Assessing the fire resilience of the savanna tree component through a functional approach
Introduction
Every year, wildfires burn more than 400 million hectares worldwide (Andela et al. 2017) and shape the structure and diversity of all vegetated biomes (Bond and Keeley 2005). However, the effects of fire on vegetation and wildlife can vary considerably, even in fire-prone ecosystems as savannas. Natural fires, mainly caused by lightning strikes, have been an important driver of the distribution of savannas over the past 25 million years (Thonicke et al. 2001; Fidelis et al. 2018). In the Cerrado (Brazilian savanna), it has been estimated that burning intervals ranged from two to nine years before European colonization (Ratter et al. 1973; Hoffmann 1998). The environmental filter promoted by these natural fires favored the evolution of fire-tolerant and fire-dependent species and increased the diversity and endemism of savannas (Bond and Keeley 2005; Pausas and Keeley 2014).
Recent studies have shown that long-term fire suppression can lead to vegetation homogenization, woody encroachment and local extinctions of many typical savanna species (Durigan and Ratter 2016; Abreu et al. 2017; Passos et al. 2018; Baker et al. 2020). Moreover, the high fuel load that accumulates in the absence of fire can lead to very intense fires, with higher temperatures and the fire reaching the upper crowns of taller trees (Miranda et al.1993; Bradstock et al. 2010). Unlike the savannas of Australia and Africa (Van Wilgen et al. 2007; Russell-Smith et al. 2009), the Brazilian Cerrado has been under conservation policies based on the suppression of fires in protected areas regardless of the type of ecosystem (Durigan 2020). However, in 2012, the Federal Law on the Protection of Native Vegetation recognized that fire management policies can benefit savannas (Durigan and Ratter 2016) and that the prescription of periodic fires should be applied to fire-prone ecosystems (Durigan 2020).
Conversely, many non-protected savannas are experiencing changes in their fire regime (i.e., intentional fires), where fire return intervals could reach one to four years (Hoffmann 1998; Júnior et al. 2014). These changes are probably due to an increase in the frequency of severe drought events and expansion of human activities (Klink and Moreira 2002; Bowman et al. 2011; Overbeck et al. 2015). Besides, these intentional fires occur most frequently during the middle and last months of the dry season (April to September), resulting in more intense fires with higher temperatures (Miranda et al. 2009). Even considering that the savanna plants have adaptations to deal with fire, very frequent fires can increase tree mortality rates, mainly of seedlings and small individuals (Hoffmann and Solbrig 2003; Medeiros and Miranda 2005; Gomes et al. 2014), and reduce the recruitment of woody species (Lenza et al. 2017). Therefore, several studies suggest that frequent fires can lead to lower tree density and diversity and the homogenization of tree communities (Coutinho 1990; Medeiros and Miranda 2005; Mews et al. 2014; Lenza et al. 2017).
The controversial debate related to the “ideal” fire regime and to what extent we must manage fire in savannas may have arisen because most studies have focused only on changes in the more “traditional” community metrics, such as species richness, composition and demographic rates (Abreu et al. 2017; Passos et al. 2018). Nevertheless, we must also understand how the underlying processes associated with these changes can affect the functioning of the ecosystem (e.g., productivity, species resource requirements and biological interactions). For instance, an increasing number of coexisting species does not necessarily mean a higher niche complementarity but rather increased competition if these species have similar traits and make use of the same resources. Under increasing fire events, dominance should gradually shift toward tree species with functional traits that favor them to persist (e.g., thicker bark) and increase performance in high light availability vegetation (lower wood density and specific leaf area) due to changes in the environmental filters. Frequent burning can also affect animal-plant interactions (Rainsford et al. 2020), leading to the dominance of tree species with less specialized traits (e.g. species that are pollinated and dispersed by wind; Martins and Batalha 2006; Cianciaruso et al. 2012; de Deus and Oliveira 2016; Kuhlmann and Ribeiro 2016). Evaluating communities' functional composition (i.e., the dominant traits in the tree community) can help us to understand potential ecosystem functions that have been lost. Moreover, understanding how functional composition changes with fire regime allows us to compare areas with entirely different assemblages, leading to broader patterns for fire ecology.
Here, we evaluated the extent to which the structure (tree density and aboveground biomass), diversity (taxonomic and functional) and resilience (functional redundancy and functional response indices) of tree communities differ between burned and unburned Cerrado savanna plots. Considering fire as an anthropogenic disturbance, we predicted that tree density, aboveground biomass, taxonomic and functional diversity, and resilience indices would be lower in the burned plots. Additionally, we predicted that burned plots would be dominated by species with traits associated with: a) fire resistance (e.g., thicker bark), b) open environments with high light incidence (e.g., low specific leaf area and wood density), and c) strategies of dispersal or reproduction that do not rely on animals interactions (e.g., wind pollination, hermaphrodite flowers, small seeds dispersed by wind).
Section snippets
Study site, fire events and vegetation sampling
The study was conducted at the Serra de Caldas Novas State Park (PESCAN), which is located in the Brazilian Central Plateau, Southeastern Goiás (17°47′ S; 48°40′ W) at an altitude ranging from 700 to 1010 m a.s.l. PESCAN has an area of about 12,000 ha (Fig. 1) and the dominant vegetation is the cerrado stricto sensu (>70% of the area of the park), a typical savanna vegetation with trees 3–7 m in height and a high density of shrubs and grasses (Ribeiro and Walter 2008). Other vegetation types
Results
The mean aboveground biomass (AGB) differed significantly between the burned and unburned plots and was 19.67 ± 4.14 Mg ha−1 (mean ± standard error) for the unburned community and 12.04 ± 2.78 Mg ha−1 for the burned community (Table 1). These values indicated a reduction in AGB stock of around 40% for the burned community compared to the unburned community. Tree density, rarefied species richness, and Fisher diversity were not significantly different between burned and unburned plots (Table 1).
Discussion
We examined how structural, taxonomic and functional diversity and resilience metrics differ between the tree component of burned and unburned Cerrado savannas, considering a period of ten years before vegetation sampling. Although tree aboveground biomass was markedly smaller (40%) in burned plots, species richness and diversity were similar between fire regimes. Burned plots had higher functional diversity of vegetative traits but smaller functional diversity of reproductive traits,
Conclusion
Despite our study sampled limitations and a low variety of fire regimes, which hinder more general conclusions on the effect of different fire-regimes in savanna tree communities, our results provide valuable insights for future research on this topic. Our results highlight the resilience of savanna tree communities to periodic fires. Although fire promotes a substantial reduction in the tree stand biomass and, consequently, in savannas carbon storage, eventual fires can generate benefits for
Funding
This work was supported by the Brazilian Council for Research and Scientific Development (CNPq) (Grant numbers 441225/2016–0 and 433828/2018–8). MA received scholarships from the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank the managers of the Serra de Caldas Novas State Park (PESCAN) for permission to conduct our research in the park.
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