Allometric equations for crown fuel biomass of Aleppo pine (Pinus ...

Research Note

CSIRO PUBLISHING

International Journal of Wildland Fire, 2007, 16, 642–647

www.publish.csiro.au/journals/ijwf

Allometric equations for crown fuel biomass of Aleppo pine (Pinus halepensis Mill.) in Greece I. D. MitsopoulosA,B and A. P. DimitrakopoulosA A Laboratory

of Forest Protection, School of Forestry and Natural Environment, Aristotle University of Thessaloniki, PO Box 228, 54124 Thessaloniki, Greece. B Corresponding author. Email: [email protected]

Abstract. Allometric equations for the estimation of crown fuel weight of Aleppo pine (Pinus halepensis Mill.) trees in the Mediterranean Basin were developed. Forty trees were destructively sampled and their crown fuels were weighed separately for each fuel category. Crown fuel components, both living and dead, were separated into size classes and regression equations that estimate crown fuel load by diameter class were derived. The allometric equation y = axb with diameter at breast height as the single predictor was chosen, because the addition of other parameters did not decrease the residual sum of squares significantly. The adjusted coefficient of determination (R2adj ) values were high (R2adj = 0.82–0.88) in all cases. Diameter at breast height was the most significant determinant of crown fuel biomass. The aerial fuels that are consumed during crown fires (i.e. needles and twigs with diameter less than 0.63 cm) comprised 29.3% of the total crown weight. Live fuels constituted ∼96.3% of total crown biomass, distributed as follows: needles 16.7% (average load 12.07 kg), branches with 0.0–0.63-cm diameter 12.6% (average load 9.18 kg), 0.64–2.5-cm diameter 37.3% (27.99 kg), 2.51–7.5-cm diameter 25.4% (18.59 kg), and >7.5-cm diameter 3.7% (2.65 kg). The equations provide quantitative fuel biomass attributes for use in crown fire behaviour models, fire management and carbon assessment in Aleppo pine stands. Additional keyword: crown fires.

Introduction Wildland fires are the most destructive disturbance of the natural lands in the Mediterranean Basin (FAO 2006). Mediterranean landscapes have always been subjected to fires, and therefore burning has become part of their dynamic natural equilibrium (Moreno and Oechel 1994). Recent changes of land-use patterns have caused a reduction or abandonment of traditional activities (extensive grazing or wood harvesting), which has resulted in the increase of the amount of available fuel (Perez et al. 2003). The pyric properties of Mediterranean fuels in combination with the climate create conditions of extreme fire hazard (Dimitrakopoulos and Panov 2001). Low-elevation Mediterranean forests of Aleppo pine (Pinus halepensis Mill.) are extremely prone to crown fires and represent ∼1/3 of the total burned area in the Mediterranean Basin. In addition, the broadleaved evergreen shrub understorey (known as ‘maquis’) below the live crown fuel complex creates ladder fuels that facilitate fire transition from the forest floor to the canopy layer (Quezel 2000). In Greece alone, the forests of Aleppo pine represent 16% of the total burned area (Dimitrakopoulos 2001). As crown fuels are the main fuel layer supporting crown fire spread, crown fuel load assessment is essential for fire management planning (Keane et al. 2001). The relevance of crown fuel load in crown fire phenomena is easily understood as it represents the potential energy source available to be released during a crown fire. Crown fuel load is essential in assessing crown fire intensity and flame length (Byram 1959; Thomas 1963), fuel consumption during crown fires (Stocks et al. 2004), crown fire © IAWF 2007

severity (Pollet and Omi 2002), crown fire effects on ecosystems (Turner and Romme 1994) and carbon emission from crown fires (Amiro 2001). Furthermore, the ability to predict crown fuel load will facilitate fire managers’ assessing of crown fire potential (Sando and Wick 1972), the effectiveness of canopy fuel treatments (Johnson et al. 1998; Stephens 1998; Agee et al. 2000; Fule et al. 2001; Scott and Reinhardt 2001), the onset of crowning (Van Wagner 1977; Alexander 1998; Cruz et al. 2004) and the crown fire rate of spread (Cruz et al. 2002, 2005). Finally, the application of fire behaviour models that predict crown fire behaviour (Van Wagner 1977, 1989; Forestry Canada Fire Danger Group 1992; Finney 1998; Scott and Reinhardt 2001; Cruz et al. 2002, 2005) entails a need for quantification of crown fuel loads. Conifer needles and fine twigs are the main aerial fuels consumed within the flaming front of a crown fire (Van Wagner 1977; Call and Albini 1997; Scott and Reinhardt 2001; Stocks et al. 2004). Although numerous studies correlate crown or foliage biomass with tree dendrometric characteristics (Kittredge 1944; Moeur 1981; Grigal and Kernik 1984; Baldwin et al. 1997; Monserud and Marshall 1999), few studies measure crown fuel load by diameter size classes (Brown 1978; Stocks 1980; Johnson et al. 1989, 1990) as it is required in fire behaviour modelling. The objective of the present study was to develop allometric equations for the crown fuel load estimation of Aleppo pine. Diameter at breast height (DBH) was chosen as the independent variable because this characteristic is highly correlated with 10.1071/WF06038

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Aleppo pine crown fuel biomass

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Table 1. Descriptive statistics of Aleppo pine sampled trees n, the number of samples; DBH, diameter at breast height

n Minimum Maximum Range Mean Standard deviation Standard error

DBH (cm)

Height (m)

Age (years)

Crown length (m)

Height to live crown base (m)

Crown width (m)

40 7 56 49 28.9 14.2 2.2

40 5 24 19 13.2 5.2 0.8

40 12 54 42 30.8 12.8 2.1

40 3.5 18 14.5 10.1 3.4 0.5

40 0.5 9.5 9 3.1 2.4 0.4

40 1.4 10.8 9.4 4.8 3.3 0.5

tree biomass, and is consistently measured during forest inventories (Ter-Mikaelian and Korzukhin 1997; Zianis and Mencuccini 2004). Methods Study area The study area is located in the north part of the island of Evia in central Greece (latitude 23◦ 13 N, longitude 38◦ 40W). This area was chosen because it is representative of coastal Aleppo pine forests in Greece. The altitude is ∼300 m and the climate belongs to the Mediterranean type, with mild winters and dry hot summers. The mean annual rainfall is 440.1 mm and the mean annual air temperature is 22◦ C. In the past, numerous fires have burned different parts of the forest. Resin collection and grazing are traditional and current management practices. The forest comprises uneven-aged stands, often with a dense understorey of broadleaved evergreen shrubs (maquis). Fuel sampling Crown fuel biomass was measured by destructive sampling of a total of 40 trees during the summer. The trees were selected by random sampling from representative stands in the forest. Trees with extremely lopsided crowns, heavily defoliated or with broken tops were excluded (Brown 1978). For each sampled tree, DBH was measured to the nearest mm. The crown width was measured on each tree as the average of two perpendicular measurements taken on the ground on felled trees. Sample trees were felled carefully so as to avoid breaking branches. After felling, the tree age was measured and measurements of total height, height from ground to living crown and crown length were taken to the nearest decimetre. Crown fuels were separated into the following diameter classes (Deeming et al. 1977; Brown 1978): needle foliage, 0.0–0.63 cm, 0.64–2.5 cm, 2.51–7.5 cm, >7.5 cm, and dead fuels 7.5 cm) was dried for at least 120 h or until constant weight. Statistical analysis Oven-dry weights for each fuel size class were initially plotted against variables such as DBH, height, age, height from ground to live crown, crown length and crown width in order to provide a visual assessment between the relation of crown fuel biomass and the independent variables. Table 1 presents the descriptive statistics for the variables used in the analysis. Least-square regression models were developed for individual variables testing several curvilinear models. The regression models with the highest R2adj (adjusted coefficient of determination) and the lowest standard error of estimate (s.e.e.) were selected. Regression analysis revealed that DBH had the highest correlation with crown fuel weight and therefore was used as the primary independent variable in the present study. The addition of other variables such as tree height, age, height to live crown, live crown length and crown width did not improve the correlation significantly. Therefore, crown fuel weight was expressed as a simple power function of DBH, y = axb , where y = ovendry weight of a crown fuel component in kg, x = diameter at breast height in cm, and a, b = estimated coefficients. Estimates of a and b were obtained by ordinary least squares (OLS) after a log/log-transformation of the above allometric equation to a linear model. The antilog of the intercept was multiplied by a correction factor in order to account for bias inherent in fitting models to the geometric mean rather the arithmetic mean (Baskerville 1972; Sprugel 1983). The correction factor (CF) was calculated as follows (Sprugel 1983): CF = exp(S2y,x /2)

(1)

where S2y,x is the standard error of the estimate (s.e.e.) calculated from: s.e.e. = [(ln YI − ln YI )2 /(N − 2)]1/2

(2)

where N is the size of the sample, Y is the observed value and Y is the predicted value. For each model, normal probability plots of residuals and plots of standard residuals v. predicted values were calculated to test the assumptions of least-squares regression. In addition, plots of standardised residuals against

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Table 2. Crown loads for each fuel component in Aleppo pine sampled trees Crown fuel category

Average load by crown fuel component (kg)

Crown load distribution by fuel component (%)

12.07 9.18 27.99 18.59 3.10 2.65 69.99 72.65

16.7 12.6 37.3 25.4 4.3 3.7 96.3 100

Needles Branches 0.0–0.63-cm diameter Branches 0.64–2.5-cm diameter Branches 2.51–7.5-cm diameter Branches >7.5-cm diameter Dead fuels 7.5 cm in diameter) branches. The variables for the

allometric regressions are given in Table 3. The relationships for live crown fuel components were able to explain more than 82% of the observed variation. The allometric model failed to provide reasonable estimates for the largest crown branches category (>7.5 cm), perhaps owing to the limited number (n = 12) of sampled trees with this fuel category. As fuels larger than 1 cm in diameter are rarely consumed during crown fires (Call and Albini 1997; Scott and Reinhardt 2001; Stocks et al. 2004), the estimation of large diameter crown fuel classes is of minor importance for fire behaviour prediction. Our results support the most common mathematical model in biomass studies y = axb , where y is the aboveground biomass or crown fuel weight and x the DBH (Brown 1978; Stocks 1980; Johnson et al. 1990; Ter-Mikaelian and Korzukhin 1997; Zianis and Mencuccini 2004). It is emphasised that the allometric regression equations presented in the present paper are only valid within the range of stem diameters (7–56 cm) covered by the sampled trees. Equation reliability This is the first crown fuel biomass study of Aleppo pine in the Mediterranean Basin employing destructive sampling of individual trees (n = 40). The data more closely represent errorfree measures of crown fuel components because they were obtained in total on each tree. A problem in most attempts to model aboveground biomass is that biomass components are estimated from samples rather than whole tree measurements. Variability of these estimates confounds the model-fitting process. Subsampling to evaluate the weight of each tree component or roundwood diameter class is less accepted (Catchpole and Wheeler 1992). Direct measurement is extremely expensive and time-consuming, and consequently, oven-dry tree biomass estimates of individual tress will continue to be calculated from subsamples taken within the crown. Several authors pointed out that the dimensional relationships, for which the allometric equations are developed, are dependent on age, density and site quality (Baskerville 1983; Long and Smith 1988). Also, the crown fuel load equations might show an over-prediction bias in forest stands with high densities and basal areas (Cruz et al. 2003). The trees sampled in the present study are representative of the sizes and conditions of Aleppo pine most commonly found in the eastern Mediterranean Basin. As biomass relationships for crown fuel

Aleppo pine crown fuel biomass


Needles

Branches 0.0–0.63 cm 3.5

3.5

3

In(branches 0.0–0.6 cm, kg)

4

In(needles, kg)

3 2.5 2 1.5 1 0.5

2.5 2 1.5 1 0.5 0 1.5

⫺0.5

0 1

1.5

3 3.5 4 2 2.5 In(diameter at breast height, cm)

4.5

4.5

4



4.5

3.5 3 2.5 2 1.5 1 0.5

3.5

4

4.5

5

4 3.5 3 2.5 2 1.5 1 0.5 0

⫺0.5

0 4.5

2

2.5

3

3.5

4

4.5

5

⫺1

5

In(diameter at breast height, cm)

Live crown biomass

Total crown biomass

6

7

5.5 5

In(total crown biomass, kg)

In(live crown biomass, kg)

3

Branches 2.51–7.5 cm 5

2 2.5 3 3.5 4 In(diameter at breast height, cm)

2.5

In(diameter at breast height, cm)

Branches 0.64–2.5 cm

1.5

2

⫺1

5

5

1

645

4.5 4 3.5 3 2.5 2 1.5 1

6 5 4 3 2 1

0.5 0

0 1

1.5


4.5

5

1

1.5


4.5

5

Fig. 1. Regressions of various crown fuel components and the diameter at breast height. Solid line is regression; dashed line represents 95% confidence intervals.

size classes have not been reported for other Mediterranean regions, these equations could be applied elsewhere on an interim basis. However, for future research, the influence of topographic and stand structure characteristics on the crown biomass–DBH relationship of Aleppo pine should be investigated, in order to render these equations more globally applicable.

Conclusions The present study produced allometric equations that allow the estimation of crown fuel biomass at tree level for Aleppo pine, based on the diameter at breast height. The log/logtransformation of the allometric model y = axb gave the best fit for all live crown fuel components except the branches with diameter >7.5 cm. The R2adj values were between 0.82 and 0.88.

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Table 3. Regressions of crown fuel components (kg) on diameter at breast height (cm) forAleppo pine trees in Greece Regression equation form: ln y = ln a + b(ln x). s.e., standard error; R2adj , adjusted multiple coefficient of determination; s.e.e., standard error of estimate; CF, correction factor (Sprugel 1983). ∗∗ , P < 0.001; ∗∗∗ , P < 0.0001 Crown fuel component Foliage 0.0–0.63 cm 0.64–2.5 cm 2.51–7.5 cm Dead fuels