Elsevier

Acta Oecologica

Volume 111, August 2021, 103728
Acta Oecologica

Assessing the fire resilience of the savanna tree component through a functional approach

https://doi.org/10.1016/j.actao.2021.103728Get rights and content

Highlights

  • - Fire regimes strongly affect aboveground biomass in savannas.

  • - Fires can benefit the balance between light-demanding and shade-tolerant species.

  • - Fire increase the dominance of species with less specialized reproductive traits.

  • - Functional redundancy and response diversity indices are useful in fire ecology.

Abstract

Changes in savanna's fire regimes, either through fire suppression or through an increase in fire frequency, can negatively affect their resilience. We evaluated the extent to which the aboveground biomass, diversity (taxonomic and functional) and resilience (functional redundancy and functional response indices) of savanna tree communities differ between burned and unburned plots. Burned plots experienced two fire events over the ten-years prior to sampling, while unburned plots experienced fire suppression over the same time period. We found that aboveground biomass was 40% smaller in burned plots, indicating that fire regimes must be included as a source of variation in models estimating the potential of savannas to store carbon. Burned plots had a higher functional diversity of vegetative traits but a smaller functional diversity of reproductive traits, indicating that generalizations about the effect of fire on tree functional diversity should be viewed with caution. Periodic fires can benefit savanna tree biodiversity by maintaining the balance between light-demanding and shade-tolerant species but can also increase the dominance of species with less specialized reproductive traits that do not rely on animal interactions. Burned plots had slightly lower functional redundancy but similar functional response diversity compared to unburned plots, suggesting that both communities harbor tree species that might respond positively or negatively to fire and, therefore, will be able to maintain the ecosystem functions considered under a future scenario of fire-suppression or increased fire frequency. Therefore, a longer-term fire suppression (>10 years) or a return fire interval of less than 4 years may be necessary to reduce the resilience of the savanna tree component, considering the ecosystem functions analyzed in this study.

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.

References (106)

  • P.E. Oliveira

    Dioecy in the cerrado vegetation of Central Brazil

    Flora

    (1996)
  • D. Sheil et al.

    Defining and defending Connell's intermediate disturbance hypothesis: a response to Fox

    Trends Ecol. Evol.

    (2013)
  • I.A. Silva et al.

    Plant functional types in Brazilian savannas: the niche partitioning between herbaceous and woody species

    Perspect. Plant Ecol. Evol. Systemat.

    (2011)
  • M. Tabarelli et al.

    Abiotic and vertebrate seed dispersal in the Brazilian Atlantic forest: implications for forest regeneration

    Biol. Conserv.

    (2002)
  • R.C.R. Abreu et al.

    The biodiversity cost of carbon sequestration in tropical savanna

    Sci. Adv.

    (2017)
  • N. Andela et al.

    A human-driven decline in global burned area

    Science

    (2017)
  • M. Ângelo Marini

    Bird movement in a fragmented Atlantic Forest landscape

    Stud. Neotrop. Fauna Environ.

    (2010)
  • J. Ansell et al.

    Contemporary Aboriginal savanna burning projects in Arnhem Land: a regional description and analysis of the fire management aspirations of Traditional Owners

    Int. J. Wildland Fire

    (2019)
  • A.G. Baker et al.

    Rainforest expansion reduces understorey plant diversity and density in open forest of eastern Australia

    Austral Ecol.

    (2020)
  • S.C. Barrett

    The evolution of plant sexual diversity

    Nat. Rev. Genet.

    (2002)
  • D. Bates et al.

    Package ‘lme4’

    Convergence

    (2015)
  • R.S. Bivand et al.

    Comparing implementations of global and local indicators of spatial association

    Test

    (2018)
  • D.M.J.S. Bowman et al.

    The human dimension of fire regimes on Earth

    J. Biogeogr.

    (2011)
  • R.A. Bradstock et al.

    Effects of weather, fuel and terrain on fire severity in topographically diverse landscapes of south-eastern Australia

    Landsc. Ecol.

    (2010)
  • D. Bruno et al.

    Impacts of environmental filters on functional redundancy in riparian vegetation

    J. Appl. Ecol.

    (2016)
  • J. Chave et al.

    Improved allometric models to estimate the aboveground biomass of tropical trees

    Global Change Biol.

    (2014)
  • R.S.C. Cooke et al.

    Global trade-offs of functional redundancy and functional dispersion for birds and mammals

    Global Ecol. Biogeogr.

    (2019)
  • J.H.C. Cornelissen et al.

    A handbook of protocols for standardised and easy measurement of plant functional traits worldwide

    Aust. J. Bot.

    (2003)
  • W.K. Cornwell et al.

    A trait-based test for habitat filtering: convex hull volume

    Ecology

    (2006)
  • L.M. Coutinho

    Fire in the ecology of the Brazilian cerrado

    Fire in the Tropical Biota

    (1990)
  • L.F. Daibes et al.

    A field perspective on effects of fire and temperature fluctuation on Cerrado legume seeds

    Seed Sci. Res.

    (2017)
  • V.D.L. Dantas et al.

    Fire drives functional thresholds on the savanna-forest transition

    Ecology

    (2013)
  • F.F. de Deus et al.

    Changes in floristic composition and pollination systems in a “Cerrado” community after 20 years of fire suppression

    Rev. Bras. Bot.

    (2016)
  • G. Durigan et al.

    The need for a consistent fire policy for Cerrado conservation

    J. Appl. Ecol.

    (2016)
  • Fao et al.

    Harmonized World Soil Database (Version 1.2). FAO, Rome, Italy and IIASA, Laxenburg, Austria

    (2012)
  • B.S. Fichino et al.

    Does fire trigger seed germination in the neotropical savannas? Experimental tests with six cerrado species

    Biotropica

    (2016)
  • A. Fidelis et al.

    The year 2017: megafires and management in the cerrado

    Fire

    (2018)
  • B. Finegan et al.

    Does functional trait diversity predict aboveground biomass and productivity of tropical forests? Testing three alternative hypotheses

    J. Ecol.

    (2015)
  • C.R. Fonseca et al.

    Species functional redundancy, random extinctions and the stability of ecosystems

    J. Ecol.

    (2001)
  • T.L. Frizzo et al.

    Contrasting effects of fire on arboreal and ground‐dwelling ant communities of a Neotropical savanna

    Biotropica

    (2012)
  • M. Gashaw et al.

    Influence of heat shock on seed germination of plants from regularly burnt savanna woodlands and grasslands in Ethiopia

    Plant Ecol.

    (2002)
  • L.C. Girão et al.

    Changes in tree reproductive traits reduce functional diversity in a fragmented Atlantic forest landscape

    PloS One

    (2007)
  • G. Gottsberger et al.

    Life in the Cerrado

    (2006)
  • G.P. Hempson et al.

    Comparing bark thickness: testing methods with bark–stem data from two South African fire‐prone biomes

    J. Veg. Sci.

    (2014)
  • K.J. Hennenberg et al.

    Phytomass and fire occurrence along forest-savanna transects in the comoé national park, ivory coast

    J. Trop. Ecol.

    (2006)
  • S.I. Higgins et al.

    Effects of four decades of fire manipulation on woody vegetation structure in savanna

    Ecology

    (2007)
  • R.J. Hijmans et al.

    Very high resolution interpolated climate surfaces for global land areas

    Int. J. Climatol.: A Journal of the Royal Meteorological Society

    (2005)
  • W.A. Hoffmann

    Post‐burn reproduction of woody plants in a neotropical savanna: the relative importance of sexual and vegetative reproduction

    J. Appl. Ecol.

    (1998)
  • W.A. Hoffmann et al.

    Elevated CO2 enhances resprouting of a tropical savanna tree

    Oecologia

    (2000)
  • W.A. Hoffmann et al.

    Specific leaf area explains differences in leaf traits between congeneric savanna and forest trees

    Funct. Ecol.

    (2005)
  • Cited by (6)

    • Assessing woody plant encroachment by comparing adult and juvenile tree components in a Brazilian savanna

      2022, Flora: Morphology, Distribution, Functional Ecology of Plants
      Citation Excerpt :

      Several studies have evaluated the effects of woody plant encroachment in savanna areas. Usually, they are based on chronosequence or dynamic data (Altomare et al., 2021; Maracahipes-Santos et al., 2018) which demand time and resources to obtain the results, or are based on remote sensing data (Goncalves et al., 2021b; Rosan et al., 2019), that does not provide data on species diversity and composition which are important metrics to evaluate encroachment effects (Abreu et al., 2017; Pellegrini et al., 2021). Since evaluating encroachment effects on ecosystems is an urgent matter, the development of other methods to detect changes in vegetation using a single-time plant inventory can help to improve the management plans and conservation policies.

    • Tree species dominance in neotropical savanna aboveground biomass and productivity

      2021, Forest Ecology and Management
      Citation Excerpt :

      We used data collected between 1997 and 2018 in 1,476 Cerrado sampling plots from 60 sites in Southeastern and Central Brazil (Fig. 1; Table A.1), which composed a dataset of 157,094 trees. Therefore, our dataset gathered Cerrado study sites from different sources (Altomare et al., 2021; Vale et al., 2009; Prado-Junior et al., 2020; Scolforo et al., 2015), with different plot sizes (ranging from 100 up to 1800 m2) and different total sampling areas (i.e., the sum of plot areas within each site, ranging from 0.24 to 17.2 ha), but which were satisfactorily sampled individually (see Fig. A.3). Trees with diameter at breast-height (1.3 m above ground; DBH) higher than 5 cm had their DBH measured in all plots and were identified when possible (see sites detailed information in Table A.1).

    • How to measure response diversity

      2023, Methods in Ecology and Evolution
    View full text