Fire behavior in an Ecotonal Grassland of the Chaco region, Argentina

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ARTICLES
RIA / Vol. 38 / N.º1
Fire behavior in an Ecotonal Grassland
of the Chaco region, Argentina
KUNST, C.1, LEDESMA, R.1, BRAVO, S.2, DEFOSSÉ, G.3, GODOY, J.1, NAVARRETE, V.1
ABSTRACT
We wanted to assess the fire behaviour of an ecotonal grassland, a fire prone ecosystem native of the Chaco
region, northwestern Argentina. The fire season extends from July to October, during the dry and cold season
of the year. The site of the experiments was an ecotonal grassland 50-100 m wide located in the Santiago del
Estero Research Station, (28º 03’ S 64º 15’ E). Fire was applied in two blocks, in October 2007, in 6 plots each.
Fine fuel load, botanical composition, and fine fuel bulk density were estimated. Fire behavior was assessed
by estimating forward rate of spread and flame length. Wind speed, wind direction, air relative humidity and
air temperature were assessed by a Kestrel digital hand-held station. Means, standard deviations and other
descriptors were used to summarize the data. Correlation among variables was assessed using the Kendall’s
τ correlation coefficient. The Student t-test was used for mean comparisons. Blocks had a different botanical
composition, presenting the plots dominated by Trichloris pluriflora, a tall, stemmy native grass larger mean
fine fuel load and mean bulk density than those dominated by Pappophorum pappipherum, a leafy bunchgrass
(p < 0.0001). These facts significantly affected the forward rate of spread (p < 0.0001) but flame length was unaffected and more related to the presence of shrubs with volatile compounds in the plots. Average forward rate
of spread was 24.0 m*min-1, a magnitude comparable to other subtropical grasslands. Average flame length
was 3.5-4 m in the fine fuel, a magnitude usually prescribed for brush control, but that needs wide blacklines
to be controlled.
Keywords: fire behavior, Chaco, brush control.
INTRODUCTION
The Chaco is a vast natural region in northern Argentina
and surrounding countries (Morello and Adamoli, 1968; Bucher, 1982). The vegetation of the Chaco region is a mosaic
of forests, woodlands, savannas and shrublands (Morello
and Adamoli, 1968; Bucher, 1982). Fire has been used
historically by Chaco aborigines for war, hunting and other
purposes (Bucher, 1982). Today, fire is used by cattlemen
and farmers to achieve proper range management and land
clearing (Bordón, 1993). Fire usually starts in the savannas
and grasslands due to the high amounts of available fine
fuel, but it may propagate to the woodlands and forests in
the form of a crown fire in extreme environmental conditions (Tortorelli, 1947; Morello and Adamoli, 1974).
Instituto Nacional de Tecnología Agropecuaria, Estación Experimental Agropecuaria Santiago del Estero, Jujuy 850, Santiago del Estero, G4200CQR, Argentina . Correos electrónicos: [email protected] [email protected]
2
Universidad Nacional de Santiago del Estero, Botany Chair, Faculty of Forestry, Av. Belgrano (S) 1912, Santiago del Estero, Argentina.
3
Centro de Investigación y Extensión Forestal Andino Patagónico (CIEFAP) y Universidad Nacional de la Patagonia San Juan Bosco,
Sede Esquel, Esquel, Chubut. [email protected]
1
Received February 11th 2011 // Accepted December 27th 2011 // Published online February 29th 2012
April 2012, Argentina
The new approach of fire management considers fire
ecology and fire prevention as two of the components of
the fire triangle (Myers, 2008; FAO, 2007). The fire behavior
in the Elionorus muticus savannas, an important vegetation
type of the Chaco, can be deducted from studies conducted by Bravo et al. (2000) and Kunst et al. (2001). The first
reported the fire regime in Elionorus muticus savanna of the
Chaco and the latter effects of fire on Acacia aroma Hook
et Arndt (tusca) a common brush species of the savanna,
from a management point of view. Fire behavior features
such as fireline intensity, flame length and rate of spread
were quite similar to those reported by Cheney and Gould
(1993), Cheney and Catchpole (1995) and Streeks et al.
(2008) for grasslands of Australia and Texas.
The specific objective of this research was to assess the
fire behavior of a Chaco grassland, a fuel complex present
in the midland range site, located between the dry forest
in the upland range site and the savanna in the lowland
range site, that could be described as an ecotone between
the latter vegetation types (Kunst et al. 2006). Since our
research is targeted toward the use of fire (prescribed fire)
the behavior of a headfire was tested and registered. By
describing the fire behavior in this vegetation type we attempted to understand the role of fire and to complete the
information on fire regime of the Chaco region. The description of fire behavior is also a basic information needed
in order to develop fire management programs considering
both prescribed burning and fire control.
MATERIAL AND METHODS
Study area. It was located in the ‘La Maria’ Experimental
Ranch, Santiago del Estero Experimental Station, Instituto
Nacional de Tecnología Agropecuaria, Santiago del Estero,
Argentina (28º 3’ S and 64º 15’ W). The climate is semiarid
subtropical. Winter is cold and dry and summer is warm
and rainy (Boletta, 1988). The mean annual precipitation
is 574 mm (Meteorological Station Santiago del Estero Experimental Station, 1990-2008 series, S. D. 208 mm; C.V
36%; confidence interval for the mean 519.88 – 618.11
mm. year-1 series 1936-2005) falling mainly from November to May.). Fire has an inherent seasonality in the Chaco
region: due to climatic features, the fire season begins in
June, at the start of the dry and cold season, and ends in
October, with the first rains and increasing air temperatures (Kunst et al. 2001). Fine fuels accumulate in Elionorus
muticus savannas and Chaco grasslands during summer
and fall; and are at appropriate burning conditions during
late winter and early spring. Atmospheric conditions include
steady winds from the north/north-east (mean velocity 2040 km/h), air temperatures up to 30 – 35 ºC, and relative air
humidity down to 20% at noon and early afternoon (Bravo
et al. 2000). These conditions are used to setting the fires,
generally to achieve objectives of shrub clearing and control, and the promotion of new grass growth.
The vegetation type studied was a native grassland located in the midland range site, an ecosystem bordered by a
forest dominated by hardwood tree species in the upland
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and a bunchgrass savanna in the lowland site (Kunst et
al., 2006). Usually the grassland occupies a ‘strip’ with a
width at least 50-100 m and. If a crownfire kills the forest
trees, the species of the grassland ‘enter’ in the upland site,
creating a ‘quemado’, a grassland with the same botanical composition of the midland grassland (Morello and Saravia Toledo, 1958). Main fuels propagating the fire in the
grassland are native tall and bunch grass species. Brush
species, specially Lippia turbinata and Schinus spp, rich in
volatile compounds, are interspersed in the grassland, and
may or may not ignite, depending on the meteorological circumstances and the firing technique used
Field work. During the 2007 fire season two blocks of
six plots each with a minimum size of 30 m x 30 m were
burned in order to assess fire behavior. The size of the plots
corresponded with the width of the grassland ‘strip’ and its
orientation in relation with the prevailing winds. Larger plots
would not allow the building of blacklines.
Fuel loads. Fine fuel loads were assessed by locating
randomly sample quadrats area = 0.25 m2, n > 5 in each
plot. The grass species present in each sampling quadrat
were identified and registered, and the data were used to
calculate species frequency (presence/absence). Aboveground plant biomass was harvested to a height of 2 cm
in each quadrat, separated by hand in standing biomass
and litter, stored in bags, taken to the lab and ovendried
48 hs at 60 oC. Moisture content of the samples was estimated gravimetrically. In selected plots standing fine fuel
was separated in three strata: 0-0,25 m, 0,25-0,50 m and
more than 50 cm. Results were expressed in kg dry matter
(DM)*ha-1. Curing, define as % of dry tissue in the plants,
was estimated visually.
Meteorological data. In each burn, air relative humidity
(%), wind speed (km*h-1) and air temperature (oC) were
registered using a Kestrel 6000 digital meter at midflame
height.
Firing techniques. Drip torches were used to lit headfires, in a line perpendicular to the prevailing wind direction,
usually N-NE.
Bulk density. Bulk density was estimated by the ratio of
fuel load, total grass height (m) and height strata (m) and
expressed as kg DM*m-3
Estimation and description of the elements of fire behavior. Fire behavior in a particular vegetation type (~ fuel model) could be estimated using several approaches: describing the weather pattern, topography and fuels (Agee 1993,
De Bano et al., 1998) or by stating where a fire burns, how
fast it travels, how much fuel is consumed and how much
heat is released (Encyclopedia of Southern Fire Science
2009). Weather was described in the previous paragraphs.
The local topography of the Study area is a mixture of very
gentle hills and flats; however, slope could be estimated @
0 for practical purposes. However, wind speed and specially direction could be modified by forests and shrublands
that act as wind funnels (Kunst, personal observation).
Fuel description was achieved by sampling. The amount
KUNST, C.1; LEDESMA, R.1; BRAVO, S.2; DEFOSSÉ, G.3; GODOY, J.1; NAVARRETE, V.1
ARTICLES
R = D/(t0 – t1) [1]
Where: D (m) = was a side or the hypotenuse of a plot;
t0 and t1 (sec) the time at the start of the fire and the time
when the fire front arrived to the end of the plot. Residual
fires were not taken into account for this calculation. Time
was estimated by a chronometer, with a 1/10 second precision. Results were expressed in m min-1. Fine fuel consumption was estimated visually.
Statistical analysis. Means, standard deviations and
other descriptors were used to summarize the data. Correlation among variables was assessed using the Kendall’s
τ correlation coefficient. The Student t-test was used for
mean comparisons. Mathematical calculations were performed using the PROC MEANS, CORR and GLM of the
SAS Statistical Package (SAS 1998).
A
VA
0
Block 2
B
Maximun
3,5
x
Minimun
1000
2000
3000
x
1,75
x
Block 1
Block 2
Figure 1. Mean and range of fuel load (A) and bulk density (B) for
each block. INTA EEA Santiago del Estero, ‘La Maria’ Experimental
Ranch.
Flame length
4000
Mean
7
5000
6000
7000
8000
Flame length (m)
VA (m*min-1)
Block 1
3,5
0
Mean
x
20
0
Minimun
4500
RESULTS
40
x
x
0
Trichloris pluriflora and Pappophorum pappipherum were
the dominant grass species in block 1 and block 2, respectively (data not shown). The first is a erect tall grass,
Maximun
9000
Fine fuel load (kg DM.ha-1)
of heat released was estimated by observing flame length,
(m) assessed visually by two independent observers at the
site of the burn and averaged. Flame length was used to
estimate fire intensity, a feature defined as the energy released along a linear fire front. It is strongly related to the
energy content of the fuel, fuel load, and fire rate of spread
(Alexander, 1982; Agee, 1993). The mean rate of forward
spread of the fire front (R) was calculated as the ratio between the distance traveled by the fire front and the time
required (formula 1):
Bulk density (kg.m-3)
6
0
9000
Fine fuel load (kg MS*ha-1)
Figure 2. Bulk density by grass species and height strata. References: Tri plu: Trichloris pluriflora; Papp papp: Pappophorum pappipherum. Columns with different letter are significantly different (p < 0.05, t-test).
7
Flame length
40,00
7,00
20,00
3,50
0,00
0,000
0,500
1,000
1,500
2,000
2,500
3,000
Flame length (m)
VA (m*min-1)
VA
0,00
3,500
Bulk density (kg MS*m-3)
Figure 3. Relationships between bulk density (BD), forward rate of spread (R) and flame length. Chaco grassland, INTA EEA Santiago
del Estero, ‘La Maria’ Experimental Ranch.
Height strata
Attribute
Study site
0-0,25 m
0,251-0,50 m
0,50-0,75 m
Total
C
1
3483,52
2547,88
286,8
6318,2
2
1786,96
968,88
140,72
2896,56
1
1.39 a
1.01 a
0.11 a
2
0.71 b
0.38 b
0.06 b
DA
Table 1. Mean fine fuel load (C, kg DM*ha-1) and mean bulk density (DA) for two study sites by height strata in a grassland of a midland
ecological site of the Chaco region. La Maria’ Experimental Ranch. INTA EEA Santiago del Estero. Different letters indicate significant differences within columns, α = 0.05.
Mean rate of speed
Date
Study site /
Plot
Fuel load
(kg MS*ha-1)
Flame length
(m)
10/12/2007
1,1
1,2
1,3
1,4
1,5
1,.
5982,48
6876,32
7752,72
6073,36
6280,88
5698,96
3--4
5--6
3--4
3--4
4--6
4--7
Mean
21/11/2007
Mean
2,1
2,2
2,3
2,4
2,5
2,6
3606,88
3647,44
2956,48
1970,96
2238,72
1986,16
4--4.5
6
6
4
4
4
m.min-1
km.h-1
16,97
1,02
sin datos
33,41
31,43
27,37
29,26
2,00
1,89
1,64
1,76
27,62
1,66
17,96
30
25,64
12,88
16,39
20
1,08
1,80
1,54
0,77
0,98
1,20
20,7
1,24
Table 2. Flame length and forward rate of spread of fires in a grassland in a midland ecological site, Chaco region. ‘La Maria’ Experimental
Ranch. INTA EEA Santiago del Estero,
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ARTICLES
with strong and lignified flowering stems, while the second
is bunch grass were leaves prevail. Curing was estimated
as 100 % in both cases, while T. pluriflora plants were taller
than P. pappipherum (Table 1). The fuel presented the highest density among blocks (p < 0.0001, Fig. 1). T. pluriflora
has the bulk of the tissue near the base of the plant (Fig. 2).
Mean forward rate of spread was significantly higher in
the first block (Tables 1 and 2) while flame lengths tended
to be larger in the second block. The shape of the fire front
was usually parabolic. Flame length in the grass averaged
3-4 m, however if volatile fuels (mainly shrubs) were present, flame length increased up to 6-7 m, irrespective of
the fine fuel load present. Fine fuels were consumed to the
ground, leaving black ashes. Mean forward rate of spread
was significantly and positively correlated with fine fuel
load (t = 0.42, p < 0.0001) and bulk density (t = 0.41, p <
0.0001), but flame length was not (Figs. 3 and 4).
by including botanical composition in fire-spread models.
Flame length was not influenced by both fine fuel load and
bulk density, probably because its was masked by the presence of shrubs with volatile compounds in the plots.
CONCLUSIONS AND MANAGEMENT IMPLICATIONS
Fires in the Chaco ecotonal grasslands move fast, with
flames at least 3.5-4 m average and fuel is consumed to
the ground. The forward fire spread was associated to botanical composition, that influences fine fuel load and bulk
density, rather than by specific weather features defined by
a prescription. However, the amount of heat released, as
estimated by the flame length, was more related to the presence of fuel components with volatile compounds.
ACKNOWLEDGEMENTS
DISCUSSION
Fine fuel loads in the Chaco grasslands were larger than
those reported by Streeks et al. (2005) for Texas but similar
to those informed by Cheney et al. (1993) and Cheney and
Gould (1995) for Australian grasslands. Differences in bulk
density among blocks are attributed to the dominant grass
species.
The use of fire in rangeland management is bound to
weather prescriptions that avoid extreme fire behavior,
especially flame length, while still fulfilling the objectives,
which usually are brush control. Weather during the burns
of this research could be considered ‘within prescription’ if
compared with those developed by Britton et al. (1987) and
Trollope (1984a and b), while somewhat inclined to the ‘red’
corner of the window. Differences in R could be attributed
to structure of grasses: a tallgrass, stemmy species such
as T. pluriflora would allow the faster propagation of the fire
front since the flames have an aerial structure to ‘catch on’.
Flame lengths were within the range reported for grassland
and savanna fires (Trollope, 1984a and b; Trollope and Tainton 1986, Kunst et al., 2001) indicating fires of high intensity,
prescribed to damage or kill the aboveground structure of
woody species of the genera Acacia, Prosopis and Schinus that an hindrance in rangelands (Trollope, 1984a and b;
Kunst et al., 2001). However, these fires would not permit
direct attack, and their control should be based in indirect
measures such the building of blacklines with a width > 30
m, as suggested by Wright and Bailey (1982).
From the point of view of the prediction of fire behavior,
there are contradictory results related to influence of fuel
loads on fire behavior. The classic view gives strong importance to fuel load in predicting R, while some studies
suggest that has no influence at all (Cheney et al., 1993).
In our study, R was significantly influences by fuel load and
bulk density, i. e. by the quantity of fuel but also by its distribution in a tridimensional space. This fact could be attributed to the different architecture of the grass species that
composed the grassland. This fact could be accounted for
Research was funded by the Project no. FP6-018505
FIRE PARADOX ‘An innovative approach of integrated
wildland fire management regulating the wildfire problem
by the wise use of fire: solving the fire paradox’, European Union, 6th Framework, and Instituto Nacional de Tecnología Agropecuaria, Specific Project 1503 ‘Incremento de
la productividad de pastizales naturales’ 2006-2009.
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Fire behavior in an Ecotonal Grassland of the Chaco region, Argentina

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