Joaquim S. Silva1,2, Mauro Nereu1,2 Luis Queirós1,2, Ernesto Deus3 and Paulo Fernandes4
1College of Agriculture, Polytechnic of Coimbra, 3040-316 Coimbra, Portugal
2Centre for Functional Ecology, University of Coimbra, 3000-456 Coimbra, Portugal
3Centre for Applied Ecology, University of Lisbon, 1349-017 Lisbon, Portugal
4Centre for Research and Technology of Agro-environmental and Biological Sciences (CITAB), University of Trás-os-Montes e Alto Douro, Apartado 1013, 5001-801 Vila Real, Portugal
Alien plant invasions may change fire regimes by increasing the fire hazard. This work presents results on fuel characteristics and fire behavior of plant communities dominated by two aggressive, fire-adapted Australian species that are rapidly expanding in Portugal and elsewhere: Hakea sericea and Acacia dealbata. This work, based on the ongoing Aliens & Flames project, aims at: (i) assessing the fuel characteristics of plant communities dominated by these two species, (ii) simulating the potential fire behavior of these fuel complexes, and (iii) comparing these results with potential fire behavior in native plant communities. Fuel samples were collected at nine different sampling sites (n=9) for each species, reasonably representative of intermediate-mature developmental stages. Hakea sericea stands were in general thick shrublands ranging between 1.5 and 4.5 m height, whereas A. dealbata stands generally had a clear separation between the canopy and the surface fuel layers, with trees up to 10 m in height. Fuel characterization followed standard methodology aimed at obtaining the fuel parameters necessary to run the Rothermel fire behavior model, using the BehavePlus 5.0.5 software. Some fuel parameters were obtained from the literature and from allometric relationships developed within the project. Fire simulations were performed using a very dry moisture scenario, with 5km/h wind speed and zero slope. Fuel characteristics and simulated fire behavior were averaged and compared with native plant communities (shrublands, broadleaves) characterized by Fernandes et al. (2009), using one-sample t-tests.
The two species presented remarkably distinct fuel characteristics. The H. sericea stands had a higher load of fine fuels than the native shrubland. The fuel complexes associated to A. dealbata stands showed complete absence of shrubs and herbaceous layer. The litter (L+F) layer was particularly compacted, with nearly 8 t ha-1, concentrated in only 2 cm of fuel depth, because of the very fine leaves that accumulate on the forest floor. The results obtained with the fire simulations were consistent with the fuel characteristics. The fuel complex dominated by H. sericea presented the highest flammability followed by the shrubland model, whereas the native broadleaf model showed the lowest flammability and there was no fire propagation in the A. dealbata stand.
These preliminary results suggest that generalizations about the higher fire hazard of plant communities in areas invaded by alien woody species may not always hold true. While areas invaded by H. sericea seem to present a high fire hazard, because of the high loads of 1h fuels and the vertical continuity of these plant formations, on the contrary, mature A. dealbata stands showed a low fire hazard, mostly because of the absence of shrubs in the understorey and the very compact fuel bed. Preliminary burn experiments in the same study plots, were consistent with the fuel data. While flame lengths in H. sericea plots reached 3 m, there was strong difficulty in burning the litter floor of A. dealbata stands, for similar environmental conditions. Our results for H. sericea are apparently contradictory to those of Van Wilgen and Richardson (1985) who obtained a higher fire hazard in native shrublands of South Africa. However, as these authors point out, the limitations of the Rothermel model must also be taken into account when interpreting these fire behavior results.
Van Wilgen B. W. & Richardson D. M. (1985) The effects of alien shrub invasions on vegetation structure and fire behaviour in South African fynbos shrublands: a simulation study. J. Appl. Ecol. 22: 955–966.