Landscapes Forest and Global Change - ESA - Escola Superior ...

P.M. Fernandes et al. 2010. Testing the fire paradox: is fire incidence in Portugal affected by fuel age
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4. Discussion
Considering the fire return intervals involved, this study reflects primarily the fire regime of
shrubland and regenerating forest. The results are coherent with the fuel dynamics described for
shrubland in northern (Fernandes and Rego 1998) and central Portugal (Fernandes et al. 2000)
which indicate increased flammability with time since fire; depending on shrubland type, fine
fuel loads in these systems are at near-equilibrium 9 to 26 years after fire.
While the typical fire return interval is shorter in N-CW, age-dependency is more apparent in
CE-S Portugal. The differences can be explained by distinct fire environments in terms of
physiography, vegetation and land use, as well as by differences in climate and ignition density.
Topography is rougher and vegetation types − namely atlantic and sub-atlantic shrubland types
(Ulex, Erica, Pterospartum, Cytisus) and Pinus pinaster and Eucalyptus globulus forest − are
more flammable in N-CE than in CW-S Portugal. Also, because of drier climate, growth rates
and fuel accumulation are expected to be slower over much of CW-S in comparison with N-CE.
Mediterranean shrubland types prevail in CW-S (limestone maquis and garrigue, Cistus), and
because their canopy is poor in dead fuels, exhibit fire behaviour patterns in relation to weather
that are distinct from atlantic and sub-atlantic shrublands and that may increase the agedependency
of fire hazard. Density of fire ignitions is also substantially higher in N-CE, where
traditional fire use at short-return intervals for pastoral purposes is common in the mountains.
Higher fire recurrence may promote a fuel complex with a significant grass component that will
burn more readily, hence facilitating more frequent fire.
Table 1: Weibull model parameters (and 95% confidence intervals) and median fire-free interval (MEI)
for the fire frequency analysis. Fire size class data respect to the N-CE sub-division.
Data set n MEI b c
N-CE 2862 12.1 13.7 (13.5-13.9) 3.0 (2.9-3.1)
CW-S 88 15.5 17.0 (16.1-17.9) 4.1 (3.5-4.7)
Fire size class
10-99 ha 2131 12.3 13.9 (13.7-14.1) 3.0 (2.9-3.1)
100-499 ha 594 11.7 13.2 (12.8-13.6) 3.0 (2.8-3.1)
500-999 ha 88 11.2 12.6 (11.7-13.5) 3.1 (2.6-3.6)
≥ 1000 ha 49 12.2 13.7 (12.4-15.1) 3.2 (2.6-3.8)
Table 2: Burnt patch size statistics by fuel age class.
Fuel age class (yrs.) n Median (ha) CV (%) Max. (ha)
3-4 26 50.2 117.7 379
5-6 197 43.4 146.1 1100
7-8 418 48.3 190.4 2196
9-12 1043 44.0 222.9 5487
13-19 1681 41.8 351.1 13119
≥20 6832 27.3 876.6 66071
Forest Landscapes and Global Change-New Frontiers in Management, Conservation and Restoration. Proceedings of the IUFRO Landscape Ecology
Working Group International Conference, September 21-27, 2010, Bragança, Portugal. J.C. Azevedo, M. Feliciano, J. Castro & M.A. Pinto (eds.)
2010, Instituto Politécnico de Bragança, Bragança, Portugal.