Foliar Moisture Content Research Articles

Relationships between moisture, chemistry, and ignition of Pinus contorta needles during the early stages of mountain pine beetle attack

Very little is known about how foliar moisture and chemistry change after a mountain pine beetle attack and even less is known about how these intrinsic foliar characteristics alter foliage ignitability. Here, we examine the fuel characteristics and ignition potential of Pinus contorta (lodgepole pine) foliage during the early stages of a mountain pine beetle attack. Foliar samples were taken periodically from multiple trees identified as green (healthy, unattacked), recently attacked, or red (dead). The fuel moisture content, chemical composition, and time to ignition of needles from each attack category were quantified. Foliar moisture contents varied by an order of magnitude between the attack categories and were lowest for red needles (∼12% on average), highest for green needles (∼109% on average), and most variable for needles of recently attacked trees. Dry matter proportions of fiber in the needles of attacked and red trees were nearly twice that of green needles. Starch and sugar levels were much lower in the needles of attacked and red trees than green trees. Crude fat contents also differed between the attack categories. Time to ignition was strongly related to time since beetle attack. Ignition times varied from as little as 11s for red needles to 41s for green needles. A combined model of foliar moisture content, fiber, and crude fat explained 92% of the variation in the foliar time to ignition. Results show that decreased moisture contents and changes in foliar chemistry increase the foliar flammability of mountain pine beetle-attacked trees. This suggests that less heat would be required to ignite the foliage of attacked trees and thus crown fire potential may be higher in attacked stands as long as foliage is retained on the tree.

Modelling the effects of surface and crown fire behaviour on serotinous cone opening in jack pine and lodgepole pine forests

A methodology has been developed for defining the various threshold conditions required for the opening of serotinous cones and viable seed release in the overstorey canopies in jack pine (Pinus banksiana) and lodgepole pine (Pinus contorta var. latifolia) forests on the basis of fireline intensity and, in turn, rate of fire spread and fuel consumption. The extent of the effects to the overstorey canopy (i.e. crown scorch height and flame defoliation) and the type of fire (i.e. low- to high-intensity surface, intermittent crown and active crown) vary at any given fireline intensity level and are principally a function of foliar moisture content, canopy base height, stand height and canopy bulk density. The viability of the seed stored in serotinous cones of the two pine species begins to decreases once the flame-front residence time at the ground level of an active crown fire exceeds 50 s.

A comparison of two methods for estimating conifer live foliar moisture content

Foliar moisture content is an important factor regulating how wildland fires ignite in and spread through live fuels but moisture content determination methods are rarely standardised between studies. One such difference lies between the uses of rapid moisture analysers or drying ovens. Both of these methods are commonly used in live fuel research but they have never been systematically compared to ensure that they yield similar results. Here we compare the foliar moisture content of Pinus contorta (lodgepole pine) at multiple sites for an entire growing season determined using both oven-drying and rapid moisture analyser methods. We found that moisture contents derived from the rapid moisture analysers were nearly identical to oven-dried moisture contents (R2 = 0.99, n = 68) even though the rapid moisture analysers dried samples at 145°C v. oven-drying at 95°C. Mean absolute error between oven-drying and the rapid moisture analysers was low at 2.6% and bias was 0.62%. Mean absolute error was less than the within-sample variation of an individual moisture determination method and error was consistent across the range of moisture contents measured. These results suggest that live fuel moisture values derived from either of these two methods are interchangeable and it also suggests that drying temperatures used in live fuel moisture content determination may be less important than reported by other studies.

The effects of sudden oak death on foliar moisture content and crown fire potential in tanoak

The introduction of non-native pathogens can have profound effects on forest ecosystems resulting in loss of species, changes in species composition, and altered fuel structure. The introduction of Phytophthora ramorum, the pathogen recognized as causing Sudden Oak Death (SOD), leads to rapid decline and mortality of tanoak ( Lithocarpus densiflorus) in forests of coastal California, USA. We tracked foliar moisture content (FMC) of uninfected tanoaks, SOD-infected tanoaks, SOD-killed (dead) tanoaks, and surface litter for 12 months. We found that FMC values differed significantly among the three categories of infection. FMC of uninfected tanoaks averaged 82.3% for the year whereas FMC of infected tanoaks had a lower average of 77.8% (ANOVA, P = 0.04). Dead trees had a significantly lower FMC, averaging 12.3% (ANOVA, P < 0.01) for the year. During fire season (June–September), dead tanoak FMC reached a low of 5.8%, with no significant difference between dead canopy fuels and surface litter (ANOVA, P = 0.44). Application of low FMC values to a crown ignition model results in extremely high canopy base height values to escape crown ignition. Remote estimation of dead FMC using 10-h timelag fuel moisture shows a strong correlation between remote automated weather station (RAWS) 10-h timelag fuel moisture data and the FMC of dead leaves ( R 2 = 0.78, P < 0.01). Results from this study will help refine the decision support tools for fire managers in SOD-affected areas as well as conditions in other forests where diseases and insect epidemics have altered forest canopy fuels.

Assessing crown fire potential in coniferous forests of western North America: a critique of current approaches and recent simulation studies

To control and use wildland fires safely and effectively depends on creditable assessments of fire potential, including the propensity for crowning in conifer forests. Simulation studies that use certain fire modelling systems (i.e. NEXUS, FlamMap, FARSITE, FFE-FVS (Fire and Fuels Extension to the Forest Vegetation Simulator), Fuel Management Analyst (FMAPlus®), BehavePlus) based on separate implementations or direct integration of Rothermel’s surface and crown rate of fire spread models with Van Wagner’s crown fire transition and propagation models are shown to have a significant underprediction bias when used in assessing potential crown fire behaviour in conifer forests of western North America. The principal sources of this underprediction bias are shown to include: (i) incompatible model linkages; (ii) use of surface and crown fire rate of spread models that have an inherent underprediction bias; and (iii) reduction in crown fire rate of spread based on the use of unsubstantiated crown fraction burned functions. The use of uncalibrated custom fuel models to represent surface fuelbeds is a fourth potential source of bias. These sources are described and documented in detail based on comparisons with experimental fire and wildfire observations and on separate analyses of model components. The manner in which the two primary canopy fuel inputs influencing crown fire initiation (i.e. foliar moisture content and canopy base height) is handled in these simulation studies and the meaning of Scott and Reinhardt’s two crown fire hazard indices are also critically examined.

Nomographs for predicting crown fire initiation in Aleppo pine (Pinus halepensis Mill.) forests

Nomographs that calculate the threshold values of surface fire parameters which lead to crown fire initiation were created by linking two separate fire behavior models: Van Wagner’s crown fire ignition criteria and Byram’s surface fire model. The nomographs were also based on the existing surface (fuel load, fuel heat content) and canopy (foliar moisture content, live crown base height) fuel models of Aleppo (Pinus halepensis Mill.) pine forests of Mediterranean Greece. The most important fire parameters for crown fire initiation that are calculated by the nomographs are the critical flame length and the forward spread rate of the surface fire. These parameters are readily observable in the field during fires. The nomographs provide a judicious way to assess whether a crown fire is likely to occur in a conifer forest stand. Although the fire behavior models used had limited testing and are based on certain assumptions, yet they are widely applied in forestry practice worldwide, as a basis for justified fire prevention and suppression planning.

Fluctuations in fuel moisture across restoration treatments in semi-arid ponderosa pine forests of northern Arizona, USA

Fuel moisture is an important variable in estimating fire behaviour and wildfire hazard. We measured three replicates each of thin-and-burn, burn-only, and control treatments in semi-arid ponderosa pine forests of northern Arizona, USA to quantify temporal changes and treatment effects on live foliar and dead fuel moisture content. Overstorey structure and canopy bulk density were reduced 40–75% in the thin-and-burn treatment v. the burn-only and control treatments. Fluctuations in foliar moisture content varied temporally and across study areas. In 2003, a significant treatment effect was found for two study sites for 1-year-old foliage, but no significant treatment effect was found for new foliage. In 2004, a significant treatment effect was found across all three study sites for both 1-year-old and new foliage. However, no clear pattern existed regarding a specific treatment and its effect on moisture content of old or new foliage. No conclusive evidence was found for a significant treatment effect on the moisture content of fuel particles in the size classes of 0–6, 6–25, and 25–100-mm diameter. Proposals regarding amplified fire behaviour as a consequence of reduced fuel moisture contents in treated v. untreated forest stands in semi-arid ponderosa pine forests of northern Arizona therefore appear to be unwarranted.

Role of Foliar Moisture Content in the Silvicultural Management of Forest Fuels

Abstract In combination with measured stand data and assumed environmental conditions, reasonable estimates of foliar moisture content (FMC) are necessary to determine and justify silvicultural targets for canopy fuels management strategies. FMC often is overlooked in fuels planning, with default values substituting for more precise estimates, but its role is operationally significant in the selection of silvicultural targets even at modest surface fire intensities. This article discusses the role of FMC in canopy ignition and summarizes the results of 11 studies on the FMC values and trends for 16 North American conifers. FMC values ranged from 73 to 480% but varied by species, foliage age, and season. The information presented here will be helpful to managers engaging in canopy fuels planning with the use of popular fire behavior and fuels management software (e.g., NEXUS, Fuels Management Analyst, and the Forest Vegetation Simulator's Fire and Fuels Extension). West. J. Appl. For. 21(4):228–231.

Predicting the ignition of crown fuels above a spreading surface fire. Part II: model evaluation

A crown fuel ignition model (CFIM) describing the temperature rise and subsequent ignition of the lower portion of tree crowns above a spreading surface fire was evaluated through a sensitivity analysis, comparison against other models, and testing against experimental fire data. Results indicate that the primary factors influencing crown fuel ignition are those determining the depth of the surface fire burning zone and the vertical distance between the ground/surface fuel strata and the lower boundary of the crown fuel layer. Intrinsic crown fuel properties such as fuel particle surface area-to-volume ratio and foliar moisture content were found to have a minor influence on the process of crown fuel ignition. Comparison of model predictions against data collected in high-intensity experimental fires and predictions from other models gave encouraging results relative to the validity of the model system.

EIB FUNDING TO UPGRADE LATVIAN PLANT, NETWORKS

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.

Foliar moisture content of Pacific Northwest vegetation and its relation to wildland fire behavior

Foliar moisture was monitored for five conifers and associated understory vegetation in Pacific Northwest forests. Decline in foliar moisture of new foliage occurred over the dry season, while less variation was evident in older foliage. Late season foliar moisture ranged from 130 to 170%. In riparian–upland comparisons, largest differences were found for understory vegetation, with less variation evident for overstory trees. Minimum foliar moisture values of 100–120% are appropriate to use in crown fire risk assessment for the Pacific Northwest.

Fire Danger Monitoring Using ERS-1 SAR Images in the Case of Northern Boreal Forests

Research was carried out to assess the potential of imaging radar systems for monitoring forest fire danger. In Canada, daily forest fire danger ratings are generated by the Canadian Forest Fire Danger Rating System (CFFDRS), based on estimates of fire weather indices (FWI) and meas- ured foliar moisture content (FMC). To evaluate the potential of imaging radar, an experiment was conducted using test sites consisting of jack pine, black spruce and white spruce stands located in the MacKenzie river basin, Northwest Territories, Canada. Radar image intensity values from ERS-1 SAR imagery collected over these stands in 1994 were compared to FWI indices and FMC data. FWI indices were calculated using data from local weather stations. Seasonal trends in radar backscatter (σ 0 ) were shown to correlate with temperature and precipitation. Significant relationships were found between σ 0 and FWI codes and indices, particularly in the case of the black spruce stands, with slow- drying fuels, like duff moisture code (DMC), drought code (DC), and build-up index (BUI). Rates of changes in σ 0 were related to rates of changes in FMC, particularly in the case of the jack pine stands for old FMC and in the case of white spruce stands for composite FMC.

Heat content variation of interior Pacific Northwest conifer foliage

Heat content with and without ash was measured for foliage of three interior Pacific Northwest conifers (Pinus ponderosa, Pseudotsuga menziesii, and Abies grandis/concolor) in five locations in Oregon and Washington. Heat content with ash did not vary by species (20.47–20.59 MJ kg–1), but ash content did vary, so that heat content without ash varied by species. Heat content also varied by location but, due to confounding with season, these differences should be viewed with caution. Foliar heat content values averaged about 10% higher than those used for surface fuels in standard fire behavior fuel models. If heat content is included in future models of crown fire behavior, errors in using heat contents that are too low may have a larger impact than most errors in estimating foliar moisture content, but sources of error might be overwhelmed by other errors in estimating crown fire energy balance.

Foliar moisture content variations in four coniferous tree species of central Alberta

Trends of foliar moisture content variations were determined for jack pine (Pinusbanksiana Lamb.), black spruce (Piceamariana (Mill.) B.S.P.), white spruce (Piceaglauca (Moench) Voss), and balsam fir (Abiesbalsamea (L.) Mill.) in central Alberta. New and old tree foliage was periodically sampled between early March and mid-September during 2 consecutive years. The seasonally recurring variations in foliar moisture contents were characterized by (i) early spring declines of moisture in the old foliage, (ii) flushing of new foliage at high bud moisture contents when the contents in the old foliage were low, and (iii) summer recoveries of moisture in the old foliage and simultaneous decreases of moisture in the new foliage until both eventually converged toward comparable moisture contents. The cumulative effects of temperature, in terms of degree days above 6 °C, were reflected in timing of the declines and the recoveries of moisture in the old foliage. These results may be applied in rating the crowning potential of local forest fires.