Possible Land Management Uses of Common Cypress to Reduce Wildfire Initiation Risk: A Laboratory Study. Della Rocca, G., Hernando, C., Madrigal, J., Danti, R., Moya, J., Guijarro, M., Pecchioli, A., & Moya, B. Journal of Environmental Management, 159:68–77, August, 2015.
doi  abstract   bibtex   
The flammability of Cupressus sempervirens has been fully characterized. Two contrasted bench-scale methodologies have been used to ratify the results. Cypress showed low ignitability and consumability and high sustainability and combustibility. The low ignitability of C.~sempervirens suggests a potential resistance to moderate wildfires. Cypress stands could be a promising land management tool to reduce the wildfire initiation risk. Accurate determination of flammability is required in order to improve knowledge about vegetation fire risk. Study of the flammability of different plant species is essential for the Mediterranean area, where most ecosystems are adapted to natural fire but vulnerable to recurrent human-induced fires, which are the main cause of forest degradation. However, the methods used to evaluate vegetation flammability have not yet been standardized. Cupressus sempervirens is a native or naturalized forest tree species in the Mediterranean area that is able to tolerate prolonged drought and high temperatures. The aim of this study was to characterize the flammability of C.~sempervirens var. horizontalis at particle level by using different bench-scale calorimetry techniques (mass loss calorimeter, epiradiator and oxygen bomb) to determine the main flammability descriptors (ignitability, sustainability, combustibility and consumability) in live crown and litter samples. Our findings indicate that this variety of cypress is relatively resistant to ignition because of the high ash content, the high critical heat flux, the high time to ignition displayed by both crown and litter samples and the ability of the leaves to maintain a high water content during the summer. We also discuss the possibility of exploiting some morphological, functional and ecological traits of the species to construct a barrier system (with selected varieties of cypress) as a promising complementary land management tool to reduce the fire spread and intensity in a Mediterranean context. [Excerpt: Discussion and conclusions] [...] The FMC of live C. sempervirens samples was relatively high and stable between April and the middle of August (84-96%). Furthermore, the low variability in the water content across the crown layers ( Fig. 3) may have important implications for crown fire potential ( Van Wagner, 1977). Madrigal et al. (2013) reported a wider range of FMC for Pinus pinaster needles (95-155%), whereas Viegas et al. (2001) indicated that the FMC of live Mediterranean forest fuels may be as low as 50-60\,% during the summer season, much lower than the value detected in the present study in live crown samples of cypress. Ignition and fire occurrence are closely related to the moisture content of live and dead fuel ( Dimitrakopoulos and Papaioannou, 2001) and fresh leaves of Mediterranean species appear to become very flammable when the moisture content decreases below 75\,% ( Chandler et al., 1983) or below 100\,% in the case of pine needles ( Van Wagner, 1977). An interaction between the physiological state of a live plant (which determines the moisture content) and the concentration of volatile compounds has also been reported ( Weise et al., 2005 and Alessio et al., 2008). During an intense wildfire, the effect due to accumulation of gasified volatile organic compounds (VOCs) may be stronger than the effect due to a low water content, thus explaining specific occurrences such as extreme rate of fire spread and eruptive fire events ( Viegas and Simeoni, 2011). [\n] The live fresh samples of C. sempervirens did not ignite in MLC tests performed at 25 kW/m2, whereas in a previous study, live P. pinaster needles with similar FMC (95%) ignited at the same heat flux ( Madrigal et al., 2013) ( Supplementary Fig. 1). Notably, the HRR curve obtained for C. sempervirens at 35 kW/m2 was similar to that obtained by Madrigal et al. (2013) for live P. pinaster needles at a lower heat flux (25 kW/m2). In EPI2, at 25 kW/m2 ( Table 5), the IF of fresh cypress live fuel was as low as 23\,%, while at 55 kW/m2, the IF was 100\,%, for both crown fresh samples and litter samples (conditioned and oven-dried). These results are consistent with the CHF values determined in MLC tests. [\n][...] [\n] The TTI and CHF are the most representative descriptors of ignitability. Both TTI (at 50 kW/m2) and CHF of live cypress fuel obtained with MLC in the present study were significantly higher than those determined by Madrigal et al. (2013) for live P. pinaster needles with the same FMC value (95%). The ignitability of common cypress was much lower than that of P. pinaster. Live C. sempervirens leaves also displayed the longest TTI in a comparative study of ornamental species Ganteaume et al. (2013a). However, based on TTI, common cypress has been reported to be the most flammable of several Mediterranean species tested ( Liodakis et al., 2002). This discrepancy may be due to different ignition devices used in the two studies. [\n] Heat content is known to be highly dependent upon the chemical composition of a fuel. The GHC values obtained in this study for live C. sempervirens samples in the oxygen bomb test were higher than those obtained for litter ( Table 3). Whereas, in contrast to the findings of Elvira and Hernando (1989), only slight differences in GHC values of live samples were found between spring and summer in the present study ( Table 4). Madrigal et al. (2011) reported a higher GHC for live P. halepensis and P. pinaster needles than for C. sempervirens and Pinus pinea. Even greater differences were observed between species on comparing the GHC of litter samples ( Supplementary Table 2). Liodakis et al. (2002) reported a lower GHC for C. sempervirens leaves that for other Mediterranean species, both conifers and broadleaf species, possibly due to differences in the proportion of chemical components in the leaves and to differences in the residual mass fraction. [\n] [...] [\n] In highly resinous species (i.e. pines) and during the heating phase, the effect of volatile flammable gases (VOCs) is crucial in priming and/or accelerating combustion (Chetehouna et al., 2009). Our findings indicate the natural resistance of live fuel of common cypress to the initial stages of combustion (ignitability). The gases produced by thermal degradation of cypress live fuel did not ignite, irrespective of the heat source (EPI or MLC) or the heat flux to which the samples were subjected. The VOCs stored in cypress leaf glands (Della Rocca, 2012 and Moya and Moya, 2013d) are probably degassed (Greenberg et al., 2006 and Ciccioli et al., 2014) when vegetation is warming up (at 60-150 °C), and thus will not contribute to ignition. Moreover, cypress is not a resinous species in the strict sense. Neither the xylem or the bark produce resin constitutively, but only as consequence of mechanical wounds or lesions due to pathogens or pests. [\n] In Mediterranean regions characterized by long dry summers, litter appears to play a crucial role in fire risk and transmission (Hogkinson, 2002). Both conditioned and oven-dried C. sempervirens litter samples always ignited (IF 100%); however, in the present study all flammability parameters (except TTI) measured at 35 kW/m2 were lower than those obtained at 25 kW/m2 for P. pinaster litter with the same FMC (11%) by Madrigal et al. (2011) ( Supplementary Table 2). Comparing different Mediterranean species, Petriccione (2006) classified the flammability of cypress litter as moderate-high, similar to that of Myrtus communis, Q. ilex, O. europea and P. halepensis, but lower than that of Quercus pubescens, Fraxinus ornus, P. pinaster and Genista aetnensis. [\n] In nature, C. sempervirens litter has a high bulk density ( Ganteaume et al., 2013b), as the small segments accumulate in thick layers, also due to a low mineralisation rate ( Della Rocca et al., 2006). These traits reduce the circulation of air in the litter, which may act like a sponge and retain humidity, thus hampering ignition. A thick litter layer is also known to favour self-extinction of fire ( Ormeño et al., 2009). [\n] In this study, the RMF at the end of the test was considered as ash content. In the MLC tests, the ash content of cypress fresh fine fuel ranged from 3.5 to 6.8\,%. This is consistent with the results reported by Dimitrakopoulos and Panov (2001) who obtained higher values of mineral ash for C. sempervirens leaves (4.73%) than for other Mediterranean species such as Q. ilex (2.9%) and P. brutia (2.8%). The heat content of plant material has been demonstrated to be negatively related to ash content. A higher ash content essentially indicates that there is less mass to burn ( Mutch and Philpot, 1970 and Philpot, 1970), thus influencing fire sustainability and consumability. [...] [Management recommendations and future research] Common cypress is known to produce a deep dense litter that is difficult to ignite and even prevents the development of understory vegetation due to a presumed allelopathic effect. This species is also often planted as a windbreak to protect valuable crops from prevailing winds in the Mediterranean area. In addition to these traits, the findings of our laboratory trials, which have demonstrated the low ignitability of crown and litter cypress samples, encourage further studies at tree and plantation levels to assess the effectiveness of cypress barriers as a silviculture measure to reduce wildfire initiation risk in sensitive sites (e.g. WUIs). Well designed plantations constructed with suitable varieties of cypress (selected for habit and resistance to pathogens causing severe dieback and resin exudation, e.g. cypress canker) and taking into account topography, flammability risk mapping, land use etc. may be useful for creating buffer zones to hinder or prevent the rapid spread of wildfire, also decreasing the likelihood that a surface fire will become a crown fire. The ability of cypress plantations to slow fire progression is derived from a combination of properties of individual trees. The need of transposing laboratory results and empirical observations to a real scale, lead the Diputación de Valencia (Spain) and the province of Siena (Italy) to plan the realization of the first experimental plantations of the 'cypress system' as a way to conduct further research aimed at improving prevention against wildfires. Useful information on setting up operational cypress plantations as firewalls is reported in Della Rocca et al. (2014).
@article{dellaroccaPossibleLandManagement2015,
  title = {Possible Land Management Uses of Common Cypress to Reduce Wildfire Initiation Risk: A Laboratory Study},
  author = {Della Rocca, G. and Hernando, C. and Madrigal, J. and Danti, R. and Moya, J. and Guijarro, M. and Pecchioli, A. and Moya, B.},
  year = {2015},
  month = aug,
  volume = {159},
  pages = {68--77},
  issn = {0301-4797},
  doi = {10.1016/j.jenvman.2015.05.020},
  abstract = {The flammability of Cupressus sempervirens has been fully characterized. Two contrasted bench-scale methodologies have been used to ratify the results. Cypress showed low ignitability and consumability and high sustainability and combustibility. The low ignitability of C.~sempervirens suggests a potential resistance to moderate wildfires. Cypress stands could be a promising land management tool to reduce the wildfire initiation risk. Accurate determination of flammability is required in order to improve knowledge about vegetation fire risk. Study of the flammability of different plant species is essential for the Mediterranean area, where most ecosystems are adapted to natural fire but vulnerable to recurrent human-induced fires, which are the main cause of forest degradation. However, the methods used to evaluate vegetation flammability have not yet been standardized. Cupressus sempervirens is a native or naturalized forest tree species in the Mediterranean area that is able to tolerate prolonged drought and high temperatures. The aim of this study was to characterize the flammability of C.~sempervirens var. horizontalis at particle level by using different bench-scale calorimetry techniques (mass loss calorimeter, epiradiator and oxygen bomb) to determine the main flammability descriptors (ignitability, sustainability, combustibility and consumability) in live crown and litter samples. Our findings indicate that this variety of cypress is relatively resistant to ignition because of the high ash content, the high critical heat flux, the high time to ignition displayed by both crown and litter samples and the ability of the leaves to maintain a high water content during the summer. We also discuss the possibility of exploiting some morphological, functional and ecological traits of the species to construct a barrier system (with selected varieties of cypress) as a promising complementary land management tool to reduce the fire spread and intensity in a Mediterranean context.

[Excerpt: Discussion and conclusions]

[...] The FMC of live C. sempervirens samples was relatively high and stable between April and the middle of August (84-96\%). Furthermore, the low variability in the water content across the crown layers ( Fig. 3) may have important implications for crown fire potential ( Van Wagner, 1977). Madrigal et al. (2013) reported a wider range of FMC for Pinus pinaster needles (95-155\%), whereas Viegas et al. (2001) indicated that the FMC of live Mediterranean forest fuels may be as low as 50-60\,\% during the summer season, much lower than the value detected in the present study in live crown samples of cypress. Ignition and fire occurrence are closely related to the moisture content of live and dead fuel ( Dimitrakopoulos and Papaioannou, 2001) and fresh leaves of Mediterranean species appear to become very flammable when the moisture content decreases below 75\,\% ( Chandler et al., 1983) or below 100\,\% in the case of pine needles ( Van Wagner, 1977). An interaction between the physiological state of a live plant (which determines the moisture content) and the concentration of volatile compounds has also been reported ( Weise et al., 2005 and Alessio et al., 2008). During an intense wildfire, the effect due to accumulation of gasified volatile organic compounds (VOCs) may be stronger than the effect due to a low water content, thus explaining specific occurrences such as extreme rate of fire spread and eruptive fire events ( Viegas and Simeoni, 2011).

[\textbackslash n] The live fresh samples of C. sempervirens did not ignite in MLC tests performed at 25 kW/m2, whereas in a previous study, live P. pinaster needles with similar FMC (95\%) ignited at the same heat flux ( Madrigal et al., 2013) ( Supplementary Fig. 1). Notably, the HRR curve obtained for C. sempervirens at 35 kW/m2 was similar to that obtained by Madrigal et al. (2013) for live P. pinaster needles at a lower heat flux (25 kW/m2). In EPI2, at 25 kW/m2 ( Table 5), the IF of fresh cypress live fuel was as low as 23\,\%, while at 55 kW/m2, the IF was 100\,\%, for both crown fresh samples and litter samples (conditioned and oven-dried). These results are consistent with the CHF values determined in MLC tests.

[\textbackslash n][...] 

[\textbackslash n] The TTI and CHF are the most representative descriptors of ignitability. Both TTI (at 50 kW/m2) and CHF of live cypress fuel obtained with MLC in the present study were significantly higher than those determined by Madrigal et al. (2013) for live P. pinaster needles with the same FMC value (95\%). The ignitability of common cypress was much lower than that of P. pinaster. Live C. sempervirens leaves also displayed the longest TTI in a comparative study of ornamental species Ganteaume et al. (2013a). However, based on TTI, common cypress has been reported to be the most flammable of several Mediterranean species tested ( Liodakis et al., 2002). This discrepancy may be due to different ignition devices used in the two studies.

[\textbackslash n] Heat content is known to be highly dependent upon the chemical composition of a fuel. The GHC values obtained in this study for live C. sempervirens samples in the oxygen bomb test were higher than those obtained for litter ( Table 3). Whereas, in contrast to the findings of Elvira and Hernando (1989), only slight differences in GHC values of live samples were found between spring and summer in the present study ( Table 4). Madrigal et al. (2011) reported a higher GHC for live P. halepensis and P. pinaster needles than for C. sempervirens and Pinus pinea. Even greater differences were observed between species on comparing the GHC of litter samples ( Supplementary Table 2). Liodakis et al. (2002) reported a lower GHC for C. sempervirens leaves that for other Mediterranean species, both conifers and broadleaf species, possibly due to differences in the proportion of chemical components in the leaves and to differences in the residual mass fraction.

[\textbackslash n] [...]

[\textbackslash n] In highly resinous species (i.e. pines) and during the heating phase, the effect of volatile flammable gases (VOCs) is crucial in priming and/or accelerating combustion (Chetehouna et al., 2009). Our findings indicate the natural resistance of live fuel of common cypress to the initial stages of combustion (ignitability). The gases produced by thermal degradation of cypress live fuel did not ignite, irrespective of the heat source (EPI or MLC) or the heat flux to which the samples were subjected. The VOCs stored in cypress leaf glands (Della Rocca, 2012 and Moya and Moya, 2013d) are probably degassed (Greenberg et al., 2006 and Ciccioli et al., 2014) when vegetation is warming up (at 60-150 \textdegree C), and thus will not contribute to ignition. Moreover, cypress is not a resinous species in the strict sense. Neither the xylem or the bark produce resin constitutively, but only as consequence of mechanical wounds or lesions due to pathogens or pests.

[\textbackslash n] In Mediterranean regions characterized by long dry summers, litter appears to play a crucial role in fire risk and transmission (Hogkinson, 2002). Both conditioned and oven-dried C. sempervirens litter samples always ignited (IF 100\%); however, in the present study all flammability parameters (except TTI) measured at 35 kW/m2 were lower than those obtained at 25 kW/m2 for P. pinaster litter with the same FMC (11\%) by Madrigal et al. (2011) ( Supplementary Table 2). Comparing different Mediterranean species, Petriccione (2006) classified the flammability of cypress litter as moderate-high, similar to that of Myrtus communis, Q. ilex, O. europea and P. halepensis, but lower than that of Quercus pubescens, Fraxinus ornus, P. pinaster and Genista aetnensis.

[\textbackslash n] In nature, C. sempervirens litter has a high bulk density ( Ganteaume et al., 2013b), as the small segments accumulate in thick layers, also due to a low mineralisation rate ( Della Rocca et al., 2006). These traits reduce the circulation of air in the litter, which may act like a sponge and retain humidity, thus hampering ignition. A thick litter layer is also known to favour self-extinction of fire ( Orme\~no et al., 2009).

[\textbackslash n] In this study, the RMF at the end of the test was considered as ash content. In the MLC tests, the ash content of cypress fresh fine fuel ranged from 3.5 to 6.8\,\%. This is consistent with the results reported by Dimitrakopoulos and Panov (2001) who obtained higher values of mineral ash for C. sempervirens leaves (4.73\%) than for other Mediterranean species such as Q. ilex (2.9\%) and P. brutia (2.8\%). The heat content of plant material has been demonstrated to be negatively related to ash content. A higher ash content essentially indicates that there is less mass to burn ( Mutch and Philpot, 1970 and Philpot, 1970), thus influencing fire sustainability and consumability. [...]

[Management recommendations and future research]

Common cypress is known to produce a deep dense litter that is difficult to ignite and even prevents the development of understory vegetation due to a presumed allelopathic effect. This species is also often planted as a windbreak to protect valuable crops from prevailing winds in the Mediterranean area. In addition to these traits, the findings of our laboratory trials, which have demonstrated the low ignitability of crown and litter cypress samples, encourage further studies at tree and plantation levels to assess the effectiveness of cypress barriers as a silviculture measure to reduce wildfire initiation risk in sensitive sites (e.g. WUIs). Well designed plantations constructed with suitable varieties of cypress (selected for habit and resistance to pathogens causing severe dieback and resin exudation, e.g. cypress canker) and taking into account topography, flammability risk mapping, land use etc. may be useful for creating buffer zones to hinder or prevent the rapid spread of wildfire, also decreasing the likelihood that a surface fire will become a crown fire. The ability of cypress plantations to slow fire progression is derived from a combination of properties of individual trees. The need of transposing laboratory results and empirical observations to a real scale, lead the Diputaci\'on de Valencia (Spain) and the province of Siena (Italy) to plan the realization of the first experimental plantations of the 'cypress system' as a way to conduct further research aimed at improving prevention against wildfires. Useful information on setting up operational cypress plantations as firewalls is reported in Della Rocca et al. (2014).},
  journal = {Journal of Environmental Management},
  keywords = {*imported-from-citeulike-INRMM,~INRMM-MiD:c-13745883,~to-add-doi-URL,cupressus-sempervirens,fire-fuel,forest-resources,wildfires},
  lccn = {INRMM-MiD:c-13745883}
}
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