PRESCRIBIEDBURNING USE IN FOREST FIRE MANAGEMENT
Francisco C. ~ e g o and l Paulo M. ~ e r n a n d e s ~
I EstaqBo Florestal Nacional, Institute Nacional de Investigagio Agriria Tapada das Necessidades, 1350 Lisboa, PORTUGAL Tel +351 1390 16 61 Fax +35 1 1 397 3 1 63; E-mail
[email protected] 2 Departamento Florestal, Universidade de Tras-0s-Montes e Alto Douro, Apartado 202,5001 Vila Real, PORTUGAL Tel. +351 59 320236 Fax +35 1 59 74480 E-mail
[email protected]
Introduction gathering, war, Fire manipulation by Man started 50,000 years ago as a way to inipmve h~~nting,
grazing and agriculture activities. However the first rational recornme~~dation for the use of fire in forest management seems to belong to Friderico Vanlhagen In 1836. This forcst officer wrote an authentic burning prescription for the pine plantations in Central Portugal, that still rcrnains largely valid: "AAer the pine forest is 20 years old there is a safe way to prevent it from being burned by summer wildfires: kuniing the litter in dry winter days will not damage the roots, and if that opcration is repeated annually there will be no dangcr of wtldfires ... The pine stands that get used to see their floor burned atlt~i~ally grow much
i I.'
faster and the benefit is great in every sense ... It is obvious that for a safe operation, even in wintertime, the forest should not have tall shnrbs in the understory ... The burn must be done with appropriate winds, and fire should be lit on the opposite side of the wind". The work of this pioneer was forgotten but prescribed fire was rediscovered and it is used nowadays in many parts of the world. The variety of purposes that prescribed burning can accotnplish makes it the most powerful and tlexible tool in forest and wildland management. The correct use of prescribed burning requires adequate planning made in advance, including the definition of objectives, constraints, desired fire effects and behaviour, ignition technique, etc. A burning plan and an evaluation report are therefore of extreme importance to achieve the management objectives and monitorize the results. This accumulatton of knowledge is fundamental, since prescribed fire needs to be done by experienced and properly trained operators.
Weather Conditions for Prescribed Burning A gencral llnderstanding of the separate and cotnbined effects of scveral weather elemalts on fire
behaviour is needed to exccuLe a good bum. Weather factors influence li~elrnolsture, a~hichis critical to success. Dccause weatllcr and I'LICI factors intcract, an experienced prcscribcd burner can corlduct a successfi~lburn cven with onc or mol-e factors slightly outside the desis~drange as lung as they are oll>ct by othcr factors. Prescnbcd lire should be accomplished during thc dorniancy season. It 1s possiblc to bum between October (after 25 mm of rainfall) and April, but the bcst wcather conditions nonnally occur bctwecn December and March. The preferred weather situa[ion follows the passage o f a cold iront that has brought
5 to 20 mrn of rain. During this tinlc, persistent winds, low relative humidities (but higher lhan 40 per cent), cool temperatures and sunny days can be anlicipared. Prcscribcd tires behave in a more prajictable manner when wind speed and direction arc steady, which is particularly important when burning critical areas with regard to file1 and topography. Relatively high winds quickly dissipate the heat of a backing lire, reducing the probability o f trec crown scorch occurrence and increasing smoke dispel-sion. Howevcr, ]nature pitre stands with sparse understory can bz humcd at vcry low wind specds, just enough to givc direction to the fire. Table I . \Veathcr and firc behaviour rangcs Tor prescribed bunling of miiritime pine (Pinirs pinoster) stands. hlinim.
Opti~nurn
2
5
blaxim.
Weather Nu days since last rain Temperature, OC
4
6 - 15
20
Kelativc humidity, %
40
50 - 70
85
Midtlarne windspeed, kmw'l~
0.5
2-6
20
Dead fine fuel moisture content, %
12
15 - 25
35
Duff moisture content, O/a
80
> 80
Katc of spread, d m i n
0.2
0.5 - 1 .0
Flame length, In
02
0.5 - I .O
1.5
Fireline intensity, kW/ni
30
70 - 200
400
Fire behaviour 3.0
* equally acccptablc for other Mediterranean pines, eucalyptus and Q I I L ~ / . c I I S / ) J ' ~ ~ ~ I ( ~ ~ C ~
The ranges of weather and fire behaviour parameters that are suitable for controlled buming are displayed in Table 1 (Silva 1984, Botelho et al. 1987, Femandes 1997). Burning conditions and ignition technique will be related to each other in the next section.
Firing Techniques Various firing patterns can be used to accomplish a bum objective. The technique chosen must be correlated closely with environmental conditions to prevent damage to resources, and can change as fuels, topography and weather change. Before buming it is necessary to prepare an area for firing and to have all needed tools and equipment in operating order and ready to go. Control lines must be lomted and established, preferably taking advantage of natural or artificial barriers. The major firing techniques will be described according to Wade and Lunsford (1989), Botelho (1990), Botclho et al. (1994) and Vcga et al. (1 994). Backing fire is the easicst and safcst type of prescribed !ire, providing that wind speed and
direction are steady. A back fire is started along a banier and allowed to back into the wind, normally descending a slope. This technique is associated to low rates of spread (0.3-0.6 m minu]) and short tlames that correspond to fireline intensities of 40- 150 kW m'l. Back fires produce minimum scorch and good smoke dispersion and can bc used in hcavy fuels and young pine stands (with diameter at the breast hcight higher than 6 cm). Major disadvantages are the slow progress of the fire (increasing costs) and the potential for root damage, with increased exposure to heat if the lower duff is not moist enough. The best conditions for backing fire are attained with steady winds of 2-6 km h-I, continuous fuel and dead fine file1 moisture contents of 12-22 per cent -- fucl consumption and fire spread will be unsatisfactory above 22 per cent and fire will stop propagating at 25 per cent. In strip-firing, a series of lines of fire are set progressively or sin~ultaneouslyup-wind o r downwind of a firebreak in such a manner that no individual line can develop to a high intensity level before it
reaches either a firebreak or other line of fire. Strips are set 5 to 50 ni apart, depending on the desired flame length and intensity (nomially between 100 and 300 kW m-I). Strip-firing is a very flexible technique and can be perfonned in a wide range of conditions, but not when fuel load is high; it is specially recommended for fuel management in short cycles after the first treatment and dead fuel moisture is in the range of 20-25 per cent. Execution is quite fast when compared to backfiring, but a higher numbcr-of personnel is required and the structure of the stand should guarantee visibility and safety.
back fires Fig. 1.
strb f k s
headfires
Prescribed fires fireline intensity (kW nl-I) according lo dead fine fuel moisture content (%) ranges for different firing techniques (Botelho et al. 1994). Occasionally, on areas with light and cven fuels, a heading fire may be allowed to move over the
entire area to better accomplish thc objectivcs. However one or more of the following conditions nus st be verified, in order to confine fireline intensity to 400 kW m-I and prevent fil-e escape and ecological damages: distance between crown basis and ground exceeding 3 meters, dcad fuel moisturc higher than 20 per cent and wind speed in the lower range. Head firing is the only ignition technique that can achieve self-sustained fire propagation when dead Fuel moisture content ranges from 25 to 35 per cent. Rate of spread of a wind-induced tire is 4 times higher than a back fire executed in the same conditions, which allows the treatment of a much broader area in the same period of time. This technique should be rescrved to highly experienced burning teams and implies additional operational costs to build, watch and secure wider control lines. The above-mentioned procedures and considerations are appropriate for prescribed under-burning of forest stands. In shrublands it is more troublesome to conduct back fires, particularly when dead fuel moisturc content is higher than 20 per cent orland in the absence of wind. Fuel bed discontinuity, lack of litter and high percentage of live fuel are the factors that explain this ditTiculty. On the other side, and because fires in shrub fuels are crown tires, high intensities are achieved during head fires, especially when dead nloisture content is near 15 per cent (1500-2500 kW m-I) but even for 25-30 per cent (5001000 kW m-I). Detrimental ecological effects are not expected for those bums, because duff (if moist) is let3 almost intact.
Other fire ignition techniques exist, such as: -
point source fires;
-
nng or perimetral firing;
-
pile burning.
However they are nnt currently used in Europe and we will not describe thcm here Nevcrtheless it is worth rncntioning aerial ignition, a technique appropriated for forest stands divided by blocks in flat terrain or when a mosaic effect is desircd. Large areas can be safely burned in a singlc burning period, including dispersed tracts, and the impact on air quality is reduced
Fuel Cunsurnption and Postburn Dynamics The effect on fuels is relatively independent from PI-escribedburning technique, Nevcrtheless backfires tend lo have a greater impact on litter and duff fuels, while headfircs cause more mnsumption of the shrub layer. Prescribed fire reduces essentially the fine fuels (less than 6 mm in diameter) load. However, the goal should be to consume the very fine fuels (< 3 mm) which represent the main wildfire hazard. The deepcr forest floor layers (F and H) have important physical (soil protcction) and biological roles and their depth and load should not be reduced in more than 50 per cent (Fig. 2). Fuel consumption by prescribed fire is dependent on prebum load and moisture content. A continuous and self-sustained fire front reduces fine fuels by more than 80 per cent (Botelho et al. 1994, Vega et al. 19941, but values of 60 per cent are considered acceptable. The presence of unbu~nedfucl patches has ecological benefits, because it facilitates postburn colonization by animals and plants. After prescribed fire, a new cycle of fuel accumulation begins following specific temporal patterns according to each vegetation type, and whose knowledge is important to plan and rationalize fire prevention activities. In maritime pine stands, litta returns to the initial amounts in 4 to 7 years, depending on site quality (Rego el al. 19931, and shrub biomass rebuilds in 7-10 years (Rego et al. 1988). Wildfires are easily controlled in northem Portugal shrublands subjected to prescribed burning intervals of 8 years (Fernandes and Rego 1996). As a general rule, fuel management is necessary when fine fuel load reaches 8 t ha-', but above 30 t ha-I understory prescribed burning should be replaced by other technique. Skillful planning of prescribed fire requires therefore the ability to estimate preburn ikel load (from easily measured field data or age records) and tbel consumption.
50
60
70
80
90
100 110 120 130 140 150 I60
Moisture contcnt, % Fig. 2,
Duff (forest floor F and H layers) consunipllon as n function of 11ioihture contrnr In Pitlrrs pinusler stands.
0 ' 0
-
4
8
--
12
-.
16
-
-
20
24
Age, yedrs
Fig. 3.
Fine fuel (< 6 mm) dynamics in low shrubland of Northzm Po~tugalas a functioli o f fuel consumed (%) by the last bum (Fernandrs 1997).
Prescribed Burning Planning
A prescribed burning operation involves 5 stages: analysis, prescrip'tio~~, preparation, cxecutiun and evaluation (Wade and Lunsford 1989). A written plan prepared by a knowledgeable person is needed for each area to be burned. The plan should be completed before the burning scason and the burning tcatns should be prepared to bum when the prescribcd weather occurs, The plan -- a prcparcd fonn with space for all information -- sliould illclude: -
explicit (and quantified, if possible) objectives;
-
scheme of the burning unit;
-
equipment and personnel;
-
fire prescription;
-
season;
-
time of day;
-
firing plan;
-
alternative prescriptions;
-
preparation work;
-
impact of smoke;
-
escaped-fire plan;
- control and mopup; -
evaluation;
-
identification of the person(s) who prepared the plan.
The first step to a successful prescribed bum is a stand by stand analys~s,determining the needs of each stand and what actions should be taken to meet these needs. Prescr~bedfire as well as other alternatives should be addressed and a decision making regarding the preferred treatment. A burning prescription consists of defining the weather conditions suitable to fulfill a given bunling
objective, the firing technique and the preparation and protection necessities. The essential points of the prescription must be defined in advance, even if slight adjustments are made when executing the burn. Preparation is the key to successfUl burning and it is essential in realizing maximum benefits at acceptable costs. It consists of all steps necessary in making the area ready for firing and of having all tools and equipment in operating order and ready to go. There are few days of good prescribed burning weather during the year (50 to 80 in northern Portugal). When those days arrive top priority should be given to burning. With adequate preparation, burning can begin without loss of opportunity. A prescribed burning crew consists normally of a leader -an experienced prescribed burner with an understanding of fire behaviour -- and 3 to 6 crew members. The following elements are important when executing a bum: -
make sure all equipment is in working order and safe;
-
cany burning plans and maps lo the job;
-
clicck thc weather before stal-tiny to bum and keep updated during the day;
-
clicck all control lines;
-
notify adjoining property owners and fire control organizations betbre starting fire;
-
check duff and soil for dampness;
-
test bum with a small fire before burning, to make sure the lire is burning as expected;
--
infonll crew of starling point and tiring sequence;
-
havc a means of instant communication with all crew rnembel-s;
-
be alert to changing conditions, and be prcpared to change burning techniques or stop the fire il' an emergency arises,
-
lnopup and patrol perimeters constantly during the operation, and tlicrcaftcr until there is no hrther danger of tire escapc.
Finally, the purposes ofcvaluating a bum are to dctcnnitie how well the objectives of the burti were I I - e An initial evaluation should be made immediately met and to gain information to be used in ~ L I ~ Lburns. after the bum. A second evaluation should be made during or aAcr the first posttire growing season. The following points should be considcrcd: -
amount of trees foliage disccloration;
-
anlount of file1 consumption;
-
smoke dispersion into upper atmosphere;
-
protection o r areas not to be burned;
--
any escape of fire;
-
any adverse public comment.
In the first growing season after the bum it is inipodant to check resin exudation from pine trees (an indicator of cambium damage or insect attack), other signs of insect attack, modality of trees, sprouting vigor of undesired vegetation, remaining duff layer, mineral soil exposed and any soil movement, and public expression regarding the burning program.
References
Botelho, H.S. 1990. Tecnicas de fog0 controlado. Pp. 57-61 In F. Rego & H. Botelho (eds.), A tecnica do fog0 controlado, UTAD, Vila Rcal. Botclho, H.S., F.C. Rego and J.M. Silva. 1987. Prescribed tire behavior in Pitt~rspinaster forests of Northern Portugal. In Com. International Symposium tor the formulation of an ctTective coninion stratcgie for the prevention and combat o f b r e s t tires in the Mediterranean Region. Athens, Greece. Botelho, H.S., J.A. Vega, P. Fernandes and F.C. Rego. 1994. Prescribed fire behavior and tine fuel consumption in Northern Portugal and Galiza maritime pine stands. In Proe. 2nd Int. Conf. on Forest Fire Research., 2 1-24 Nov. 1994, Coimbra.
Fensham, R.J. 1992. The management implications of fine he1 dynamics in bushlands surrounding Hobart, Tasmania. Journal of Environmental Management, 36: 301-320. Fernandes, P. 1997. 0 LISO da tecnica do fogo conlrolado: porqu&, quando e como. Revistn Florcstal lO(1): 70-78. Fernandes, P.and F.C. Rego. 1996. Changes in fuel structure and tire behaviour with healhland aging in No~themPortugal. In Proc. 13th Conf. on Fire and Foresl Meteorology, Out. 1996, Melbourne, Australia. Rego, F.C., M.G. Barreira and S. Bunting. 1988. Effects of prescribed fire on Cliamaesprrrtircm tridentalum ((L. P. Gibbs) in Pinus pinasler (Aiton) forests. Journal of Iiange Managcmcnt, 41(S):
410-412. Rego, F.C., H. Botelho, L. Ruas and P. Fernandes. 1993. Final Report of the STEP project "Forest Fire Prevention Through Prescribed Burning: Experimental Study on Fire Effects on Litter and Soil". UTAD, Vila Real. Silva, J.M. 1984. Fogo controlado: podcroso aliado do floreslal. Pp. 37-52 In Notas Ticnico-Cientilicas, . Vega, J.A., J.C Valette, F.C Rego, C. Hernando, D. Gillon, J. Ventura, S. Bara, V. Gomendy, H. S. Botelho, M. Guijarro, C. Houssard, L. Ruas, P. CuiAas, J. Marechal, J. Mendes-Lopes, R. Diez, P Fernandes, M. Fonturbel, J. Santos, M. Kozados & M. Beloso. 1994. Forest Fire Prevention ThrougI1 Prescribed Burning: an International Cooperative Project Carried Out in the European S.T.E.P. Program. 2" Int. Conf. on Forest Fire Research. Coimbra, 21-24 Nov. 1994. Wade, D. and J. Lunsford. 1989. A guide for prescribed firc in Southem forests. USDA For. Sen. Southern Region Techn. Public. R8-TP I I . Atlanta, Florida.
