## 1 Introduction  
The increase in greenhouse gases (GHG) in the atmosphere and consequential global climate change are major concerns for today's society. According to [PERSON] et al. [1], and [PERSON] et al. [2], forest fires release a very large amount of CO\({}_{2}\) (corresponding to approximately 2 to 3 PgC/year), significantly contributing to the concentration of GHG in the atmosphere. Several studies based on remote sensing were carried out to assess the severity of wildfires and to monitor postfire dynamics of forest ecosystems [3, 4, 5, 6] or to quantify gross primary production (GPP) [7, 8]. GPP is the amount of carbon fixed by vegetation through photosynthesis and is the key element in the carbon balance between biosphere and atmosphere [9]. After extreme weather events or anthropogenic disturbances, for example, forest fires, GPP suffers variations that could significantly impact the net carbon exchange of the ecosystem (net ecosystem exchange (NEE)) [10, 11]. Therefore, changes in GPP before and after a wildfire event are a way to assess its severity. In the Mediterranean region, over the last few decades, forest fires have become larger in area and of higher severity [2, 12]. In Southern Europe, wildfires are becoming a frequent occurrence, compromising the development of forest ecosystems. Socioeconomicchanges that occurred in the second half of the last century are reflected both in land use and population lifestyle. The migration of inhabitants of rural areas to larger cities, a consequence of the abovementioned socioeconomic changes, resulted in the increased neglect of agricultural land and forestry, and consequently the accumulation of biomass over the years, leading to large-scale and intense fires. Another important contributing factor for the occurrence of these fires is the increase in highly combustible, easily cultivated, and fast-growing plant species stands, such as _Eucalyptus globulus_ Labill. [13; 14; 15; 16; 17]. Prior to the 1980s, forest fires in Portugal never exceeded 10,000 ha of burnt area in a single occurrence [17]. The Portuguese Institute for Conservation of Nature and Forests (ICNF) regularly publishes statistics of large wildfires over 10,000 ha [18], which are the fires discussed in this study. The first major forest fire over 10,000 ha that had occurred in Portugal was on 13 July 1986 in the municipality of Vila de Rei (district of Castelo Branco), with 10,032 ha of burnt area. On 6 August 1992, Arganil was hit with a large fire that destroyed 12,814 ha; after that, there was a pause of 11 years without any other occurrence affecting more than 10,000 ha. The year 2003 was ominous, with large wildfires burning areas over 20,000 ha, and affecting many municipalities in the Central and Alto Alentejo regions, namely, the municipalities of Chamusca, Proenca-a-Nova, Oleiros, Serta, Vila de Rei, Macao, Gaviao, Nisa, Ponte de Sor, Alter do Chao, and Crato. In the same year, the Algarve region faced the great fire of Monchique with more than 66,000 ha of burnt area, striking the municipalities of Silves, Aljezur, Portimao, Monchique, and Odemira, the latter in the Lower Alentejo region. The year 2017 was the worst year in terms of loss of human lives, housing, and forests caused by several major forest fires. On 31 October 2017, a total of 412,781 ha was burnt [19], with 112 fatalities and more than 500 homes destroyed by the fires [20; 21]. The centre of Portugal was the most affected area of mainland Portugal during the 2017 fires. The Pedrogao Grande (Leiria district) fire that broke out on 17 June destroyed 27,364 ha [19] and killed 64 people [20]. This fire merged with the Gois fire (Coimbra district) that burnt 17,521 ha [19]. The fire of Serta (Castelo Branco district) on 23 July 2017 consumed 29,758 ha. In the same year, the worst month was October, with fires that burnt 16,610 ha in Figueira da Foz and 35,806 ha in Lousa (Coimbra district), 30,142 ha in Serta (Castelo Branco district), 57,534 ha in Seia (Guarda district), and 15,687 ha in Alcobaca (Leiria district) [19].