1. Introduction
The 2017 fire season in Portugal was unprecedented, with a record of 540,000 ha burned, a total of 119 fatalities and millions of euros in losses and damages, resulting from several extreme wildfires [
1,
2,
3]. The weight of these numbers shocked society in general, leading to widespread recognition of the need for a paradigm shift in forest and fire management. Other countries also suffered extreme wildfires in recent years, such as Chile [
4], Brazil and Bolivia [
5], Australia [
6] and the USA [
7].
The Portuguese 2017 fire season was amplified by a severe drought and the occurrence of atmospheric conditions conducive to large wildfires [
8,
9,
10,
11,
12]. Adding to these factors, Portugal has extensive areas of undermanaged forests and shrublands that facilitate the occurrence of frequent, very large and uncontrolled wildfires [
13]. Overall, climate change will likely create conditions for more frequent wildfires and extreme fire behavior in the future [
14], potentially leading to severe fire seasons, such as that experienced in 2017.
Effective strategies are necessary to reduce the likelihood of severe fire seasons in the future. The landscape needs to be shaped to promote fire-resiliency in the medium and long term. One of the possibilities is to reduce landscape flammability and fuel continuity by managing fuels at the landscape level and, therefore, potentially offsetting the effects of current and future weather conditions on wildfire spread and behavior [
15]. To be an effective wildfire hazard reduction tool, landscape fuel management alternatives should be considered as part of a holistic and integrated approach, interconnecting the actors and phases of complex wildfires [
16,
17,
18].
Science can contribute with knowledge and tools for more effective landscape fuel management, thus improving planning and decision-making. Previous research has shown how fire spread simulation tools can be used to assess wildfire exposure at the landscape level [
19,
20,
21], quantify associated risk [
22,
23,
24], study wildfire transmission [
25,
26] and identify optimal fuel treatment location [
27,
28]. These simulation tools have also proven useful to quantify the potential impact of climate change on wildfire incidence [
29] and of mitigation measures on post-fire erosion and water contamination [
30,
31].
In the aftermath of the 2017 wildfires in Portugal, a group of landowners from the civil parish of Alvares, municipality of Góis, in central Portugal, requested support from the Forest Research Centre (University of Lisbon) to develop a plan for the rehabilitation of an extensively burned area in a way that would reduce its vulnerability to large wildfires. In central Portugal, large wildfires are very often associated with high intensities, severe damages and pose important threats to human health and both economic and forest sustainability [
10,
11]. Besides the 2017 wildfire, which burned 60% of its area, wildfires in Alvares over the last 40 years burned the equivalent to the total area of the parish twice. Such a short fire cycle is driven by long-term demographic and land-use changes common to other parts of rural Portugal, namely population decrease and aging [
32], abandonment of agricultural lands and expansion of forest and shrubland area [
16] and increasing frequency of droughts and heatwaves as a result of climate change [
33]. In addition, highly fragmented land ownership, with numerous small land properties owned by a very large number of landowners [
34] with low-income levels, leads to undermanagement of forests and pasture areas. These environmental and socioeconomic factors have contributed to developing a fire regime dominated by large wildfires.
The overarching project had the main goal of developing proposals to reduce the vulnerability of the Alvares parish to large wildfires based on three more specific objectives: (1) to reduce the frequency of large fires; (2) to improve the safety of people and assets; and (3) to strengthen the local economy. This integrated approach contributes to supporting the necessary change in the forest management paradigm and developing fire-resilient landscapes [
35]. Here, we evaluated how different fuel treatment strategies can reduce wildfire hazard in Alvares. Parallel studies have addressed the other two pillars [
34,
36,
37]. Two main strategies were analyzed: fuel breaks (linear treatment units) and dispersed random fuel treatments in the landscape (areal treatment units). Both strategies had different levels of implementation (i.e., extent in the landscape) and, when combined, resulted in twelve different fuel management scenarios for the parish. These strategies were chosen because they have been commonly applied in Portugal [
25,
38], and references therein] and because they allow achieving the other two specific objectives for the Alvares parish [
35]. In this work, the objective was to understand how these fuel management scenarios can change future wildfire hazard, then maintaining the landscape similar to conditions prevailing at the time of the 2017 wildfire. It is beyond the scope of the current work to optimize fuel treatments location and quantify the potential impacts of climate change on wildfire hazard.
2. Study Area
The Alvares parish has an extent of 10,057 ha and is located in the center of Portugal (
Figure 1). It has rugged terrain, ranging in elevation from 300 m in the south to about 1200 m in the north, coinciding with the Lousã mountain. Along this elevation gradient, precipitation ranges from 1100 mm to 1700 mm per year [
39]. The summer months (July and August) are usually dry and receive, on average, around 15–20 mm of precipitation each.
The Alvares landscape has suffered profound changes in the last century, shifting from a landscape dominated by shrubland, pastures and agricultural areas, with less than 10% forest area [
35], to a forest-dominated landscape (ca. 90%), composed mainly of
Eucalyptus sp. (mainly Tasmanian blue gumand shinning gum, hereafter eucalypt, covering 53%) and
Pinus pinaster Aiton (maritime pine ca. 30%, hereafter pine) (
Figure 1; Portuguese Land Cover Map 2015: COS2015, Direção Geral do Território). Both are harvestable commodities: eucalypt is used mainly for the pulp and paper industry, and pine is mainly used for timber. Like many other regions of the country, the Alvares parish underwent a pronounced population loss, with a 75% decrease in the number of inhabitants from 1960 to 2011 [
40,
41]. More than 96% of the lands are privately owned, by more than 3000 landowners, including two paper industry companies [
35].
The Alvares parish had 42 wildfires in the 1975–2017 period, which burned more than 20,000 ha, the equivalent to twice of the parish extent. About 90% of the burned area resulted from 10 very large wildfires that burned over 1000 ha each. Many areas of the parish burned more than 3 times over the past 43 years (see
Figure 2a). The last very large wildfire occurred in June 2017 and was the most destructive on record, burning around 60% of the parish area. These large wildfires are usually associated with high severity [
10,
11], are an important threat to the safety of people and assets [
37] and hamper economic sustainability [
36].
5. Discussion
This work introduced an innovative approach to estimate wildfire hazard at the landscape level by considering uncertainties and variability in the fuel distribution associated with different management strategies. FUNC-SIM proved very useful to understand the impact of relatively small changes in fuel treatment areas on wildfire hazard. It revealed important differences in estimated burn probability and fireline intensity, especially in treated forest stands, highlighting the suitability of the approach to effectively consider the impact of fuel management on wildfire hazard. It also provided a more realistic understanding of the impact of fuel breaks on wildfire hazard by considering that fuels in these areas change over time, rather than assuming time-invariant barriers [
25]. The approach can be extremely useful to quantify how different efforts and spatial configurations of fuel treatment units can affect wildfire hazard. Additionally, it can also be used to uncover the role that different surface fuels (related to less fire-prone cover types, e.g., as determined by forest composition) have on wildfire hazard and risk assessment at the landscape level.
Model calibration results agreed well with historical data regarding fire size distribution and spatial patterns of fire activity. A slight underestimation occurred, particularly for smaller fire size classes, but is not expected to have a relevant impact on the overall results, considering that almost 90% of the burned area has been historically determined by very large wildfires (>1000 ha). The use of different ignition and fuel maps for the BAU option, as opposed to the baseline simulation, had very little impact on the estimated fire descriptors (not shown). This provided added confidence in FUNC-SIM’s ability to provide useful insights on the impact of different fuel management strategies on reducing wildfire hazard.
Nevertheless, the stochastic approach of FUNC-SIM still is affected by large uncertainties regarding both present and future fuel model distribution in the landscape. For example, results will be sensitive to the large uncertainties associated with the total forest area under fuel management and its frequency. Barreiro et al. (2021) [
36] provided a rough estimate of 30% of managed eucalypt stand area based on information from the forest owners association and a group of landowners. Santos et al., (2021) [
34] estimated, for a sample of 221 owners, who managed 36% of the forest area, that 29% of the owners had treated fuels at least once in the last ten years. Both probably overestimate the area under frequent fuel treatment because there is much less information available regarding the spatial coverage of “absent” and “quasi-absent” FMAs. The lack of information on the location of individual properties and associated FMA introduced additional uncertainties. This is a common problem, not only in the Alvares parish but throughout most of rural Portugal and is aggravated by the fact that most of the area is private. The stochastic approach of FUNC-SIM should better cope with such uncertainties compared to more traditional approaches. Nevertheless, future work will benefit from having more detailed information on where how and when fuel is treated on the landscape, particularly for the larger-size properties.
If its landscape remains unchanged and similar to conditions prevailing before the 2017 wildfire, the Alvares parish will continue to suffer the consequences of frequent, very large and uncontrollable wildfires. The analysis indicated which areas are more likely to suffer very large and intense wildfires over the next 40 years. This can provide insights on which areas are more vulnerable and require priority efforts in the near future, for example, to protect villages with very high wildfire risk [
37]. These analyses also showed that wildfire transmission is an important problem in Alvares, as it also has been shown for other areas of the country (e.g., [
25]). Results strongly suggest that areas contiguous to the northern border of the parish are an important risk. Thus, treating fuels to reduce wildfire hazard should go beyond the limits of the parish and need to be planned and implemented over a broad area.
As expected, results suggest that the fuel management strategies may lead to relevant decreases in wildfire intensity, burn probability and frequency of large wildfires. Even with minor increases in the fuel treatment area, either through fuel breaks or in scattered forest stands, the impacts can be relevant. These results indicate that it is very important to increase the fuel treatment area in an under-managed landscape, such as Alvares. For example, combining the lowest FBN priority with a moderate increase in the area treated in forest stands (FB 1/3 and Moderate Mngt) reduced the total simulated burned area by 20.8% and reduced the probability of wildfires larger than 5000 ha between 20% to 70%.
Considering the two fuel management strategies, results point to a slightly higher impact of fuel breaks on reducing wildfire exposure, particularly in their “area of influence”. Some considerations are warranted. First, results are highly dependent on the fuel distribution that was assumed for fuel breaks. We tested a different distribution, assuming a higher probability of a wildfire stopping in the fuel breaks (i.e., non-burnable), and it nearly doubled the total burned area reduction. The additional effectiveness will depend on how firefighters use fuel breaks to suppress wildfires. In this respect, our results are a worst-case scenario, i.e., no fire suppression takes place in the fuel breaks. Conversely, spotting was also not simulated and can significantly reduce fuel break effectiveness, particularly in areas with high fire intensity and/or vertical continuity, typical of unmanaged eucalypt and pine stands. Comparison with the empirically determined return for effort of fuel treatments in eucalypt landscapes, where spotting is a relevant fire spread mechanism, suggests our results could be optimistic [
53]. Finally, a simulation-based analysis is needed to complement the expert knowledge used in defining the priority segments of the fuel break network. This analysis should consider the complementary effect of different segments and other fuel management strategies.
Any fuel management strategy should be analyzed in terms of its effectiveness. Results suggest that the linear fuel break strategy seems to be more effective than random and scattered areal fuel treatments, particularly the “top priority” part of the network (FB 1\3). These results were expected considering that (i) the main purpose of fuel breaks is to reduce burned area and (ii) the fuel break locations were determined using expert knowledge and were not randomly dispersed over the landscape. Still, expert knowledge is subjective, and this stresses the necessity of identifying the optimal treatment locations at the landscape level [
27,
28,
54], possibly combining different strategies to improve the effectiveness of both linear and area-wide fuel treatments. Additionally, the annual increases in the managed area are in the same order of magnitude as the annual decreases in burned areas [
46] for FBN 1\3 and 2\3 and a moderate increase in forest management. This suggests that the effectiveness of fuel management scenarios needs to be carefully evaluated, using a holistic and integrated approach, which can be separated into two main aspects: (1) moving beyond the concept of relying on burned area alone as an indicator of wildfire impact [
16]; (2) taking into account the direct and indirect impacts of fuel management strategies on the safety of people and assets [
37], wildfire costs [
34] and economic revenues [
36].
Overall, the approach presented here has the necessary flexibility to integrate uncertainties associated with fuels and estimate wildfire hazard in other areas of Portugal, as well as in other Mediterranean areas. The detailed forest management approaches and associated fuel distributions for eucalypt stands cover a wide range of possibilities and can be applied to a large extent of the country. The fuel distributions for shrublands would need to be adapted to the regional characteristics, for example, separating Atlantic from Mediterranean species. For pine forests, more information is necessary regarding the existing forest management approaches and the fuel distribution in areas with natural regeneration (e.g., understand the fuel dynamics over time) associated with wildfires that occurred in the last decades. Regarding fuel breaks, the approach presented can be applied to most of the Portuguese territory, easily adapted to cover additional variations (e.g., different treatment frequencies) or applied to other fuel management strategies. Regarding the results presented, results should be valid at least for forest-dominated landscapes, with a large fraction of eucalypt forests, rugged terrain and a short fire cycle. This definition covers a large extent of center Portugal, the most fire-prone area of the country.
6. Conclusions
In the aftermath of the extreme 2017 wildfire season in Portugal, it is crucial to find smart and effective solutions to create fire-resilient landscapes. In this work, we developed a tool (FUNC-SIM) to evaluate how different fuel treatment strategies may affect wildfire hazard in the Alvares parish in the next 40 years. We followed an innovative approach based on fuel distributions and stochastic simulation that provides a more realistic approach to integrate different fuel treatment strategies.
If the landscape remains unchanged, Alvares will continue to be affected by frequent, large wildfires, with larger probabilities estimated to occur in the north, northeast and center-east areas of the parish. They will be associated with fireline intensities that require aerial resources and sometimes are beyond suppression capabilities. Increasing fuel treatment area in the parish is critical to reducing exposure, intensity and the likelihood of very large wildfires. Fuel treatment scenarios decreased burned area between 12.1 and 31.2% and significantly reduced the likelihood of very large wildfires affecting the parish, for example, 10% to 40% for fire sizes larger than 5000 ha, depending on the scenario.
About 8% of the eucalypt forest area in Alvares is treated annually, and depending on the fuel treatment scenario, this area increased between 1% and 4.6%, decreasing the total burned area between 12.1 and 31.2%, respectively. On average, as an indicative figure, simulated burned area decreased 0.22% per ha treated, and fuel treatment cost-effectiveness decreased with increasing area treated. Overall, both fuel treatment strategies can effectively reduce wildfire hazard and be part of a larger, holistic and integrated plan to reduce the vulnerability of the Alvares parish to wildfires in the future.