FIRE EFFECT ON THE SEASONAL FOREST STRUCTURE IN THE CERRADO BIOME

There are few studies on the effects of fire on the seasonal semideciduous forest of the Cerrado region. This study aimed to assess the effect of fire on Semideciduous forest fragments located in the Altamiro Moura Pacheco State Park (AMPSP). The study area is located in the central state of Goiás. A total of 10 plots (50 x 20 m; 1,000 m) were drawn in the remaining forests seeking to portray two areas: area which underwent forest fire (burned area; B) and unburned area (unburned; UB). A total of 771 individuals with DBH (≥ 5 cm) were identified. A total of 303 individuals were recorded in the burned area (B), comprising 28 families, 41 genera and 47 species, while the unburned area (UB) had 468 individuals comprising 25 families, 43 genera and 50 species. The species Anadenanthera colubrina (Vell.) Brenan, Emmotum nitens (Benth.), Zanthoxylum rhoifolium Lam, Tapirira guianensis Aubl., Myracrodruon urundeuva Allemão and Luehea candicans Mart. had the highest Importance Value Index (IVI) values and occur in both areas. The percentage of dead trees differed between the areas, where the burned area had a mortality of 12.69% and the unburned area of 7.29% in regards to the number of individuals. Density per plot was the only parameter that differed between the areas, where the burned area (B) had the lowest values. The fire affected all diameter classes, demonstrating the fragility of this vegetation type in regards to fire. The fire had a greater effect on trees in the smaller diameter classes, reducing the frequency of individuals in the burned area and changing height distribution, which may have favored the establishment of lianas and invasive species.


INTRODUCTION
The Brazilian Cerrado region occupies about 25% of Brazil with an original area of approximately 203 million km 2 (RIBEIRO; WALTER, 2008), of which only 47% remains (about 103 million km 2 ) (BRASIL, 2014).Low humidity conditions at a certain time of the year (dry season), in addition to other factors such as humidity of the fuel, presence of invasive species, relief and vegetation type, may favor the spread of forest fires in the Cerrado (FREIRE et al., 2002TORRES, 2006).The fire acts directly on the plant communities and may modify the floristic composition, forest structure, plant recruitment (COCHRANE; SCHULZE, 1999) and reduce tree density per area (LIMA, et al., 2009).
Effects of fire in the Cerrado biome are regarded as a determining factor for the structure and functioning of the biome (SIMON et al., 2009).The occurrence of changes in plant communities after the passage of fire is strictly related to three main factors: (i) the number of individuals present in the area, (ii) the arrangement of these individuals in the area and (iii) the specific growth of each species (LOPES et al., 2009).
In this work, we hypothesized that the occurrence of the fire changes the community structure in Semideciduous Seasonal Forest Cerrado.Therefore, the objective of this study was to assess the effect of fire occurrence on the tree community structure of the Semideciduous Seasonal Forest in the Altamiro de Moura Pacheco State Park (AMPSP) using floristic surveys of the burned and unburned areas of the Park.

Characterization of the study area
This study was carried out in the Altamiro de Moura Pacheco State Park (AMPSP), located between the geographic coordinates 16° 30' -16° 35' S and 49° 07' -49°13' W, Goias state.The park is a full protection conservation unit established by the Law num.11.878 on December 30 1992, with a current area of 2,132 hectares and prevailing vegetation comprising dry forest, riparian forest, gallery forest, cerrado strict sense, Cerradão (Higth cerrado) and capoeira (Medium secondary sucession vegetation), in addition to some areas covered with pastures (SEMARH, 2015).
The climate of the region is Aw according to the Koppen classification, i.e., Sub-humid savannah with two distinct seasons: the rainy and dry periods (PEEL et al., 2007).The rainy period occurs in the summer (October to April), and the dry period in the winter (May to September, where July and August are the driest months).
The average temperature of the coldest month is above 18º C. Precipitation is greater than 750 mm per year, reaching at maximum 1,800 mm (CARDOSO, 2015).The prevailing soils are Oxisols (Red and Dusky Red Latosols), and Red-Yellow Argisols according to the Brazilian System of Soil Classification (EMBRAPA, 2013).The topography varies from slightly to sharply steep (SEMARH, 2015).

Floristic data collection
The first area in which the survey of the species was carried out was located near the headquarters of the Altamiro Moura Pacheco State Park, an area that was burned (B) in 2013.The second area did not suffer the passage of fire (unburned area; UB).A total of five plots of 0.1 ha (20 x 50 m) were established in each area (FELFILI et al., 1994).The inventory was conducted between the months of February to April 2015.
The trees with circumference at breast height equal to or above 15.7 cm (or diameter at breast height DBH ≥5 cm) were measured in each plot using a graduated tape (cm).All branches of a single tree, and all the bifurcations below the height of 1.30 m were measured separately being realized average values for branches.
The plots were temporarily marked with a barricade tape, with all the tree specimens included in the sample being identified with aluminum platelets in a sequential order.The geographical coordinates were obtained in the center of each plot using a Global Positioning System (GPS).
The species not identified by the survey team were collected using a trimmer, and stored in plastic bags to subsequently be sent to the Allometry and Forest Inventory laboratory (Laboratório de Alometria e Inventário Florestal; LAIF) of the Federal University of Goiás.The angiosperms classification system of the Angiosperm Philogeny Group III (APG III, 2009) was used.

Phytosociological analysis
The indexes that show the horizontal structure of vegetation (i.e., density, dominance, frequency and the coverage index -CI) were compared to assess changes in species composition and community structure.The Soresen index (Ss -Eq1) was used to calculate the similarity among sites and the Pielou index (J -Eq2) to assess the distribution of individuals in regards to the species (VENTUROLI et al., 2016) Where a = number of species occurring in part 1 or 1 community; b = number of species that occur in the portion 2 or 2 community; c = number of common species in two installments or communities J = H'/Hmáx (Eq2) Where H'= Shannon-Weaver diversity index; Hmax = Ln(S); S = number of total species sampled.

Statistical Analysis
The nonparametric Mann-Whitney test was used with a 95% significance level to compare the studied areas (burned and unburned areas).A box-plot with the quantiles, maximum and minimum values and the median of each area analyzed was built for the variable which significantly differed between the areas (number of individuals).
Species occurrence pattern and the number of individuals in the burned (B) and unburned areas (UB), were evaluated separating the different groups and discriminating the number of individuals of each species found.Then, a principal component analysis was carried out.

Height and diameter structure
The frequency distribution among the diameter and height classes were calculated and represented as histograms for the horizontal and vertical characterizations of the fragments sampled.The amplitude of the diameter classes was defined in 5 cm and of the height classes in 2 m for comparison purposes.These amplitude definitions follow Haidar (2008) and Souza et al., (2012) in studies carried out in a Semideciduous Seasonal Forest (SSF).
The Fabaceae family had the highest species richness in both areas, with 13 species and 11 genera (three species and three genera of the subfamily Caesalpinioideae, seven species and five genera of Mimosoideae, and three species and three genera of Papilionoideae).The second most species rich families were Apocynaceae with five species and one genus and Malvaceae with five species and five genera.Anacardiaceae, Combretaceae Sapindaceae, Meliaceae and Myrtaceae were represented by three species and three genera each.Bignoniaceae had three species and two genera, while Annonaceae had three species and one genus.
The Soresen index calculated for this study was 0.58, and the Pielou index (J) was 0.42 and 0.58 for the burned and unburned area, respectively.A higher concentration of individuals UB areas were recorded in the smaller diameter classes.In addition, frequency of occurrence decreases as diameter class increases, showing a "reverse J" shaped distribution (Figure 1).Subtitle: AD = Absolute density; RD = Relative density, AF = Absolute frequency, RF = Relative frequency, ADo = Absolute dominance, RDo = Relative Dominance and IVI = Importance Value Index.

DISCUSSION
The relative amount of dead tree trunks in the unburned area (UB) is similar to the observed for Brazilian forest formations with 7.29%.In the burned area the number of dead trees reaches 12.69%.The Family Fabaceae had the highest occurrence in both areas.This Family has important species for the ecological succession of a forest, given its primary function in nitrogen fixation (GUSSON et al, 2009).Pioneer species were recorded (highlighting P. gonoacantha), in addition to late secondary species (A. colubrina, Copaifera langsdorffii and Hymenaea courbaril).
The number of families recorded in the entire area was similar to the reported by Haidar (2008) in seasonal semideciduous forest fragments (SSFF) in Distrito Federal, and lower than the encountered by different authors in the same vegetation type in different regions of Brazil.The total basal area of 23.18 m 2 .ha - , is similar to that of other studies in seasonal semideciduous forest in central Brazil.However, the tree density per hectare was lower than the values recorded by different authors (IMAÑA-ENCINAS et al., 2007;HAIDAR, 2008).
The genus Aspidosperma had the second largest species richness (five species).Aspidosperma species are heavily exploited due to good quality wood and have currently become rare due to the intense exploitation to which individuals were submitted to, in addition to deforestation (INSTITUTO BRASILEIRO DE GEOGRAFIA E ESTATÍSTICA (IBGE), 2002).Several genera of interest for the logging industry were also sampled, for example Tabebuia, Handroanthus, Myracrodruon, Hymenaea, Anadenanthera and Cedrella.
The occurrence of such genera shows the importance of maintaining the AMPSP, which functions as a gene bank (germplasm) of species threatened by exploitation without proper management or by the human pressure on natural communities (IBGE, 2002;HAIDAR, 2008).The ten species with the highest importance value indices (IVI) in the UB were, in a descending order: Anadenanthera colubrina (Vell.)Brenan, P. gonoacantha (Mart.),Emmotum nitens (Benth), Z. rhoifolium Lam., M. rostrata DC., Tapirira guianensis Aubl., Myracrodruon urundeuva Alemmão, Aspidosperma subincanum Mart ex DC., Luehea divaricata Mart.and Psidium sartorianum (O.Berg) Nied.These species add up to 55% of the total IVI recorded for this tree community.
A higher concentration of individuals was recorded in the smaller diameter classes (DBH > 5 ≤ 10 cm), which is considered normal for Brazilian rainforests (MACHADO et al., 2010;SOUZA, 2012).However, the number of individuals in the smaller size class differs between the areas, where the burned area has fewer individuals than the unburned areas.This difference could be due to the action of fire during forest fire occurred because the effects of the flames are higher in smaller trees (RIBEIRO et al, 2012), leading therefore to a reduced density patches of smaller diameter in the burned area.
The fire in the burned area also affected harshly the trees in the larger diameter classes, showing that species of the seasonal semideciduous forests are more susceptible to the passage of fire, in contrast with species occurring in the savanna vegetation types.The basal area occupied by dead trees in the B area was 3.20 m 2 /ha while in the UB area was1.20 m 2 ha -1 .The B area had 72 ind.ha -1 and UB area had 74 ind.ha -1 .The diameter distribution in both areas indicates that new individuals are entering the forest community.Self-regeneration of individuals in the burned area (B), may be inferred once the diameter distribution had an "inverted J" shape, which is typical of a largest number of individuals occurring in the smaller diameter classes (MARTINS et al., 2016).
The burned area (B) had density of individuals ranging from 51 to 91 individuals per plot while the unburned area (UB) had a variation of 74 to 119 individuals per plot.Therefore, burned area (B) had a density of individuals 36% lower than the unburned area (UB).The effect of fire modified the height distribution within the fragment, what affected light entrance.The light input may have been a determining factor for a greater occurrence of lianas and species of exotic grasses with invasive characteristics in the burned área.Lianas and invasive exotic grasses are mainly heliophytic and proliferate very effectively in illuminated environments (SILVA-NETO et al., 2015;VENTUROLI et al., 2015).

CONCLUSION
 The fire acted more severely on the smaller diameter classes, changing the density of individuals and the distribution among height classes.The fire also affected the trees in the larger diameter classes, showing the fragility of the species of this vegetation type in regards to forest fires. The clearings formed by the death and fall of the tree component in the burned area enabled a higher occurrence of colonizing species, especially lianas and grasses.Such higher occurrence changes the natural structural characteristics of the seasonal semideciduous forest fragments of the AMPSP.

Figure 1 .
Figure 1.Number of individuals per diameter class (DBH > 5 cm) register in the study areas (burned or unburned) of seasonal semideciduous forest of the Altamiro de Moura Pacheco State park, Goiás.Figura 1. Número de indivíduos por classe de diâmetro (DAP > 5 cm) registrados nas áreas de estudo (queimadas e não queimadas) de floresta estacional semidecidual do Parque Estadual Altamiro Pacheco, Goiás.The distribution of individuals in height classes tends to statistical normality in the unburned area, with a higher frequency of individuals in the smaller size classes (<10 m).The distribution of height classes in burned area had an asymmetric distribution, with the highest concentration of individuals in the smaller height classes (Figure2).

Table 1 .
Average of the parameters analyzed per study area, burned area (B) and unburned area (UB), in the Altamiro Moura Pacheco State Park, state of Goiás.The parameters represented by J and Ss are the Pielou and Soresen indexes, respectively.Tabela 1. Média dos parâmetros analisados por área de estudo, area queimada (B) e área não queimada (UB), no Parque Estadual Altamiro Pacheco, estado de Goiás, Os parâmetros apresentados por J e Ss são os indices de Pielou e Soresen, respectivamente.