Effects of experimental fires on the phylogenetic and functional diversity of woody species in a neotropical forest

https://doi.org/10.1016/j.foreco.2019.117497Get rights and content

Highlights

  • Fire-induced mortality is more random for small compared to large trees.

  • High phylogenetic and functional diversity losses related to fire.

  • Fire-related declines in tree diversity occurred mostly after high-intensity fires.

Abstract

Although tropical forest fires are naturally rare, they have become more frequent and intense in response to recent changes in land use and climate. This shift in fire regime may drive widespread forest degradation in Amazonia, with important consequences not only for species richness but also for functional and phylogenetic diversity. Here, we test the overall hypothesis that fire-induced tree mortality causes more losses in phylogenetic and functional diversity than in taxonomic diversity, because fire kills trees non-randomly. To test this hypothesis, we established a large-scale, long-term (nine-year) fire experiment in southeast Amazonia in three 50-ha plots, with unburned Control and two different fire regimes (burned annually and burned every three years), between 2004 and 2010. Overall, tree assemblages exposed to experimental fires lost more phylogenetic and functional diversity compared to unburned areas, especially where the experimental fires were more intense (in areas burned every three years). The rate of loss of phylogenetic and functional diversity per species-unit was higher for larger trees than for small ones, which indicates that fire-induced mortality is more random for small trees. Our results indicate that fire acts as a selective pressure, filtering species with similar phylogenetic and functional traits. Given that forest fires are likely to become more common and frequent in the region, it is essential to understand their impact above and beyond taxonomic diversity.

Introduction

Fire, logging, fragmentation, and their interactions threaten Amazonia’s biodiversity (Nepstad et al., 1999, Laurance et al., 2018, Silvério et al., 2019). As people clear, degrade, and fragment forests, they have become more flammable and more vulnerable to higher intensity and more frequent fires (Morton et al., 2013, Brando et al., 2014, Balch et al., 2015, Silvério et al., 2019, Brando et al., 2019a, Brando et al., 2019b), causing potential losses in forest biodiversity and functions (Bradshaw et al., 2009, Dirzo et al., 2014, Trumbore et al., 2015, Barlow et al., 2016). Droughts increase the likelihood of more widespread forest fires, which can further reduce forest diversity (Brando et al., 2014, Balch et al., 2015, Trumbore et al., 2015). Between 1999 and 2010, for example, more than 85,500 km2 of Amazonian forest burned, almost 3% of the total forested area of the Amazonian basin (Morton et al., 2013). Given that tropical forest fires peak during warm, dry periods, and that sources of ignition are not lacking, projected climate change scenarios strongly suggest that forest flammability will increase in coming decades across much of eastern Amazonia (Duffy et al., 2015). However, the potential effects of this change in fire regime on regional plant diversity remains unclear.

Although Amazonian tree species tend to be vulnerable to understory fires (Barlow et al., 2003, Brando et al., 2014), post-fire tree mortality can vary widely (Balch et al., 2011, Flores et al., 2014, Brando et al., 2016). Two interrelated processes explain this high variability. First, fire intensity is controlled by processes that vary spatially and temporally, such as fuel characteristics and forest understory microclimate (Cochrane et al., 1999, Cochrane and Laurance, 2008, Parsons et al., 2015, Brando et al., 2014, Brando et al., 2016). Second, the severity of fire damage is strongly influenced by plant traits such as bark thickness, size, rooting mat density, and wood density, all of which vary widely within and across tree species and over the Amazon (Gerwing, 2002, Balch et al., 2011, Brando et al., 2012, Martins et al., 2012).

Fire-induced tree mortality influences different facets of species diversity but most studies on tropical forests have quantified only its effects on species richness (Balch et al., 2008, Balch et al., 2011, Brando et al., 2014). However, a broader viewpoint that also includes phylogenetic and functional diversity could yield additional insights into the causes and consequences of observed community responses to fire (Cavender-Bares and Wilczek, 2003, Arroyo-Rodríguez et al., 2012, Cianciaruso et al., 2012). In general, fire-induced tree mortality occurring at random (i.e., not selective) will cause proportional reductions in species richness, functional diversity, phylogenetic diversity, and the number of individuals (Cianciaruso et al., 2012, Dantas et al., 2013). Nevertheless, even in non-fire adapted ecosystems, such as Amazonian forests (Cochrane and Schulze, 1999, Flores et al., 2014), selective fire-induced tree mortality is common (Brando et al., 2012, Lawes et al., 2013). If fire-induced tree mortality does not occur at random, because some traits provide more fire resistance (e.g., thick bark) (Van Nieuwstadt and Sheil, 2005, Brando et al., 2012), then losses in functional and phylogenetic diversity are expected to be higher than losses in taxonomic diversity (Rosell et al., 2014).

Fire-induced tree mortality is highly variable in the tropics, resulting in rates that range from 8% to 90% (Balch et al., 2008, Balch et al., 2011, Barlow and Peres, 2008, Brando et al., 2014, Flores et al., 2014). To better understand how this high variability affects biodiversity, we studied two experimental fire regimes (annual and triennial) in southeast Amazonia. Previous studies conducted in the same experimental area showed that tree mortality following experimental fires was higher for small, thinner-barked individuals and for those trees growing in treatments burned every three years (B3yr), compared with trees growing in the treatment burned every year (B1yr) (Brando et al., 2014). Given these results, we hypothesized that: (1) fire-induced losses in functional and phylogenetic diversity will be higher than those in taxonomic diversity because fire-induced tree mortality tends to select species with thick bark and high wood density and because such traits converge in the phylogeny; (2) overall losses in diversity and abundance will be higher in B3yr than in B1yr treatments, because of the increased tree mortality in B3yr than in B1yr areas; (3) losses in biodiversity for trees in small class sizes will be more randomly distributed compared with larger trees, leading to more even losses in taxonomic, phylogenetic, and functional diversity, because small trees have thinner bark and leaves that are more exposed to fires.

Section snippets

Study area and fire experiment

The fire experiment was established in 2004 in a transitional forest located between the Cerrado and Amazonia boundary, southern Amazon basin (13° 04′ S, 52° 23′ Ν). The vegetation is classified as seasonal evergreen forest (Kunz et al., 2009), typically ~20 m in height, with most species common to more moist Amazon forests and a few to the Cerrado (Balch et al., 2011). The experiment was comprised of three 50-ha plots: an unburned Control, a plot that was experimentally burned every three

Results

Cumulative tree mortality from 2004 to 2010 was higher in the burned plots for both small (59–64%) and large trees (30–39%) compared to the unburned Control (30% and 16%, respectively; Fig. 2). As a result, the yearly loss of taxonomic, functional and phylogenetic diversity from 2004 to 2012 was significantly higher in the burned plots compared to the control for both small and large size trees (Table S1; Fig. 2). Fire-related losses in biodiversity were particularly higher after 2009, when

Discussion

This study tested the hypothesis that surface fires drive losses in functional and phylogenetic diversities of Amazonian trees at higher rates than in taxonomic diversity. Supporting this hypothesis, we found that tree sub-assemblages in the burned forests lost more phylogenetic and functional diversity per species-unit than sub-assemblages in the unburned Control. This indicates that the experimental fires preferentially killed trees with certain traits that are convergent in the phylogeny.

Declaration of Competing Interest

Author declares that there is no conflicts of interest.

Acknowledgements

We thank the Instituto de Pesquisa Ambiental da Amazônia (IPAM) for institutional support, in particular the IPAM field staff for sampling and monitoring of plots. Special thanks to Grupo Amaggi for allowing the research to take place on their farm and providing infrastructure support. This research was supported by the CNPq (#400893/2014-2; #441703/2016-0) and Capes-Brazil via scholarships to CCN and LM. LM and DVS had postdoctoral fellowships (#439801/2016-8; #405800/2013-4), and PMB and PDM

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