

This paper constitutes a digest and critique of the currently available information pertaining to the influence of live fuel or foliar moisture content (FMC) on the spread rate of crown fires in conifer forests and shrublands. We review and discuss the findings from laboratory experiments and field-based fire behaviour studies. Laboratory experimentation with single needles or leaves and small conifer trees has shown an unequivocal effect of FMC on flammability metrics. A much less discernible effect of FMC on crown fire rate of spread was found in the existing set of experimental crown fires carried out in conifer forests and similarly with the far more robust database of experimental fires conducted in shrubland fuel complexes. The high convective and radiant heat fluxes associated with these fires and the lack of appropriate experimental design may have served to mask any effect of FMC or live fuel moisture on the resulting spread rate. Four theoretical functions and one empirical function used to adjust rate of fire spread for the effect of foliar or live fuel moisture were also concurrently examined for their validity over a wide range of FMC conditions with varying outcomes and relevancy. None of these model functions was found suitable for use with respect to dead canopy foliage.

Comments

Simard et al. (2011) have produced a comprehensive data set and analysis concerning mountain pine beetle (MPB; Dendroctonus ponderosae)-caused mortality and associated crown fire feedbacks in lodgepole pine (Pinus contorta)-dominated forests. Misapplication of the NEXUS fire modeling system (Scott and Reinhardt 2001) results in the suspect conclusion that active crown fire (perpetuation of flames through the canopy) probability is reduced in all post-mortality stages. Simard et al. (2011) assert that the loss of canopy fuel following tree mortality overwhelms the concomitant loss of foliar moisture content (FMC) but do not fully account for this drop in moisture or the resulting increase in surface fuels. Here, we show how to account for decreases in FMC and increases in surface fuels within NEXUS and report findings contrary to Simard et al.’s (2011) conclusions for the red stage (dead needles still within canopy). Overall, NEXUS is a questionable choice for this research due to its inherent lack of crown fire predictive capability (Cruz and Alexander 2010) and empirically derived crown fire models developed using living canopies (Van Wagner 1977, Rothermel 1991, Scott and Reinhardt 2001). NEXUS and related fire modeling systems (e.g., FARSITE, FlamMap, BehavePlus, and FFE-FVS) rely on the integration of Rothermel’s (1972) surface and crown (1991) fire spreadmodels with VanWagner’s (1977, 1993) crown fire transition and propagation models (Scott 2006, Cruz and Alexander 2010). The fire modeling community recognizes the need for calibration of model input variables and parameters, including custom surface fuel models, which Simard et al. (2011) employ, to achieve accurate representations of observed, and thus predicted, fire behavior. Without calibration, quantitative fire behavior output values can be wildly unrealistic, as large under-prediction biases are prevalent (Cruz and Alexander 2010). For example, necessitating 1000 km/h wind speeds to initiate crown fire in undisturbed lodgepole pine forests (Simard et al. 2011). However, relative comparisons of disturbed and undisturbed forests’ fire behavior are possible when full consideration is given to the primary drivers of fire behavior.