Climate change is increasing fire activity in many ecosystems worldwide (Flannigan et al. 2009), emphasizing the need to understand the drivers and mechanisms that influence fire activity (Bowman et al. 2020; Shuman et al. 2022). Two factors are currently limiting our understanding of fire at regional (sub-continental) scales. On the one hand, the coarse resolution of systematic, spatially explicit, global fire datasets (250–500 m; Giglio et al. 2018; Lizundia-Loiola et al. 2020) hinders the identification of patterns at higher resolution (Chuvieco et al. 2020). On the other hand, the spatial drivers of fire are frequently correlated, since they influence each other, limiting a more mechanistic understanding of where and why fires occur (Wood et al. 2011). In this context, detailed analyses of fire patterns and drivers based on systematic records of fire at medium resolution (~ 30 m) may help to overcome these limitations. Moreover, the description of fire regimes may serve as a baseline to detect the future changes in fire activity.
Fire activity is strongly determined by climatic conditions and vegetation (fuels). The varying constraints hypothesis proposes that fire activity follows a hump-shaped relationship with productivity: at low productivity, fire is limited by low fuel amount and horizontal continuity, while at high productivity it is limited mostly by high fuel moisture content (Krawchuk et al. 2009; Krawchuk and Moritz 2011; Pausas and Ribeiro 2013). Hence, intermediate-productivity conditions are proposed to be the most fire-prone. This framework largely explains the temporal and spatial variation in fire activity.
Temporal variation in fire activity at short temporal scales, ranging from days to a few years, is closely related to climatic variability. In low-productivity systems, fires occur when a rainy or productive period, that allows fuel accumulation, is followed by a dry and or warm period that promotes fuel dessication (Kitzberger et al. 1997; Villagra et al. 2024). In turn, in high-productivity systems, where fuel amount is not a limiting factor, fires occur during dry and or warm periods, when fuel moisture decreases (Sommerfeld et al. 2018; Gaboriau et al. 2022; Cawson et al. 2024). But climate also impacts fire at much longer temporal scales, from centuries to millennia, by affecting vegetation structure and composition, which are tightly linked to productivity (Woodward and McKee 1991; Box and Fujiwara 2013). Consequently, temporal variations in climate over extended periods may influence fire activity similarly to spatial variations in climate. Thus, insights into the effects of long-term climate can often be inferred from analyses of spatial patterns of fire activity in relation to climate.
The spatial variation of climate generates vegetation variability that is much larger than the variation in fuel amount and moisture observed at at short temporal scales. At large spatial scales, the productivity gradient exhibits a wide range of vegetation physiognomies, from deserts to rainforests (Churkina and Running 1998; Chen et al. 2007; Bilgili et al. 2020). At smaller scales, within a more narrow productivity range, vegetation may present sharp changes in composition and structure, defining vegetation types with contrasting flammability (Mitchell et al. 2009). For example, a region of woody vegetation may present tall forests with closed canopy, displaying high vertical fuel discontinuity and a microclimate that promotes high fuel moisture, or short-statured forests or shrublands, with high vertical fuel continuity and lower moisture content (Kitzberger et al. 2016). Hence, spatial climatic variation may affect fire activity in two ways. On the one hand, climate may control fuel amount, continuity and moisture, irrespective of vegetation types (e.g., shrublands may be more or less open depending on productivity, but maintaining the shrubland structure). On the other hand, climate may determine the spatial distribution of vegetation types with contrasting flammability. This double effect of climate, and its correlation with vegetation type, hinders the understanding of how vegetation and climate affect fire activity. Interestingly, the same limitation is found for any other fire driver that affects the spatial distribution of vegetation types.
In mountain regions, water availability and temperature vary along topographic gradients, so topography may also have a double effect on fire. Temperature decreases with elevation and towards poleward slopes, thus reducing evapotranspiration (Holden and Jolly 2011; Méndez-Toribio et al. 2016; Jucker et al. 2018; Simon et al. 2024). Water availability is lower in steep slopes compared to plains or valley bottoms, and the latter usually present deeper soils with higher water-holding capacity compared to mid slopes or ridges (Holden and Jolly 2011; Dobrowski 2011; Jucker et al. 2018). Hence, topography, by affecting local climate and soil conditions, may also affect fire activity by controlling the spatial distribution of vegetation types or by affecting fuel amount, continuity and quality within them (Iniguez et al. 2008; Holsinger et al. 2016; Flatley et al. 2011). Moreover, slopes promote the upslope spread of fire and limit it downslope (Rothermel 1983), which is an even more direct effect of topography on fire activity.
Besides these biophysical controls, fire and vegetation are also influenced by human activities. Humans cause ignitions and suppress fires, but they can also exert an indirect effect by altering vegetation (Bowman et al. 2011; Bar-Massada et al. 2014). In regions of high productivity, forests are frequently logged or burned to promote the growth of grass for cattle raising (Curtis et al. 2018). This shift towards open forests, shrublands or grasslands, despite reducing biomass, may increase the system flammability through a reduction in fuel moisture, an increase in vertical continuity, and an increase in fine fuel density (Pausas and Ribeiro 2013). However, if fuel amount and continuity are significantly reduced, these human-caused transitions may decrease the flammability of the system, instead of increasing it (Blackhall et al. 2017).
Understanding how climate, topography and humans influence spatial patterns of fire activity either by shaping vegetation or by more direct effects requires assessing the effect of these drivers collectively across all vegetation types in the landscape and independently (e.g., Wood et al. 2011). These relationships between fire, vegetation and fire drivers that affect vegetation may also be evaluated from the vegetation type perspective: the contrasting fire activity among vegetation types may be explained by the physical-human environment where they occur or by intrinsic fuel properties. Again, this may be understood by comparing fire activity among vegetation types occurring in similar environments.
Fire activity plays an important role in the dynamics of the temperate forests in northern Patagonia. These ecosystems, characterized by tree species exhibiting low fire resilience, are susceptible to positive fire-vegetation feedbacks, amplifying their vulnerability to changing environmental conditions (Kitzberger et al. 2012, 2016, 2022). While there is a growing understanding of patterns and drivers of fire activity at landscape and regional scales, most previous studies have been based on non-systematic records of fire (e.g., large, well-documented fires), often performed over smaller administrative units (e.g., specific national parks or other protected areas) and mostly during the late 20th century (Mermoz et al. 2005; de Torres Curth et al. 2008; Holz et al. 2012; Mundo et al. 2013; Paritsis et al. 2013). Given the strong warming and drying trends in the region there is an urgent need to understand how these conditions are impacting regional fire activity. Only one study analyzed drivers of fire activity during the 21st century over a large regional scale, with the aim of predicting future fire activity (Kitzberger et al. 2022). Since the authors focused on prediction, they did not evaluate how different drivers of fire, correlated among them, influence each other, which limits a more mechanistic understanding of spatial controls of fire activity. Moreover, they used mostly coarse-resolution fire scar data (MODIS, 500 m), thus hindering the detection of fine scale patterns (e.g., some topographic effects).
In this part of the world, fire activity is lower in forests and higher in shrublands (Mermoz et al. Veblen 2005; Paritsis et al. 2013; Morales et al. 2015; Tiribelli et al. 2019; Kitzberger et al. 2022). Forests present lower amount of fine fuel, lower vertical continuity and moister dead fuels compared to shrublands (Paritsis et al. 2015; Tiribelli et al. 2018; Barberá et al. 2023). However, most forests tend to occur in areas of higher precipitation and elevation compared to shrublands. Under these conditions fuels are expected to be moister compared to areas of lower precipitation and elevation, where shrublands occur. Moreover, shrublands are more abundant near human settlements, where higher fire activity could be explained by higher ignition rate. These patterns suggest that differences in fire activity among vegetation types may be partially explained by the physical and human conditions where they occur, and not only by the differences in fine fuel amount and continuity.
Here we present an updated description of temporal and spatial patterns of fire activity in north-western Patagonia in the first decades of the 21st century, with an emphasis on understanding the effects of correlated fire drivers and how they influence each other. To this end, we developed a systematic record of fire spanning 24 years (July 1998 – June 2022), based on Landsat imagery (30 m resolution). Specifically, we aimed to (1) describe the fire regime in terms of fire size distribution, fire return interval, and fire seasonality; (2) describe interannual variability in burned area proportion and number of fires in relation to climatic variability; (3) disentangle the role of vegetation and physical and human factors in determining the spatial patterns of fire activity.