Forest Research: Open Access
Open Access

Initial Investigation of Seasonal Flammability of three Invasive East Texas Forest Understory Fuels using Thermogravimetric Analysis

Michael B. Tiller¹, Brian P. Oswald^1*, Alyx S. Frantzen², Warren C. Conway³ and Kuai Hung I^{1 *}Correspondence: Brian P. Oswald, Stephen F. Austin State University, Arthur Temple College of Forestry and Agriculture, USA, Tel: +9366457990, Email:

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Research Methodology

Species selection and sampling

Understory species selection was based on local invasive plant abundance relative to greater fuel loading in pine and hardwood forest ecosystems in East Texas. Yaupon was the primary species studied in xeric, upland pine-dominated sites, whereas, Chinese privet and tallow were sampled on more mesic, mixed hardwoodpine sites.

Samples were collected at the Pineywoods Native Plant Center located on the Stephen F. Austin State University campus, Alazan Wildlife Management Area managed by Texas Park and Wildlife Department near Alazan, Texas, and the U.S. Forest Service- Southern Research Station Experimental Forest located on the Angelina National Forest. Study sites were visited during the growing (August) and dormant (February) seasons to account for any seasonal physiological differences in flammability. Foliage and stem samples were collected from five separate specimens of each target species ranging from 0-2 m in height. Specimen height ranges were chosen based on greater susceptibility to prescribed fire [36]. Small diameter ( 0.635 cm) stem samples of Chinese tallow were collected during both dormant and growing seasons to account for “leaf off” during the dormant season, and to maintain consistency with potential seasonal variances relative to chemical composition of wood.

Green, undried foliage samples were treated with liquid nitrogen to increase the rigidity of leaf tissue to facilitate grinding. Wood samples were ground in a Thomas Model 4 Wiley mill utilizing a 2 mm, sieve to maintain consistent sample texture. Ground foliage and wood samples were dried at 70°C in an air-convection dryer prior to testing [8].

Thermogravimetric procedure

Standard TGA techniques were performed on a Perkin Elmer Simultaneous Thermal Analyzer 6000 calibrated to manufacturer specifications [37]. Proximate analysis and flammability testing were based on five individual plant samples. Proximate analysis data was estimated using a standard TGA procedure to obtain values for volatile matter (VM%), fixed carbon (FC%), and ash percent (ash%) [38]. Flammability data was estimated using TGA in an oxidative atmosphere by using air as a purge gas at 20 mL min-1 with a 10°C min-1 linear heating rate ranging from 35-650°C. Subsequent data was further analyzed using Pyris 13.2 software to produce differential thermogravimetric (DTG) degradation profiles that were used to estimate flammability parameters based on DTG peak correlation with gas- and solid-phase combustion [7]. First derivative DTG signatures represent mass loss as a function of temperature and time, and were evaluated for relative spontaneous ignition temperature (RSIT) (Figure 1) and gas-phase maximum mass loss rate (GP-MMLR) and combustion duration (GP-CD) by measuring peak onset, maxima, and endset values [7] (Figure 2). The addition of a second derivative peak was also used to elucidate the first onset and endset reaction peak occurring on the first derivative DTG peak representing gas-phase combustion, which greatly improved the accuracy of intersecting tangential lines used to estimate RSIT values (Figure 1). Based on the relative importance of flaming combustion in terms of wildfire propagation [8,9], this study focused on gas-phase combustion rates and durations as relative measures of flammability.

Figure 1: Pyris 13.2 software was used to estimate relative spontaneous ignition temperature (RSIT) for dormant season yaupon using reaction peak tangential lines from first and second derivative thermogravimetric mass loss curves (DTG).

Figure 2: Gas- and solid-phase maximum mass loss rates (GP-MMLR, SP-MMLR) were estimated using the first derivative thermogravimetric mass loss (DTG) curves onset and endset maxima from the two primary DTG peaks.

Resulting RSIT, GP-MMLR, and GP-CD data were analyzed using a one- and two-way ANOVA (p<0.05) to determine if any significant differences exist between species and season. Yaupon and Chinese privet are evergreen shrubs; therefore, foliage samples were tested using a two-way ANOVA with species and season serving as independent variables. A one-way ANOVA was conducted on Chinese tallow stem with season serving as the independent variable due to leaf off conditions in the dormant season. Descriptive statistics were also used for comparative analysis between flammability and proximate analysis data.

Results

Similar to previous TGA studies [6,7], two distinct DTG peaks representing gas- and solid-phase combustion were observed with all species. Previous studies associated greater RSIT’s with lower flash points and piloted ignition temperatures and are therefore representative of more ignitable fuels [39]. According to this assumption, yaupon exhibited the greatest mean RSIT in both growing and dormant seasons (Table 1), classifying it as the most ignitable foliage, followed by Chinese privet and tallow foliage. Yaupon and Chinese privet also exhibited significant differences (p<0.0001) with respect to species and season. Seasonal RSIT’s for Chinese tallow stem were significantly different (p<0.0001) and comparable to seasonal ignitability’s of yaupon foliage. Chinese tallow foliage and stem exhibited a considerable difference in mean RSIT, with a 50.86°C differential, thus indicating very low foliage ignitability as compared to highly ignitable stem. All species mean foliage and stem RSIT’s trended significantly higher in the growing season with the exception of Chinese privet’s greater mean dormant season RSIT. Mean VM% was consistently higher in species with greater ignitability, whereas mean ash% was generally lower in species with greater RSIT (Tables 1 and 2). In summation, yaupon is significantly more ignitable than Chinese privet, which is consistent with past studies suggesting understory species flammability is greater in pine ecosystems as opposed to hardwood ecosystems [40].

Table 1: Mean growing and dormant season flammability results for Yaupon, Chinese Privet, and Chinese tallow stems.

Table 2: Mean growing and dormant season proximate analysis results for Yaupon, Chinese Privet, and Chinese tallow.

Combustibility measures the rapidity in which a material burns and is correlated with greater GP-MMLR’s and lower GP-CD’s [7,39]. Mean GP-MMLR data for all foliage samples ranged from 2.67-4.51% min-1 during both seasons, with yaupon exhibiting the greatest GP-MMLR followed by Chinese privet and tallow (Table 1). In terms of evergreen shrub combustibility, yaupon exhibited a significantly greater mean GP-MMLR (p<0.0001) compared to Chinese privet. Chinese tallow stem was the most combustible species with mean GP-MMLR’s ranging from 9.96-11.45% min-1 in the dormant and growing seasons (Table 1). Mean VM% was consistently higher in species with commensurate increases in GPMMLR, while mean ash% was generally lower in species with greater GP-MMLR (Tables 1 and 2). Similar to previous findings, species foliage combustibility rankings were consistent with ignitability rankings, with yaupon exhibiting the greatest overall flammability. As expected, all species exhibited greater combustibility in the growing season with the exception of yaupon that had a 0.16% min-1 increase in dormant season GP-MMLR.

Measures of sustainability are related to the how long a fuel supports flaming combustion and is correlated with GP-CD. Sustainability is also synonymous with fire front residence time, which can impact fire intensity and spread depending on duration of flaming combustion and varying degrees of heat intensity dictated by species combustibility characteristics [15]. Most TGA studies measure sustainability as a function of time beginning at the onset of spontaneous ignition and concluding at the terminus of the corresponding DTG peak [7,39]. Our results yielded a mean GP-CD of 10.88-15.84 min for all species (Table 1), with Chinese tallow foliage exhibiting the greatest GP-CD followed by Chinese privet and yaupon. Mean GP-CD’s for yaupon and Chinese privet were significantly different (p<0.0001) with respect to species (p<0.0001) and season (p<0.03). Similarly, mean seasonal Chinese tallow stem GP-CD’s were also significantly different (p<0.0001). All species mean GP-CD’s and GP-MMLR’s were inversely related and in agreement with previous studies combustibility metrics [7,39]. Among GP-CD results, lower values were associated with greater GP-MMLR’s, and were generally associated with growing season conditions with the exception of Chinese privet’s slight increase in dormant season GP-CD. Another noteworthy result was the combination of yaupon’s higher dormant season MMLR and GPCD, which indicates both high combustibility and sustainability.

Discussion

Seasonality had the greatest effect on ignitability for all species. As expected from previous studies [28,29], yaupon was the most ignitable species during both seasons and poses the greatest ignition risk. In contrast, mean dormant season yaupon and Chinese privet RSIT and VM% were fairly comparable, which suggests dormant season prescribed fires targeting Chinese privet may be more effective given adequate fine fuel loading and desirable live fuel moistures. Chinese tallows’ low foliage ignitability compared to its high stem wood ignitability may potentially hinder prescribed burn efforts in exiting stands due to the inability to carry fire. Conversely, Chinese tallow that has been masticated or cut and cured in place may be more susceptible to prescribed fire resulting in greater potential top-kill of stump sprouts and improved seed bank control. Proximate analysis results indicating greater VM% were in agreement with RSIT ignitability indices, but ash% indices were somewhat inconsistent as a determinant of ignitability and are consistent with previous studies [30-38].

Foliage combustibility indices were not significantly affected by season; however, Chinese tallow stem was considerably more combustible in the growing season with the overall greatest GPMMLR. Consequently, the combined effects of Chinese tallows growing season stem combustibility and ignitability would likely improve prescribed fire effects as previously mentioned. Yaupons seasonal combustibility metrics were considerably higher as compared to Chinese privet and tallow foliage, which presents a significant wildfire risk in both seasons. With the exception of Chinese privet, all species exhibited a trend of increasing GPMMLR based on the inverse relationship of VM% and ash%. Thus, the combination of increasing VM% with subsequent decreases in ash% may serve as an additional metric for confirming combustibility metrics.

Sustainability metrics for all species were significantly affected by season (p<0.03). Similar to previous studies, short duration GP-CD’s correlated well with greater GP-MMLR’s [39,40], thus validating combustibility estimates. Interestingly, yaupon exhibited greater dormant season GP-CD and GP-MMLR, which may potentially amplify dormant season fire intensity based on longer periods of high intensity heat release from flaming combustion. In addition to increased fire intensity, detached foliage may burn for extended time periods in convective columns posing a greater risk for spot fires. Consequently, yaupons’ combined high flammability metrics pose a significant hazard to both wildfire and prescribed fire operations. Similar to yaupon, Chinese tallow stem also exhibits combined high flammability metrics which may become hazardous in dense stands under severe drought conditions.

Conclusion

Flammability parameters were estimated for three East Texas invasive forest understory species using standard TGA and proximate analysis methodologies. Results were consistent with previous TGA studies and provide new baseline flammability data for locally problematic species. Species flammability metrics can be used for future comparative analysis, as well as an initial quantitative flammability assessment that can serve as hazard classifications for fire resistant plant listings and potential inputs for future fuel models with high densities of invasive species. Yaupon was confirmed as a highly flammable species that poses a year round risk in the wildland-urban interface and is capable of intensifying fire behavior beyond prescribed fire treatment parameters. Chinese privets’ greater dormant season ignitability may improve prescribed fire effectiveness earlier in the year; however, early season drought may present a significant wildfire risk during strong wind events. Chinese tallows’ highly flammable growing season stem wood may also improve integrated control measures using prescribed fire. Collectively, these data provide greater knowledge of species flammability based on fuel chemistry, but further research is necessary to validate and compare experimental flammable indices to different testing methodologies and full scale experimentation.

Author Contributions

Conceptualizations, MBT, BPO; methodology, MBT, BPO, ASF; formal analysis, MBT ASF; investigation MBT; data curations MBT, BPO; writing-original draft preparation, MBT, BPO; writingreview and editing, MBT, ASF, WCC, BPO, IH; supervision, BPO; project administration and funds acquisition BPO. All authors have read and agreed to the published version of the manuscript.

Funding

The research was funded by the McIntire-Stennis Cooperative Forest Research Program and Welch AN-0008.

Acknowledgement

The authors extend gratitude to Stephen F. Austin State University’s Arthur Temple College of Forestry and Agriculture and Department of Chemistry and Biochemistry for supporting this research.

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