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Article

Flammability and Combustibility of Two Mediterranean Species in Relation to Forest Fires in Croatia

by
Roman Rosavec
1,
Damir Barčić
1,*,
Željko Španjol
1,
Milan Oršanić
1,
Tomislav Dubravac
2 and
Alan Antonović
1
1
Faculty of Forestry and Wood Technology, University of Zagreb, 10000 Zagreb, Croatia
2
Croatian Forestry Institute, 10450 Jastrebarsko, Croatia
*
Author to whom correspondence should be addressed.
Forests 2022, 13(8), 1266; https://doi.org/10.3390/f13081266
Submission received: 20 July 2022 / Revised: 7 August 2022 / Accepted: 9 August 2022 / Published: 10 August 2022
(This article belongs to the Section Natural Hazards and Risk Management)
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Abstract

:
Climatic conditions are extremely important for the start and spread of forest fires. Flammability and the spread of fire are sensitive to the interactions of variables within ecosystems, such as weather, fuel, and topography. Certain variables are highly significant for forest fires and are applied in various models, particularly the moisture content of potential fuel, and its flammability or combustibility. However, such these models cannot determine the true values of the required variables, unlike actual empirical research. Areas with the highest number of fires show significant variability and differences in air temperature, humidity, and precipitation. These factors have a strong influence on flammability, combustibility, and moisture content of Mediterranean species. This study analyses the flammability and combustibility of two Mediterranean species important for the Adriatic area in Croatia: climazonal holm oak (Quercus ilex L.) and Aleppo pine (Pinus halepensis Mill.) as an important conifer for afforestation and reforestation. The results of linear correlation coefficients of flammability of these species at the studied localities show a statistically significant and very strong correlation between flammability and the moisture content of the tested sample, with the exception of Aleppo pine on the island of Rab. The analysis of variance of combustibility showed that there were no statistically significant differences between nearly all variables tested. The results indicate the significant effect of live fuel moisture content on the flammability and combustibility of natural fuels.

1. Introduction

As pointed out by the Centre for Research on the Epidemiology of Disasters (CRED), Munich Reinsurance Company and the Swiss Re Group, forest fires are generally classified into a group of natural climatological disasters where wildfires are perceived as forest fires and land fires [1]. The Mediterranean region is often affected by forest fires, and climate change is causing an increasing incidence of fires, with more land area burnt [2]. Climate change forecasts for the area of Southeast Europe and the Mediterranean are predicting temperature increases above the global average, particularly in the summer months. Some 60,000 fires occur every year in all Mediterranean regions, affecting on average about 500,000 hectares of forested area [3]. Therefore, forest fires can be considered one of the most important natural destabilisers, causing long-term changes [4,5,6] that are most evident in the degradation of forests and forest habitats [7,8]. The intensity and damage activity of forest fires depend mostly on their intensity and frequency [9,10].
Different interactions within ecosystems have an effect on the conditions for the start and spread of forest fires. The most significant are climate conditions and flammable materials, though geological structure, relief, and pedological properties are also important. Climate conditions play a key role in the determination of the fire regime of an area [11,12,13,14]. The Mediterranean region is characterised by long dry periods in the summer months with warm periods during the winter months [15]. This can have a stressful effect on vegetation and increase the likelihood of fire [16,17]. Considering these effects, and particularly in the sense of climate change, forest stands will become more vulnerable to forest fire [18] and damages will increase [19].
Damage from forest fires depends above all on the characteristics of forest fuels. The most important characteristics are flammability and the classification of vegetation in terms of flammability [20], followed by combustibility of vegetation and the moisture content of vegetation fuel. Throughout history, different proposals of classifications of vegetation with respect to their flammability have been made. The rate of spread of forest fires depends on both climate conditions and the quantity and properties of fuel, and this is inversely proportionate to the moisture content of forest fuel [21]. Forest fuel is any substance or mixture of substances, or flammable biomass that is found in the forest, that can ignite and burn [22]. Ref. [23] stated that forest fuel is the entire quantity of plant material, alive and dead, lying above the mineral layer of soil. This fuel can differ in terms of its flammability [24], and both the spatial horizontal and vertical composition of fuel will influence the further spread of a fire [25].
The term “flammability” has no strict and clear definition. In the literature, it is often used to indicate the ability of a given fuel to ignite and support fire. The authors of [26,27,28] define flammability as a combination of the following aspects: ignitability, sustainability, and combustibility. Most authors agree that flammability and combustibility should be measured in laboratory conditions, where the ecological parameters of fuel can be controlled and monitored [29,30,31].
The moisture content of fuel is known to be one of the most critical factors that influence the occurrence and spread of fire [32,33,34]. Several authors have confirmed the strong correlation between the flammability of Mediterranean species and their moisture content [35,36,37,38].
Weather conditions affect the moisture content of fuel over time [39,40,41], and this also depends on the physiological and chemical properties of the fuel [42,43]. Numerous studies have revealed different approaches and methods to determine the characteristics of flammability and to compare the flammability of living and dead forest fuel of Mediterranean species [44,45,46]. The results obtained varied substantially, indicating the need to develop a standardised method to examine flammability, and to define a classification system for test results [47,48].
The aims of this study are to ascertain the flammability and combustibility of holm oak (Quercus ilex L.) and Aleppo pine (Pinus halepensis L.) in the Adriatic region in Croatia, and to analyse the impacts of climatic factors (temperature, humidity, and precipitation) on the flammability and combustibility of these two species. This research is important since the obtained results provide a better understanding of the issues surrounding forest fires and are applicable in fire control activities. This research contributes to preserving the biological and landscape diversity and to protecting natural values and the environment.

2. Materials and Methods

2.1. Study Area

The research was conducted on the island of Rab (44°46′53″ N 14°46′01″ E), in the Rab Teaching and Experimental Forest of the Faculty of Forestry and Wood Technology, University of Zagreb, and in Makarska (43°17′38″ N 17°01′20″ E), at the experimental laboratory contained within the main Makarska weather station. The island of Rab is part of the northern Croatian coast, while Makarska lies on the southern Croatian coast (Figure 1). The coastal area belongs to the maritime-subtropic climate type, which is a type C climate according to the Koppen climate classification [49], with a moderately warm and rainy climate, with several transitional climate types and subtypes. The mean annual air temperature on the island of Rab is 15.5 °C with a mean annual precipitation of 1085.4 mm, while the mean annual air temperature in Makarska is 17.0 °C with a mean annual precipitation of 993.4 mm, for the period 1981–2019, according to data from the State Hydrometeorological Institute Croatia.

2.2. Research Methods

The danger of fire is a rather complex variable influenced by weather elements (major determinants of the exposure to fire risk and the spread of fire), the local topography, and vegetation properties. Two species were tested: holm oak (Quercus ilex L.) and Aleppo pine (Pinus halepensis Mill.). Holm oak (Quercus ilex L.) is widely distributed in the western Mediterranean, and in Croatia, this is a climazonal species that covers a narrow edge of southwestern and southern Istria, passing to the southernmost part of the island of Cres, then to Rab and Pag, and further on the islands south of Lošinj and on the mainland south of Zadar. Aleppo pine (Pinus halepensis Mill.) is a typical Mediterranean species that accounts for about 10% of all forests in the Mediterranean [50]. In Croatia, it grows naturally on the Dalmatian islands south of Šibenik and along the mainland south of Split [51,52]. It is intensively used for the reforestation of burnt areas. The occurrence and spread of forest fires is influenced by topography. For forest fires, in some cases, natural obstacles or benefits are different landforms. Uphill, a forest fire will develop faster because it develops in the direction of hot air and vice versa. Since the temperature is lower at higher altitudes, the intensity of the fire element is lower there.
The testing of the flammability, combustibility, and moisture content of the tested tree species was conducted monthly at each location over a two-year period (2017–2019). Meteorological data were obtained from the State Hydrology and Metrology Service and were measured at the Rab weather station and the Makarska main weather station.
To test the flammability and combustibility of live forest fuel, the methodology described by [53], and applied in other research [54,55], was used. In this method, samples of leaf mass as live forest fuel were collected, always at identical places at both locations. For each species, about 150 g of leaf mass was collected in each monthly sampling session from each location. In order to prevent any loss of moisture of the sample between collection and testing, samples were placed in containers and hermetically sealed. The time between collection and testing was not longer than 30 min. One container was used for testing, while the others were placed in a refrigerator to avoid any sample moisture loss. After testing one species, the container with the other species was removed from the refrigerator. Testing was performed in two series, each with 25 samples. Therefore, per testing session at each location, 50 samples of each species were tested. This sample size can be considered representative and sufficient for further analysis. Samples were layered in plastic containers, each with a defined weight of approximately 1 g.
Flammability and combustibility were tested using a Quartz Saint-Gobain epiradiator (laboratory electrical heater; type 534 Rc2, power 500 W). This consists of a metal spiral situated in a 10 cm disc made of pure silicon. The electrical resistance provides infrared radiation of 3 μ (3 × 10−6) with 7.5 W (7.5 J/s) per cm2 (Figure 2). Both values (flammability and combustibility) were measured in seconds using stopwatches. The moisture content of the tested samples was obtained using standardised equations for the measurement of moisture content (percent of dry weight) by drying the materials in a dryer. When measuring the weight of samples, four samples of live forest fuel weighing from 5.00 ± 0.05 g were prepared, weighing was performed on KERN 440 electrical scale with a precision of 0.01 g. These samples were then dried for 24 h at a temperature of 105 °C and then reweighed.
The equation is:
LFMC = F W − D W D W ∗ 100
where: LFMC—live fuel moisture content, FW—fresh weight of the sample, and DW—dry weight of the sample.
A linear correlation analysis was used to determine the relationships between variables. We used multivariate linear regression to determine the correlations among LFMC, mean monthly air humidity (%), mean monthly air temperature (°C), mean monthly maximum air temperature (°C), mean monthly minimum air temperature (°C), and mean monthly precipitation (mm) (as the independent variables) with two dependent variables, time to ignite (TI) (s) as the measure of flammability and duration of combustion (DC) (s) as the measure of combustibility. All statistical analyses were performed and graphical representations made in the statistical packages SAS and STATISTICA 7.1.

3. Results

The results as shown in Table 1 indicate that both tested species had a higher flammability index and combustibility index in Makarska than on the island of Rab. The combustibility of Aleppo pine (Pinus halepensis Mill.) was also higher in Makarska than on the island of Rab, unlike the combustibility of holm oak (Quercus ilex L.), which was higher on Rab.
In Makarska, the TI for Aleppo pine (Pinus halepensis Mill.) was statistically significant, positive, and strongly correlated with LFMC (0.78) and statistically significant, positive, and moderately correlated with mean precipitation (0.46). The remaining statistically significant correlations were strong and negative (−0.75, −0.76 and −0.73), i.e., TI decreases with rising temperature. On the island of Rab, correlations between the TI for Aleppo pine (Pinus halepensis Mill.) with the tested variables were not significant (Table 2).
The TI of holm oak (Quercus ilex L.) on the island of Rab was statistically significant, positive, and moderately correlated with LFMC (0.64), while in Makarska, the TI of holm oak (Quercus ilex L.) was statistically significant, positive, and strongly correlated with LFMC (0.80) and moderately correlated with mean monthly precipitation (0.60) (Table 2).
The results of the multivariate regression analysis for the TI of Aleppo pine (Pinus halepensis Mill.) on the island of Rab are shown in Table 3, and there were no statistically significant associations between TI and the tested variables.
The results of the multivariate regression analysis for the TI of Aleppo pine (Pinus halepensis Mill.) in Makarska as shown in Table 3 indicated a statistically significant dependence of TI on LFMC, which explained 83% of the TI.
The results of the multivariate regression analysis for the TI for holm oak (Quercus ilex L.) on the island of Rab as shown in Table 4 indicated that the TI was statistically significantly dependent on LFMC, mean monthly air temperature, and mean monthly minimum air temperature. These variables together explained 65% of the TI of holm oak (Quercus ilex L.) on the island of Rab.
The results of the multivariate regression analysis for the TI for holm oak (Quercus ilex L.) in Makarska as shown in Table 4 indicated that the TI was statistically significantly dependent on LFMC and the mean monthly precipitation. These together explained 83% of the TI of holm oak (Quercus ilex L.) in Makarska.
In Makarska, the DC of Aleppo pine (Pinus halepensis Mill.) was statistically significantly, negatively, and moderately correlated with LFMC (−0.45). Other statistically significant correlations were moderate and positive (0.43, 0.44 and 0.43), with the except of the correlation between DC and the mean monthly precipitation (−0.42), which was moderate but negative. The DC of Aleppo pine (Pinus halepensis Mill.) on the island of Rab showed no statistically significant correlations with the tested variables (Table 5).
The DC of holm oak (Quercus ilex L.) in Makarska was statistically significantly, negatively, and moderately correlated with LFMC (−0.49). On the island of Rab, the DC of holm oak (Quercus ilex L.) showed no statistically significant correlations with the tested variables (Table 5).
Table 6 show the results of the multivariate regression analysis for the DC of Aleppo pine (Pinus halepensis Mill.) on the island of Rab and in Makarska. The results indicate that there were no statistically significant associations between DC and the tested variables.
The results of the multivariate regression analysis for the DC of holm oak (Quercus ilex L.) on the island of Rab are shown in Table 7, and indicate that there were no statistically significant dependencies of DC on the tested variables.
The results of the multivariate regression analysis for the DC of holm oak (Quercus ilex L.) in Makarska shown in Table 7 indicate that the DC was statistically significantly dependent on LFMC and mean monthly maximum air temperature. Together, these variables explained 47% of the DC of holm oak (Quercus ilex L.) in Makarska.

4. Discussion

Holm oak (Quercus ilex L.) is a climazonal species along the Croatian coast, while Aleppo pine (Pinus halepensis Mill.) is the main pioneer species that colonises unforested or degraded surfaces. Research has shown that both species are highly to extremely flammable [56]. Climatic conditions are exceptionally important in the start and spread of forest fires. Flammability and the spread of fires are also vulnerable to different interactions within the ecosystem, such as weather, fuel, and topography. According to [57], tree species has the strongest influence in the determination of forest composition (as potential fuel). Climate plays a key role in both flammability and combustibility, and so they are very sensitive to climate change [58]. The effects of climate change on forest fires were also highlighted by [59]. In 2022 [60], the Intergovernmental Panel on Climate Change (IPCC) indicated that global warming will increase the number of forest fires around the world, and there are many studies outlining the potential impacts on climate change on the risk of forest fires [61,62,63].
The moisture content of fuel has an exceptionally important role in both flammability, measured as TI, and combustibility, measured as DC. At both sites, most of the variation in TI and DC was explained by this single factor alone. The moisture or water content indeed is the best determinant of the capability of a fuel to burn, and if it does ignite, determines how effective and sustainable its combustion will be. Changes in moisture content are associated with atmospheric conditions and the availability of moisture in the soil on the one hand, and the ecophysiological traits of the vegetation on the other, in combination with past living conditions. A low fuel moisture content is the main cause of fires breaking out in early autumn as fuel material dries out during the summer, and of fires in spring before the new leaf mass begins its activity [64]. According to [65], the appearance of fires with catastrophic consequences that break out in dry conditions are more the result of the impact of reduced moisture content than the result of high air temperatures. The significant impact of moisture content on fuel flammability can also be shown through the evaporation and exclusion of oxygen from the combustion zone [66]. It would appear that ignition energy is lower when the moisture content is higher. On the other hand, moisture content affects the behaviour of fire, as further burning is limited when burning is reduced due to the moisture of fuel [67].
Flammability as TI, combustibility as DC, and LFMC are complex phenomena that differ among tree species. In local living conditions, they cannot be adequately assessed and established, due to the direct impacts of environmental factors. Laboratory testing has provided the opportunity to develop different models that can be used to assess the time to ignition and duration of combustion, since, in laboratory testing, the test sample has a known moisture content. According to [68,69], the results of this research reflect the actual time to ignition in nature, as the values of the moisture content of the test samples are within the range that can be present in the natural environment.
In recent years, mathematical models, systems, and simulators have been developed with the aim of assessing fire behaviour and fire risk, risk of the start and spread of fires, vulnerability and the consequences that forest fires can cause.
The research of [70] showed that ranking the flammability of species can improve the fire risk assessment index. Additionally, [71] stressed that the use of a flammability index, including the natural dynamics of live fuels, can give precise index values that indicate a threat of fire in the Mediterranean region. Therefore, the results of this study could contribute to a wide range of services (fire-fighters, protection and rescue services, local and regional governments, and other individual users). Additionally, they can serve as guidelines aimed at improving fire control policies and improving forest ecosystem conservation.

5. Conclusions

Areas in which the largest number of forest fires occur show significant variability and differences in air temperature, humidity, and precipitation levels. These factors have a strong effect on flammability, combustibility, and the moisture content of live fuels of Mediterranean species. The results obtained in this study could represent a step towards associating the influence of flammability and combustibility on the fire characteristics of a given area. Primarily, in determining the fire properties of a certain area, the emphasis should be placed on the potential of the forest fuel, while climate conditions are a limiting factor given the pronounced changeability of conditions in the micro-location. This study confirmed the need for future research that should strive to examine the relationships between flammability, combustibility and live fuel moisture content [72], and to recognise the quantity of potential forest fuel. This depends on performing silvicultural in existing stands on the one hand, and the proper selection of species for reforestation of burnt areas on the other. This should facilitate mapping fire risk (with utilization of Geographic Information System tools and remote sensing-based methodologies), particularly in forest ecosystems with a known threat of fire. Additionally, this type of research facilitates the improvement of fire control issues, such as mapping vegetation with rankings of flammability and combustibility. This would significantly contribute to conserving the biological and landscape diversity and protect the vegetation features of the Mediterranean.

Author Contributions

Conceptualization, R.R. and D.B.; methodology, R.R.; validation, A.A. and T.D.; formal analysis, R.R.; investigation, R.R.; resources, Ž.Š. and M.O.; writing—original draft preparation, R.R.; writing—review and editing, D.B.; visualization, R.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

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Forests 13 01266 g001 550
Figure 1. Geographic position of the study area. Left side—position of the island of Rab in the Croatian Adriatic coast; right side—position of Makarska in the Croatian Adriatic coast.
Figure 1. Geographic position of the study area. Left side—position of the island of Rab in the Croatian Adriatic coast; right side—position of Makarska in the Croatian Adriatic coast.
Forests 13 01266 g001
Forests 13 01266 g002 550
Figure 2. Testing with the epiradiator.
Figure 2. Testing with the epiradiator.
Forests 13 01266 g002
Table
Table 1. Flammability (TI), combustibility (DC), and moisture content (LFMC) of the tested species.
Table 1. Flammability (TI), combustibility (DC), and moisture content (LFMC) of the tested species.
SpeciesTI (s)DC (s)LFMC (%)
MakarskaRabMakarskaRabMakarskaRab
Aleppo pine
(Pinus halepensis Mill.)		9.69–16.3410.22–16.757.12–13.366.38–13.79108.88–153.48109.31–142.73
Holm oak (Quercus ilex L.)5.29–9.596.05–9.2710.22–14.679.64–13.9448.99–91.3766.36–102.86
Table
Table 2. Linear correlation coefficients of the time to ignite (TI) for Aleppo pine (Pinus halepensis Mill.) and Holm oak (Quercus ilex L.) on the island of Rab and in Makarska. Correlations in bold are significant at p < 0.05; N = 25.
Table 2. Linear correlation coefficients of the time to ignite (TI) for Aleppo pine (Pinus halepensis Mill.) and Holm oak (Quercus ilex L.) on the island of Rab and in Makarska. Correlations in bold are significant at p < 0.05; N = 25.
VariableTILFMCMean Monthly Air Humidity (%)Mean Monthly Air Temperature (°C)Mean Monthly Maximum Air Temperature (°C)Mean Monthly Minimum Air Temperature (°C)Mean Monthly Precipitation (mm)
Aleppo pine (Pinus halepensis Mill.)
Rab–TI1.000.150.08−0.11−0.11−0.120.13
Makarska–TI1.000.780.22−0.75−0.76−0.730.46
Holm oak (Quercus ilex L.)
Rab–TI1.000.640.04−0.09−0.09−0.080.38
Makarska–TI1.000.800.11−0.19−0.20−0.190.60
* Statistically significant results are bolded.
Table
Table 3. Results of the regression analysis of the time to ignite (TI) as the dependent variable for Aleppo pine (Pinus halepensis Mill.) on the island of Rab and in Makarska. Correlations in bold are significant at p < 0.05.
Table 3. Results of the regression analysis of the time to ignite (TI) as the dependent variable for Aleppo pine (Pinus halepensis Mill.) on the island of Rab and in Makarska. Correlations in bold are significant at p < 0.05.
Island of Rab
DFSSMSFPr > FR2Parc. R2Coef.
Var.		RMSE
model63.616470.602740.180.97930.0561−0.258513.920131.83840
VariableDFProc.
Param.		Standard
Error		tPr > |t|
Intercept19.154088.561901.070.2991
LFMC10.022000.031050.710.4878
Mean monthly humidity10.015180.084980.180.8602
Mean monthly air temp.11.106662.114680.520.6071
Mean monthly max. air temp.1−0.356311.14764−0.310.7598
Mean monthly min. air temp.1−0.809691.51922−0.530.6006
Mean monthly precipitation10.001050.008040.130.8972
Makarska
DFSSMSFPr > FR2Parc. R2Coef.
Var.		RMSE
model676.6404712.7734115.12<0.00010.83440.77927.321480.91926
VariableDFProc.
Param.		Standard
Error		tPr > |t|
Intercept12.742965.487040.500.6232
LFMC10.106210.024054.420.0003
Mean monthly humidity10.006290.042950.150.8853
Mean monthly air temp.1−0.180091.08663−0.170.8702
Mean monthly max. air temp.1−0.088700.74173−0.120.9061
Mean monthly min. air temp.10.119350.588920.200.8417
Mean monthly precipitation1−0.003650.00430−0.850.4067
Table
Table 4. Results of the regression analysis of time to ignite (TI) as the dependent variable for holm oak (Quercus ilex L.) on the island of Rab and in Makarska. Correlations in bold are significant at p < 0.05.
Table 4. Results of the regression analysis of time to ignite (TI) as the dependent variable for holm oak (Quercus ilex L.) on the island of Rab and in Makarska. Correlations in bold are significant at p < 0.05.
Island of Rab
DFSSMSFPr > FR2Parc. R2Coef.
Var.		RMSE
model68.446591.407775.460.00230.64540.52726.615750.50772
VariableDFProc.Param.Standard
Error		tPr > |t|
Intercept13.879892.710971.430.1695
LFMC10.048760.011564.220.0005
Mean monthly humidity10.000676070.025300.030.9790
Mean monthly air temp.1−1.212340.56074−2.160.0443
Mean monthly max. air temp.10.270080.299370.900.3789
Mean monthly min. air temp.11.050310.415572.530.0211
Mean monthly precipitation10.002670.002361.130.2743
Makarska
DFSSMSFPr > FR2Parc. R2Coef.
Var.		RMSE
model626.576894.4294814.85<00010.83190.77597.591670.54617
VariableDFProc.
Param.		Standard
Error		tPr > |t|
Intercept11.652932.147240.770.4514
LFMC10.077160.011476.73<0001
Mean monthly humidity1−0.003500.02544−0.140.8921
Mean monthly air temp.10.131480.643590.200.8404
Mean monthly max. air temp.1−0.066470.43337−0.150.8798
Mean monthly min. air temp.1−0.082130.31742−0.260.7988
Mean monthly precipitation10.007580.002632.880.0100
Table
Table 5. Linear correlation coefficients of duration of combustion (DC) for Aleppo pine (Pinus halepensis Mill.) and Holm oak (Quercus ilex L.) on the island of Rab and in Makarska. Correlations in bold are significant at p < 0.05; N = 25.
Table 5. Linear correlation coefficients of duration of combustion (DC) for Aleppo pine (Pinus halepensis Mill.) and Holm oak (Quercus ilex L.) on the island of Rab and in Makarska. Correlations in bold are significant at p < 0.05; N = 25.
VariableTILFMCMean Monthly Air Humidity (%)Mean Monthly Air Temperature (°C)Mean Monthly Maximum Air Temperature (°C)Mean Monthly Minimum Air Temperature (°C)Mean Monthly Precipitation (mm)
Aleppo pine (Pinus halepensis Mill.)
Rab–DC1.00−0.120.060.270.250.270.02
Makarska–DC1.00−0.45−0.250.430.440.43−0.42
Holm oak (Quercus ilex L.)
Rab–DC1.00−0.35−0.070.240.230.24−0.05
Makarska–DC1.00−0.490.15−0.18−0.20−0.170.06
Table
Table 6. Results of the regression analysis of the duration of combustion (DC) as the dependent variable for Aleppo pine (Pinus halepensis Mill.) on the island of Rab and in Makarska.
Table 6. Results of the regression analysis of the duration of combustion (DC) as the dependent variable for Aleppo pine (Pinus halepensis Mill.) on the island of Rab and in Makarska.
Island of Rab
DFSSMSFPr > FR2Parc. R2Coef.
Var.		RMSE
model618.448773.074790.650.68750.1788−0.095020.605652.16978
VariableDFProc.
Param.		Standard
Error		tPr > |t|
Intercept17.3749710.105180.730.4749
LFMC1−0.013920.03665−0.380.7085
Mean monthly humidity10.069010.100300.690.5002
Mean monthly air temp.12.019282.495850.810.4290
Mean monthly max. air temp.1−1.117421.35450−0.820.4202
Mean monthly min. air temp.1−0.745981.79306−0.420.6823
Mean monthly precipitation10.003760.009490.400.6962
Makarska
DFSSMSFPr > FR2Parc. R2Coef.
Var.		RMSE
model621.454713.575781.310.30190.30430.072415.599321.65084
VariableDFProc.
Param.		Standard
Error		tPr > |t|
Intercept119.385399.853891.970.0648
LFMC1−0.052720.04319−1.220.2379
Mean monthly humidity1−0.039110.07714−0.510.6183
Mean monthly air temp.1−0.383921.95142−0.200.8462
Mean monthly max. air temp.10.068211.332040.050.9597
Mean monthly min. air temp.10.401331.057610.380.7088
Mean monthly precipitation1−0.003230.00772−0.420.6809
Table
Table 7. Results of the regression analysis of the duration of combustion (DC) as the dependent variable for holm oak (Quercus ilex L.) on the island of Rab and in Makarska. Correlations in bold are significant at p < 0.05.
Table 7. Results of the regression analysis of the duration of combustion (DC) as the dependent variable for holm oak (Quercus ilex L.) on the island of Rab and in Makarska. Correlations in bold are significant at p < 0.05.
Island of Rab
DFSSMSFPr > FR2Parc. R2Coef.
Var.		RMSE
model64.407580.734600.690.65700.1880−0.08268.955881.02824
VariableDFProc.
Param.		Standard
Error		tPr > |t|
Intercept117.023505.490303.100.0062
LFMC1−0.035380.02342−1.510.1482
Mean monthly humidity1−0.030790.05125−0.600.5554
Mean monthly air temp.10.406971.135610.360.7242
Mean monthly max. air temp.1−0.379250.60628−0.630.5395
Mean monthly min. air temp.10.017510.841620.020.9836
Mean monthly precipitation10.003740.004790.780.4453
Makarska
DFSSMSFPr > FR2Parc. R2Coef.
Var.		RMSE
model613.865502.310922.690.04840.47250.29667.523070.92741
VariableDFProc.
Param.		Standard
Error		tPr > |t|
Intercept120.000053.646055.49<0.0001
LFMC1−0.054920.01948−2.820.0113
Mean monthly humidity1−0.001670.04320−0.040.9697
Mean monthly air temp.12.013401.092831.840.0820
Mean monthly max. air temp.1−1.804690.73587−2.450.0246
Mean monthly min. air temp.1−0.030420.53898−0.060.9556
Mean monthly precipitation10.001030.004470.230.8200
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Rosavec, R.; Barčić, D.; Španjol, Ž.; Oršanić, M.; Dubravac, T.; Antonović, A. Flammability and Combustibility of Two Mediterranean Species in Relation to Forest Fires in Croatia. Forests 2022, 13, 1266. https://doi.org/10.3390/f13081266

AMA Style

Rosavec R, Barčić D, Španjol Ž, Oršanić M, Dubravac T, Antonović A. Flammability and Combustibility of Two Mediterranean Species in Relation to Forest Fires in Croatia. Forests. 2022; 13(8):1266. https://doi.org/10.3390/f13081266

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Rosavec, Roman, Damir Barčić, Željko Španjol, Milan Oršanić, Tomislav Dubravac, and Alan Antonović. 2022. "Flammability and Combustibility of Two Mediterranean Species in Relation to Forest Fires in Croatia" Forests 13, no. 8: 1266. https://doi.org/10.3390/f13081266

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