INTRODUCTION
Human-induced land use and climate change are having negative impacts over most of the Earth's tropical biodiversity (Sala et al. Reference Sala, Chapin, Armesto, Berlow, Bloomfield, Dirzo, Huber-Sanwald, Huenneke, Jackson, Kinzig, Leemans, Lodge, Mooney, Oesterheld, Poff, Sykes, Walker, Walker and Wall2000; Berenguer et al. Reference Berenguer, Ferreira, Gardner, Aragão, De Camargo, Cerri, Durigan, Oliveira Junior, Vieira and Barlow2014). Most tropical species are declining and changing their geographical distributions as a result of human activities, leading to biodiversity erosion processes (Parmesan & Yohe Reference Parmesan and Yohe2003). In this scenario, human-induced changes may be acting as non-aleatory filters in the selection of disturbance-tolerant species, either by survival or regeneration after disturbances (Smart et al. Reference Smart, Thompson, Marrs, Le Duc, Maskel and Leslie2006; Tabarelli et al. Reference Tabarelli, Aguiar, Ribeiro, Metzger and Peres2010), and these species may gradually replace disturbance-sensitive species, thus increasing in abundance and geographical distribution. This process, known as taxonomic homogenization (TH), is part of a larger process of temporal simplification of originally different biotas and erosion of biodiversity, called biotic homogenization (BH), which also takes into account functional and genetic homogenization (Olden Reference Olden2006; Olden & Rooney Reference Olden and Rooney2006).
The drivers of BH are either direct, such as biological invasions and species extinctions (Olden & Poff Reference Olden and Poff2003; Olden Reference Olden2006 and references therein), or indirect, such as urbanization (Kühn & Klotz Reference Kühn and Klotz2006; McKinney Reference McKinney2006) and forest fragmentation (Lôbo et al. Reference Lôbo, Leão, Melo, Santos and Tabarelli2011). Nonetheless, the characterization of the mechanisms that drive BH is far from complete (Olden Reference Olden2006). Many other degradation processes may have a relevant role in the temporal simplification of biotas, like disturbance of sensitive environments, such as wildfires in traditionally fire-free vegetation types (Barlow & Peres Reference Barlow and Peres2008).
Among tropical forests, seasonally flooded forests are less resilient to fire, with negative and long-term effects on vegetation structure and composition, as well as the rapid loss of soil fertility (Flores et al. Reference Flores, Holmgren, Xu, van Nes, Jakovac, Mesquita and Scheffer2017). These factors aggravate the susceptibility of these forests to the more frequent recurrence of fire due to habitat fragmentation, neighbouring agriculture or pasture management and climate change. Habitat fragmentation induces canopy openness, thus allowing for greater light incidence that favours herb entrance (grasses (Poaceae) and sedges (Cyperaceae)) and faster vegetation moisture loss, resulting in increased flammability (Cochrane et al. Reference Cochrane, Alencar, Schulze, Souza, Nepstad, Lefebvre and Davidson1999; Silvério et al. Reference Silvério, Brando, Balch, Putz, Nepstad, Oliveira–Santos and Bustamante2013). Pasture management uses regular burning for renewing of grasses, and these fires accidentally escape to the neighbouring forests. There is also evidence of changing climate, and while in the Amazon basin the mean annual pattern of precipitation remains unchanged (Gloor et al. Reference Gloor, Brienen, Galbraith, Feldpausch, Schöngart, Guyot, Espinoza, Lloyd and Phillips2013), dry periods are becoming longer and more intense, which is resulting in an increasing occurrence of fire events (Gatti et al. Reference Gatti, Gloor, Miller, Doughty, Malhi, Domingues, Basso, Martinewski, Correia, Borges, Freitas, Braz, Anderson, Rocha, Grace, Phillips and Lloyd2014); even in the wetter years, the transitional forest in the eastern Amazon is more vulnerable to fire occurrence due to human actions in the landscape change (Alencar et al. Reference Alencar, Brando, Asner and Putz2015). Increased fire recurrence in Amazonian forests normally results in drastic changes in species composition and in forest structure and functioning (Cochrane et al. Reference Cochrane, Alencar, Schulze, Souza, Nepstad, Lefebvre and Davidson1999; Cochrane & Laurance Reference Cochrane and Laurance2002; Nepstad et al. Reference Nepstad, Stickler, Soares-Filho and Merry2008; Davidson et al. Reference Davidson, Araújo, Artaxo, Balch, Brown, Bustamante, Coe, DeFries, Keller, Longo, Munger, Schroeder, Soares-Filho, Souza and Wofsy2012) and could theoretically induce TH by replacing fire-sensitive species with a small group of fire-tolerant species with the ability to survive or regenerate after fire, as well as to colonize burned environments (Barlow & Peres Reference Barlow and Peres2008; Veldman & Putz Reference Veldman and Putz2011).
There are many vegetation types that have been experiencing increased fire recurrence in recent decades in Brazil. However, seasonally flooding forests are thought to be particularly sensitive to burning, which occurs mostly during the dry season (Maracahipes et al. Reference Maracahipes, Marimon, Lenza, Marimon-Junior, Oliveira, Mews, Gomes and Feldpausch2014). In the Araguaia floodplain (Planície do Rio Araguaia), Brazil's largest continuous flooding area at c. 90 000 km2 (Martini Reference Martini, Silva and Abdon2006), the seasonally flooding forests known as impucas (natural forest patches scattered in earth mound grassy fields; Eiten Reference Eiten1985; Marimon et al. Reference Marimon, Colli, Marimon-Junior, Mews, Eisenlohr, Feldpausch and Phillips2015) are a distinctive vegetation type. These forests are increasingly being affected by fires that escape from the neighbouring savannas used for grazing and are therefore regularly burnt (Marimon et al. Reference Marimon, Marimon-Júnior, Lima, Jancoski, Franczak, Mews and Moresco2008, Reference Marimon, Marimon-Junior, Mews, Jancoski, Franczak, Lima, Lenza, Rossete and Moresco2012, Reference Marimon, Colli, Marimon-Junior, Mews, Eisenlohr, Feldpausch and Phillips2015). In these systems, surface fires are frequent and often trigger smouldering belowground combustion, which slowly consumes the carbon stored in the soil and the trees’ rooting system, resulting in very high mortality rates (Flores et al. Reference Flores, Piedade and Nelson2014; Maracahipes et al. Reference Maracahipes, Marimon, Lenza, Marimon-Junior, Oliveira, Mews, Gomes and Feldpausch2014). Field observations also show that impucas recurrently affected by fire normally have their seed bank and seedling community drastically reduced or even eliminated. These forests are the habitat of many animal species (Brito et al. Reference Brito, Martins, Oliveira-Filho, Silva and Silva2008; Marimon et al. Reference Marimon, Marimon-Júnior, Lima, Jancoski, Franczak, Mews and Moresco2008), as well as connecting the extensive water network of rivers and lakes during the wet season (Martins et al. Reference Martins, Soares, Silva and Brites2002), and therefore their conservation is of paramount importance for the hydrological balance of the region.
This study focuses on six impucas in the Amazonia–Cerrado transition. The aim is to investigate whether these forests are suffering from TH as a cause of recurrent wildfires in short time intervals. Specifically, we ask the following questions: (1) Are impucas suffering from TH as a result of recurring fire events? (2) Which are the winner and the loser species on this TH? Regarding the first question, we hypothesize that TH is rapidly occurring in these forests, and we predict that TH is driven by high mortality rates of fire-sensitive species (loser species), which are rapidly being replaced by the survival or regeneration of a few fire-tolerant species (winner species). Regarding the second question, we hypothesize that if TH is observed in such a short time scale, it will be driven by reducing the number of native species, without replacement by invasive alien or introduced species (Tabarelli et al. Reference Tabarelli, Peres and Melo2012).
METHODS
Study area
We sampled six seasonally flooded forests (impucas) in areas of high ecological value in the Araguaia State Park (ASP), Novo Santo Antônio municipality, Mato Grosso, Brazil (Fig. 1 and Table 1). ASP has 223 619.54 ha and is located between two important rivers, Rio das Mortes and Rio Araguaia, in the transitional area between the Amazonia and Cerrado biomes (Ratter Reference Ratter1987; Marimon & Lima Reference Marimon and Lima2001; Marimon et al. Reference Marimon, Colli, Marimon-Junior, Mews, Eisenlohr, Feldpausch and Phillips2015). This is a flat region with an average altitude of 200 m above sea level on seasonally flooding and low-drainage plinthosols and gleysols (Mato Grosso Reference Grosso2007). The climate is classified as Aw Köppen, with mean annual temperatures of 25.7–27.3 °C, mean annual precipitation of 1800–2200 mm and well-defined rainy (October to March) and dry periods (April to September) (Silva et al. Reference Silva, Assad, Evangelista, Sano, Almeida and Ribeiro2008).
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Figure 1 Geographical location of the impucas (IMP) studied in the Araguaia State Park, Novo Santo Antônio municipality, Mato Grosso, Brazil.
Table 1 Geographical coordinates and years of fire events (Neves Reference Neves2015) in the studied flooding forests (impucas) in the Araguaia State Park, Novo Santo Antônio municipality, Mato Grosso, Brazil.
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Vegetation in ASP is composed of a mosaic of campos de murundus (literally ‘fields of earth mounds’, a type of termite savanna), impucas, patches of cerrado stricto sensu (typical savanna) and cerradão (dense woodland savanna) (Marimon et al. Reference Marimon, Marimon-Júnior, Lima, Jancoski, Franczak, Mews and Moresco2008, Reference Marimon, Colli, Marimon-Junior, Mews, Eisenlohr, Feldpausch and Phillips2015). Impucas are found within a grassy mosaic and are characterized by a thick layer of litter, formed by the slow decomposition of the leaves, which favours the accumulation of fine fuel, and therefore are flammable systems (Marimon et al. Reference Marimon, Marimon-Júnior, Lima, Jancoski, Franczak, Mews and Moresco2008, Reference Marimon, Marimon-Junior, Mews, Jancoski, Franczak, Lima, Lenza, Rossete and Moresco2012, Reference Marimon, Colli, Marimon-Junior, Mews, Eisenlohr, Feldpausch and Phillips2015; Santos & Nelson Reference Santos and Nelson2013; Maracahipes et al. Reference Maracahipes, Marimon, Lenza, Marimon-Junior, Oliveira, Mews, Gomes and Feldpausch2014). The impucas’ vulnerability to fire is enhanced by the entrance of graminoids or sedges such as Scleria spp. (Cyperaceae) (Barbosa et al. Reference Barbosa, Marimon, Lenza, Marimon-Júnior, Oliveira and Maracahipes2011), which contribute to high fine fuel loads that dry quickly in the dry season, favouring the ignition of the litter layer and of the organic soil layer. Irrespective of annual fluctuations in the flood regime, the impucas always remain flooded during the rainy season because they are located in relief depressions (Marimon et al. Reference Marimon, Marimon-Júnior, Lima, Jancoski, Franczak, Mews and Moresco2008).
Data collection
We inventoried six impucas between August 2007 and October 2008 (Time 1 – T1). At each impuca, we established 50 10 × 20-m plots along five parallel and transverse transects separated by 50 m. In July 2014 (Time 2 – T2), the areas were re-censused for surviving individuals, dead standing and new recruits (individuals who reached the minimum criteria for inclusion – see below). At each census, we recorded diameter at breast height (DBH) of all individuals (including palms and lianas) with DBH ≥10 cm in all plots. We collected botanical vouchers from all species that were taken to the Herbarium NX of the State University of Mato Grosso (UNEMAT, Nova Xavantina, Brazil), where all taxa was identified and verified according to the Brazilian Flora Species List (Lista de Espécies da Flora do Brasil 2015).
On the first vegetation census in 2007/2008, impucas 1, 5 and 6 showed evidence of fire and were occupied by Scleria spp. In 2014, only impuca 3 did not show clear evidence of fire nor presence of Scleria spp. Between 2000 and 2013, there were four fire events detected by satellite images (Terra/MODIS) in the area (Neves Reference Neves2015; Table 1). However, not all the fire occurrences detected by the satellite reached the impucas, as observed in T1 in impucas 2, 3 and 4, which did not show signs of fire degradation. Similarly, not all fire events occurring in the impucas were detected by the satellites. An example of this was recorded in impuca 2 in 2008, when a burn occurred but was not detected by the satellite (Maracahipes et al. Reference Maracahipes, Marimon, Lenza, Marimon-Junior, Oliveira, Mews, Gomes and Feldpausch2014). In any case, the impucas are surrounded by grassy fields in which fire scars can be clearly detected, even using low-resolution satellite images.
Data analysis
To explore the potential loss of species over time, we compared species richness in the two censuses using rarefaction techniques based on number of individuals, with 1000 permutations and a 95% confidence level (Krebs Reference Krebs2014). To test for changes in the mean number of individuals per impuca between T1 and T2, we performed paired t tests. To identify potential TH, we considered the species compositions at T1 and T2 and we examined the temporal variation in the similarity (using species presence or absence) with paired comparisons (Olden & Poff Reference Olden and Poff2003). We used the Jaccard index to account for floristic similarity between the two periods, because this is the most common index used in BH studies (Olden et al. Reference Olden, Poff, Douglas, Douglas and Fausch2004). We also compared the mean similarity values between T1 and T2 using a t test for dependent samples. Statistical analyses were performed with PAST 2.17 (Hammer et al. Reference Hammer, Harper and Ryan2001).
RESULTS
At the six impucas, number of species and families varied between T1 and T2. At T1, we recorded 47 species and 29 families, while at T2, we recorded 39 species and 25 families (Supplementary Table S1; available online). Therefore, 17% of the species were lost, but the loss was not homogeneous across the impucas: while impuca 3 did not lose any species, other impucas lost between one and five species (Table 2). Mortality was lowest at impuca 3, with 6% of dead individuals at T2, and highest at impuca 2, with 42%. Recruitment was highest at impuca 5, with 21% of new individuals recorded at T2, and lowest at impuca 6, with only 2% (Table 2). The mean number of individuals decreased between T1 and T2 in four of the six studied impucas (Table 2). Species richness decreased between T1 and T2 for impucas 1, 2, 4 and 5 (Fig. 2(a), (b), (d) and (e)), but not for impucas 3 and 6 (Fig. 2(c) and (f)).
Table 2 Forest structure variables in the six seasonally flooded forests (impucas) studied in the Araguaia State Park, Novo Santo Antônio municipality, Mato Grosso, Brazil, between time 1 (T1, 2007/2008) and time 2 (T2, 2014). n = number of living individuals ha–1; M = mean of number of individuals ± standard deviation; t test = paired comparison of the mean number of individuals between T1 and T2 in each impuca; S = total species richness; SS = number of shared species between the six impucas; D = number (and percentage) of dead individuals between T1 and T2; R = number (and percentage) of new recruits between T1 and T2; df = degree of freedom.
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Figure 2 Species richness in six seasonally flooded forests (impucas) sampled in the Araguaia State Park, Novo Santo Antônio municipality, Mato Grosso, Brazil, in time 1 (2007/2008, black) and time 2 (2014, grey). Black and grey bands around the central lines represent the 95% confidence intervals. (a) Impuca 1, (b) impuca 2, (c) impuca 3, (d) impuca 4, (e) impuca 5 and (f) impuca 6.
Species that had very low densities at T1, such as Alchornea discolor, Eugenia spp., Erythroxylum anguifugum, Maytenus spp., Mauritiella armata and Pterocarpus rohrii, disappeared at T2 (Supplementary Table S1). The number of species present at only one impuca was lower at T2 and the number of species present at more than one impuca did not increase between censuses (Table 2 and Supplementary Table S1). These changes were more evident when we compared the species that had restricted distribution to up to three impucas (62% at T1 and 54% at T2). Similarly, number of species occurring in four impucas increased from 38% at T1 to 46% at T2 (Table 2). Among the species that occurred in four or more impucas, we highlight Calophyllum brasiliense, Licania apetala and Mouriri acutiflora, which showed an increase in their populations in impucas 1 and 5, in which species were lost (Supplementary Table S1).
At T1, the values of floristic similarity ranged from 0.14 to 0.70, while at T2 they ranged from 0.19 and 0.78 (Table 3). Average similarity values were higher at T2 than at T1 (t = 2.69; df = 14; p = 0.017), with 73% of all possible comparisons having higher floristic similarity values at T2 (Table 3).
Table 3 Jaccard's floristic similarity matrix among six impucas sampled in the Araguaia State Park, Novo Santo Antônio municipality, Mato Grosso, Brazil, at time 1 (T1, lower diagonal) and time 2 (T2, upper diagonal).
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DISCUSSION
Our results showed new evidence that Neotropical, seasonally flooding forests show TH and an increase in floristic similarity as a result of increased fire frequency. The role of fire as a modifying driver of tropical forests has already been recognized, and there is plenty evidence that fire reduces species richness and acts as a local extinction filter of species that are not tolerant to fire (Cochrane & Schulze Reference Cochrane and Schulze1999; Barlow & Peres Reference Barlow and Peres2008; Oliveras et al. Reference Oliveras, Malhi, Salinas, Huaman, Urquiaga-Flores, Kala-Mamani, Quintano-Loaiza, Cuba-Torres, Lizarraga-Morales and Román-Cuesta2014; Devisscher et al. Reference Devisscher, Malhi, Rojas Landívar and Oliveras2016; Flores et al. Reference Flores, Fagoaga, Nelson and Holmgren2016), as well as causing long-term shifts in forest structure and species composition of tropical forests (Pinnard et al. Reference Pinard, Putz, Rumiz, Guzman and Jardim1999; Barlow et al. Reference Barlow, Peres, Lagan and Haugaasen2003; Balch et al. Reference Balch, Nepstad, Brando, Curran, Portela, Carvalho and Lefebvre2008; Flores et al. Reference Flores, Piedade and Nelson2014; Oliveras et al. Reference Oliveras, Malhi, Salinas, Huaman, Urquiaga-Flores, Kala-Mamani, Quintano-Loaiza, Cuba-Torres, Lizarraga-Morales and Román-Cuesta2014; Devisscher et al. Reference Devisscher, Malhi, Rojas Landívar and Oliveras2016; Durigan & Ratter Reference Durigan and Ratter2016). Given the short period of time (i.e., 7 years) between censuses and the profound differences that we found in the species composition and forest structure between the two censuses, we highlight the important role of fire as a BH agent in seasonally flooding forests.
In the Araguaia State Park, wildfires usually start at campos de murundus grasslands and spread into the impucas, especially during the dry season (Marimon et al. Reference Marimon, Marimon-Júnior, Lima, Jancoski, Franczak, Mews and Moresco2008, Reference Marimon, Colli, Marimon-Junior, Mews, Eisenlohr, Feldpausch and Phillips2015). Our results suggest that a first fire event in Neotropical, seasonally flooded forests clearly causes greater mortality of individuals and that species loss increases gradually with increasing wildfire recurrence. For example, impuca 2 showed no signs of fire at T1 (Barbosa et al. Reference Barbosa, Marimon, Lenza, Marimon-Júnior, Oliveira and Maracahipes2011), but had the highest tree mortality rate of all impucas at T2 (42%) due to a wildfire that occurred in 2008 (Maracahipes et al. Reference Maracahipes, Marimon, Lenza, Marimon-Junior, Oliveira, Mews, Gomes and Feldpausch2014). On the other hand, impuca 1, which was already degraded by fire at T1 (Barbosa et al. Reference Barbosa, Marimon, Lenza, Marimon-Júnior, Oliveira and Maracahipes2011) and soon after this had a new record of fire (Maracahipes et al. Reference Maracahipes, Marimon, Lenza, Marimon-Junior, Oliveira, Mews, Gomes and Feldpausch2014), lost five plant species. Effects of fire on plant mortality have also been found recently in other types of tropical forests. For example, in igapó (blackwater) forests, the first fire event also caused high tree mortality and the recurrence of fire adversely affected the recovery of these forests (Flores et al. Reference Flores, Fagoaga, Nelson and Holmgren2016). Likewise, in non-flooding forests of the Amazon basin, areas burned three times showed a reduction in saplings and tree species richness (Silveira et al. Reference Silveira, Louzada, Barlow, Andrade, Mestre, Solar, Lacau and Cochrane2016), and areas burned five times showed a substantial reduction in species diversity and stem density (Balch et al. Reference Balch, Massad, Brando, Nepstad and Curran2013). In other tropical forests around the world, species losses were also associated with fire (Slik et al. Reference Slik, Verburg and Kebler2002; Nangendo et al. Reference Nangendo, Stein, ter Steege and Bongers2005).
The species C. brasiliense, L. apetala and Vochysia divergens remained common or widely distributed in the impucas, even in those impucas that had greater mortality of individuals. However, species with a narrower distribution, such as A. discolor, Eugenia spp., E. anguifugum, Maytenus sp., M. armata and P. rohrii, disappeared from several impucas where they had a low number of individuals (five or fewer stems) at T1. In the Amazonian Forest, 1.4% of tree species are responsible for more than half of the total number of trees, therefore being hyperdominant species (ter Steege et al. Reference ter Steege, Pitman, Sabatier, Baraloto, Salomão, Guevara, Phillips, Castilho, Magnusson, Molino, Monteagudo, Vargas, Montero, Feldpausch, Coronado, Tim, Mostacedo, Vasquez, Assis, Terborgh, Wittmann, Andrade, Laurance, Laurance, Marimon, Marimon, Vieira, Amaral, Brienen, Castellanos, López, Duivenvoorden, Mogollón, Matos, Dávila, García-Villacorta, Diaz, Costa, Emilio, Levis, Schietti, Souza, Alonso, Dallmeier, Montoya, Piedade, Araujo-Murakami, Arroyo, Gribel, Fine, Peres, Toledo, Aymard, Baker, Cerón, Engel, Henkel, Maas, Petronelli, Stropp, Zartman, Daly, Neill, Silveira, Paredes, Chave, Filho, Jørgensen, Fuentes, Schöngart, Valverde, Di Fiore, Jimenez, Peñuela Mora, Phillips, Rivas, van Andel, von Hildebrand, Hoffman, Zent, Malhi, Prieto, Rudas, Ruschell, Silva, Vos, Zent, Oliveira, Schutz, Gonzales, Nascimento, Ramirez-Ângulo, Sierra, Tirado, Medina, van der Heijden, Vela, Vilanova Torre, Vriesendorp, Wang, Young, Baider, Balslev, Ferreira, Mesones, Torres-Lezama, Giraldo, Zagt, Alexiades, Hernandez, Huamantupa-Chuquimaco, Milliken, Cuenca, Pauletto, Sandoval, Gamarra, Dexter, Feeley, Lopez-Gonzalez and Silman2013), and these species have a higher chance of maintaining their populations after a disturbance than those with lower population sizes (Cochrane & Schulze Reference Cochrane and Schulze1999). Thus, in this context, the local extinction of the less abundant species can increase species similarity among different areas, depending on whether one common species or several species disappear (Olden & Poff Reference Olden and Poff2003).
The process of BH described by McKinney and Lockwood (Reference McKinney and Lockwood1999) takes into account the fact that exotic species that are highly resilient (winners) may become dominant in ecological communities after a disturbance, while most native species (losers) are not able to compete with exotic species and may become extinct. However, BH can also be the result of shifts in native species distributions after anthropogenic disturbances (Lôbo et al. Reference Lôbo, Leão, Melo, Santos and Tabarelli2011; Tabarelli et al. Reference Tabarelli, Peres and Melo2012; Silveira et al. Reference Silveira, Louzada, Barlow, Andrade, Mestre, Solar, Lacau and Cochrane2016). In this case, according to the referred authors, any biotic community naturally hosts winner species that would be favoured by disturbance and loser species that would be sensitive to such disturbances. In some areas of the Atlantic Forest of the Brazilian coast, there has been a 28% increase in plant species similarity between different areas, and this similarity is driven by the increase of pioneer species in fragmented areas (Lôbo et al. Reference Lôbo, Leão, Melo, Santos and Tabarelli2011). Similarly, in the non-flooded forest area of the Brazilian Amazon, the compositions of birds, dung beetles and sapling species were homogenized by recurrent fire events (Silveira et al. Reference Silveira, Louzada, Barlow, Andrade, Mestre, Solar, Lacau and Cochrane2016). Our results showed strong evidence that TH can also be modulated by fire, even in a short period of time, which favours the substitution of fire-sensitive species by fire-tolerant species, similarly to what has already been reported for tropical forests (Pinard et al. Reference Pinard, Putz, Rumiz, Guzman and Jardim1999; Barlow & Peres Reference Barlow and Peres2008; Balch et al. Reference Balch, Nepstad, Curran, Brando, Portela, Guilherme, Reuning-Scherer and Carvalho2011, Reference Balch, Massad, Brando, Nepstad and Curran2013; Oliveras et al. Reference Oliveras, Malhi, Salinas, Huaman, Urquiaga-Flores, Kala-Mamani, Quintano-Loaiza, Cuba-Torres, Lizarraga-Morales and Román-Cuesta2014; Devisscher et al. Reference Devisscher, Malhi, Rojas Landívar and Oliveras2016; Silveira et al. Reference Silveira, Louzada, Barlow, Andrade, Mestre, Solar, Lacau and Cochrane2016).
Plant mortality was higher than recruitment in five of the six studied impucas. This disequilibrium suggests that the effects of fire on seasonally flooding forests can last for long periods of time, as has already been found in other similar forests in the Amazon basin that still presented evidence of fire scars 19 years afterwards (Flores et al. Reference Flores, Piedade and Nelson2014).
Seasonally flooded forests have a characteristic litter layer mixed by a thin, fine root matrix (Barbosa et al. Reference Barbosa, Marimon, Lenza, Marimon-Júnior, Oliveira and Maracahipes2011; Flores et al. Reference Flores, Piedade and Nelson2014), which is particularly important for nutrient cycling (Stark & Jordan Reference Stark and Jordan1978). During the dry season, this organic layer dries enough to sustain surface fires that can lead to smouldering belowground combustion, consuming the roots of adult trees and therefore killing them (Flores et al. Reference Flores, Piedade and Nelson2014; Maracahipes et al. Reference Maracahipes, Marimon, Lenza, Marimon-Junior, Oliveira, Mews, Gomes and Feldpausch2014). As a result, the forest canopy opens, allowing light gaps and increasing light penetration into the forest, which in turn favours the establishment of pioneer species such as graminoids and sedges (Silvério et al. Reference Silvério, Brando, Balch, Putz, Nepstad, Oliveira–Santos and Bustamante2013; see also Silveira et al. Reference Silveira, Louzada, Barlow, Andrade, Mestre, Solar, Lacau and Cochrane2016). In this study, we observed an increase of Scleria grass in the impucas in T2 with respect to T1 (data not shown). An increase in the grass biomass is directly translated into an increase in fine fuel load, which increases fire severity (Nepstad et al. Reference Nepstad, Verissímo, Alencar, Nobre, Lima, Lefebore, Schlesinger, Potter, Moutinho, Mendoza, Cochrane and Brooks1999; Silvério et al. Reference Silvério, Brando, Balch, Putz, Nepstad, Oliveira–Santos and Bustamante2013).
With longer dry seasons and increased fire frequencies as a result of climate change (Aragão et al. Reference Aragão, Malhi, Barbier, Lima, Shimabukuro, Anderson and Saatchi2008; Marengo et al. Reference Marengo, Tomasella, Alves, Soares and Rodriguez2011; Gatti et al. Reference Gatti, Gloor, Miller, Doughty, Malhi, Domingues, Basso, Martinewski, Correia, Borges, Freitas, Braz, Anderson, Rocha, Grace, Phillips and Lloyd2014), seasonally flooding forests in southern Amazonia will likely suffer significant changes in their species compositions and ecosystem functioning with BH. The diversity of vegetation types in ASP demands a careful fire management plan if one aims to maintain the vegetation diversity in the area, since it is composed of a mosaic of vegetation with very different tolerance to fire, including fire thrivers and fire-tolerant and fire-intolerant vegetation communities (Pinard et al. Reference Pinard, Putz, Rumiz, Guzman and Jardim1999; Marimon et al. Reference Marimon, Marimon-Júnior, Lima, Jancoski, Franczak, Mews and Moresco2008, Reference Marimon, Marimon-Junior, Mews, Jancoski, Franczak, Lima, Lenza, Rossete and Moresco2012, Reference Marimon, Colli, Marimon-Junior, Mews, Eisenlohr, Feldpausch and Phillips2015; Oliveras et al. Reference Oliveras, Malhi, Salinas, Huaman, Urquiaga-Flores, Kala-Mamani, Quintano-Loaiza, Cuba-Torres, Lizarraga-Morales and Román-Cuesta2014; Pinto et al. Reference Pinto, Mews, Jancoski, Marimon and Bomfim2014). Changes in the fire regime compromise the maintenance of the ecological processes and the biodiversity of the ecosystems of the Cerrado (Pivello Reference Pivello2011; Durigan & Ratter Reference Durigan and Ratter2016). The absence of fire in these environments can have a negative effect through the homogenization of the vegetation mosaic that makes up the Cerrado (Durigan & Ratter Reference Durigan and Ratter2016), as well as greater accumulation of fuel, which increases the risk to large-scale forest fire-sensitive ecosystems (Pivello Reference Pivello2011).
We conclude that seasonally flooding forests can experience TH over short periods of time if fire frequency is high. Therefore, fire is an important driver of the process of TH in Neotropical forests surrounded by savanna vegetation or pastures.
ACKNOWLEDGEMENTS
We are thankful to Secretaria de Estado de Meio Ambiente de Mato Grosso (SEMA-MT) for the authorization to conduct research in the Araguaia State Park. We thank Bianca Oliveira, Fernando Elias, Eder C. Neves and Laís F.S. Neves for assistance in data collection.
FINANCIAL SUPPORT
The study was funded by the project CNPq/PELD – Transição Cerrado-Floresta Amazônica: bases ecológicas e sócio-ambientais para a conservação (etapa II, proc. 403725/2012-7), and the authors received scholarships from FAPEMAT (proc. 164131/2013), CNPq and Rede Amazônica de Inventários Florestais (RAINFOR).
CONFLICT OF INTEREST
None.
ETHICAL STANDARDS
None.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1017/S0376892918000127.