Abstracts

Tree structure and richness in an Atlantic Forest fragment: distance from anthropogenic and natural edges

Maíra Taquiguthi Ribeiro^I; Flavio Nunes Ramos^II; Flavio Antonio Maës Dos Santos^I

^IDepartamento de Botânica, Instituto de Biologia, Universidade Estadual de Campinas _ UNICAMP, Campinas, SP, Brazil, E-mail: maira.taqui@gmail.com e fsantos@unicamp.br

^IIDepartamento de Ciências Biológicas e da Terra, Universidade Federal de Alfenas _ UNIFAL-MG. Rua Gabriel Monteiro da Silva, 714, Centro, 37130-000. Alfenas, MG, Brazil,

E-mail: fnramos@gmail.com

ABSTRACT

Approximately 7.2% of the Atlantic rainforest remains in Brazil, with only 16% of this forest remaining in the State of Rio de Janeiro, all of it distributed in fragments. This forest fragmentation can produce biotic and abiotic differences between edges and the fragment interior. In this study, we compared the structure and richness of tree communities in three habitats - an anthropogenic edge (AE), a natural edge (NE) and the fragment interior (FI) - of a fragment of Atlantic forest in the State of Rio de Janeiro, Brazil (22°50'S and 42°28'W). One thousand and seventy-six trees with a diameter at breast height > 4.8 cm, belonging to 132 morphospecies and 39 families, were sampled in a total study area of 0.75 ha. NE had the greatest basal area and the trees in this habitat had the greatest diameter:height allometric coefficient, whereas AE had a lower richness and greater variation in the height of the first tree branch. Tree density, diameter, height and the proportion of standing dead trees did not differ among the habitats. There was marked heterogeneity among replicates within each habitat. These results indicate that the forest interior and the fragment edges (natural or anthropogenic) do not differ markedly considering the studied parameters. Other factors, such as the age from the edge, type of matrix and proximity of gaps, may play a more important role in plant community structure than the proximity from edges.

Keywords: Forest fragmentation. Natural edge. Vegetation structure.

RESUMO

Restam aproximadamente 7,2% da Mata Atlântica no Brasil distribuída em fragmentos, com apenas 16% dela no Estado do Rio de Janeiro. Essa fragmentação florestal pode produzir diferenças bióticas e abióticas entre bordas e interior de fragmentos. Neste estudo, comparam-se a estrutura e riqueza das comunidades arbóreas de três ambientes bordas antrópica (BA), bordas naturais (BN) e interior (IF) de um fragmento de Mata Atlântica no Estado do Rio de Janeiro, Brasil (22°50'S e 42°28'O). Mil e setenta e seis árvores com diâmetro à altura do peito > 4,8 cm, pertencentes a 132 morfoespécies e 39 famílias, foram amostradas em uma área estudada total de 0,75 ha. BN apresentou a maior área basal, e as árvores nesse hábitat tiveram coeficiente alométrico maior, enquanto BA apresentou menor riqueza e maior variação da altura do fuste em relação à altura total. Densidade de árvores, diâmetro, altura e proporção de árvores mortas em pé não diferiram entre ambientes. Foi observada alta heterogeneidade entre repetições de um mesmo tipo de hábitat. Outros fatores, como a idade da borda, o tipo de matriz e a proximidade de clareiras, podem desempenhar papel mais importante na estrutura da comunidade de plantas do que a proximidade das bordas.

Palavras-chave: Borda natural. Estrutura da vegetação. Fragmentação florestal.

1. INTRODUCTION

The Atlantic forest is the natural biome of almost the entire eastern region of Brazil and covers close to 1,300,000 km² in sixteen Brazilian states (MORELLATO and HADDAD, 2000). It is characterized by high levels of biodiversity and endemism, as well as intense human activity, and contains the largest urban centers of Brazil (MORELLATO and HADDAD 2000). The devastation and fragmentation of this native vegetation is a consequence of unplanned land occupation and exploitation of natural resources since colonial times. Currently, only 7.2% of the Brazilian Atlantic forest remains, all of which is distributed in fragments and, in the State of Rio de Janeiro, where this vegetation once covered almost the entire territory, barely 16% remains (MORELLATO and HADDAD, 2000).

Forest fragmentation, resulting from the reduction and isolation of native forests, produces an increased region of forest edge (MURCIA, 1995; HARPER et al., 2005). The forest edge is the area exposed to the anthropogenic landscape (matrix) formed after fragmentation (METZGER, 1999). Consequently, compared to the interior of the forest fragment, a forest edge can experience changes in microclimatic conditions such as increase in temperature, light exposure and wind intensity, and decrease in air and soil moisture (MURCIA, 1995; TABARELLI et al., 1999; NELSON and HALPERN, 2005). These microclimatic changes may cause differences in the tree density and structure of these forest remnants (METZGER et al., 1997; OLIVEIRA-FILHO et al., 1997). For example, since the incidence of light is elevated at the forest edges, the number of plants can increase, but with a lower basal area. Furthermore, microclimatic modifications can produce biotic differences in plant composition and diversity because of tree mortality and recruitment (LAURENCE et al., 1998; OLIVEIRA et al., 2004).

Tree structure can also be affected by the availability of light and space, which determine plant forms (HOLBROOK and PUTZ, 1989; KOHYAMA and HOTTA, 1990; MOURELLE et al., 2001). Plant form is the result of a tradeoff between the vertical growth to reach better light and spatial conditions and the horizontal growth required to support the plant´s own weight and allow efficient energy assimilation (KING, 1990; HENRY and AAERSSEN, 1999). Hence, plant allometry is related to the immediate environmental conditions. Natural edges, such as forest borders with rivers and lakes, can exhibit similar abiotic and biotic modifications to those seen at anthropogenic edges of forest fragments (CORBET, 1990; MATTLACK, 1994). However, there is little information regarding these variations.

The aim of this study was to compare the structure of the tree communities, as well as its richness, among anthropogenic edges, natural edges and forest interior of a fragment of Atlantic forest in southeastern Brazil. The expectation of this study is that the tree community structure from both anthropogenic and natural edges will be similar among them, because of the probable akin abiotic conditions, and their structure will be very different from fragment interior.

2. MATERIALS AND METHODS

2.1 Study area

The study area was located in the coastal mountain range of the Serra do Palmital, Saquarema, in the State of Rio de Janeiro, Brazil. This Atlantic forest fragment of approximately 1,200 ha is located mostly on private properties. This study was done in 180 ha of the fragment (22°50'S and 42°28'W), at altitudes ranging from 30 m to 600 m. The forest bordered pastures and cropland of private properties, producing anthropogenic edges. Within the study area, a stream 2-5 m wide and 900 m long creates a natural edge with the forest.

The regional climate is classified as Cwa according to the Köppen system (VIANELLO and ALVEZ, 1991), and is characterized by warm, wet summers and dry winters. The annual rainfall for 2003 was 1,210 mm, with the greatest precipitation occurring between November and April (RAMOS and SANTOS, 2005). The vegetation is classified as evergreen forest or Tropical Moist Forest ("Floresta Ombrófila Densa") (RADAMBRASIL, 1983).

2.2 Methods

Three habitats of the forest fragment were studied: (1) forest at 50 m from the stream (NE = natural edge), (2) forest at 50 m from areas modified by human action (AE = anthropogenic edge) and (3) forest 200 m or more from any edge (FI = forest interior). Five areas were randomly chosen within each habitat. Five sample plots of 10 m x 10 m were sampled per area (0.05 ha/ area), resulting in a sampling area of 0.25 ha per habitat and a total area of 0.75 ha.

All trees with a diameter at breast height (DBH, at 1.3 m) > 4.8 cm in each plot were tagged, their height and perimeter were measured, and voucher specimens were collected for identification. The diameter (D) was obtained from the perimeter (P) as D = P•p^-1. The tree diameter (D) and height (H) were used to estimate the basal area (Ba) as Ba = p(0.5•D)² and the cylindrical volume (V) as V = Ba•H. However, when a tree had more than one trunk at breast height, the diameter was obtained from the sum of the basal areas (Ba, in cm²) of each trunk, using the formulas Ba = P²•(4p)^-1 and D = 2(SBa•p^-1)^0.5.

2.3 Statistical analysis

The basal area, diameters, tree heights, and tree density were analyzed by a two-level nested ANOVA, after testing for normality and homoscedasticity (ZAR 1996). The factors tested were the habitat (fixed factor) and its five replicates (nested within habitats). The proportion of standing dead trees with a DBH e > 4.8 cm relative to the total of sampled trees was compared among and within habitats using a chi-square () test (ZAR, 1996).

To assess the variation in tree forms among habitats, we did a morphometric analysis based on the ratio of tree height to tree diameter, and on the ratio of total height to the height at first branching). The diameter (D) and height (H) were related by the expression H = a•D^b. The r² and the allometric coefficient (b) of the regression equations were compared among habitats. The allometric coefficients were compared by ANCOVA followed by the Scheffé test (p = 0.05) (ZAR, 1996).

The trees were divided into three classes based on the ratio of the first branching height to total tree height: Class I plants with a first branch at <1/3 of the total height, Class II plants with a first branch between >1/3 and <2/3 of the total height, and Class III plants with a first branch at >2/3 of the total height. The proportion of plants in each class relative to the total number of sampled plants in each habitat was compared by a chi-square test (ZAR, 1996). The species richness was estimated using rarefaction curves based on species, with the Rarefact module from the Krebs program (KREBS, 1989).

3. RESULTS

3.1 Tree structure

3.1.1. Among habitats

In the total area (0.75 ha), 1,168 trees were recorded; 1,076 of them were living plants (1,434.7 ± 80.8 trees.ha^-1), and 92 (7.9%) were standing dead trees (122.7 ± 62.3 trees.ha^-1). The natural edge (NE) had 347 living trees (32.2%; 1,388.0 ± 177.5 trees.ha^-1), whereas the forest interior (FI) had 382 (35.5%; 1,528.0 ± 510.0 trees.ha^-1) and the anthropogenic edge (AE) had 347 (32.2%; 1,388.0 ± 223.0 trees.ha^-1). The total basal area was 30.9 ± 8.1 m² ha^-1 and the total cylindrical volume was 433.2 ± 168.3 m³ ha^-1.

The density of living trees (F_2,12 = 0.280, p = 0.760), the DBH (F_2,12 = 2.750, p = 0.100), the total tree height (F_2,12 = 1.300, p = 0.300) and the proportion of standing dead trees ( ₂ = 0.556, p = 0.757) did not vary significantly among habitats. However, there was a significantly greater basal area in the NE (F_2,12 = 5.200, p = 0.023), while the FI and AE did not differ significantly among each other (Figure 1).

3.1.2 Within habitats

Within each habitat, tree height (F_12,1061 = 2.500; p = 0.002) and density (F_12,60 = 3.500; p = 0.001) differed significantly, with the greatest internal variation in density occurring in the FI (coefficient of variation: AE = 16.1%, FI = 33.4%, NE = 12.8%). This variation occurred principally because one of the FI replicates was established in a gap environment and had an elevated number of thin trees. However, the DBH (F_{12, 1061} = 1.100; p = 0.330), basal area (F_12,60 = 0.600, p = 0.840), and the proportion of standing dead trees (AE ₄ = 3.006; p = 0.557, FI ₄ = 8.356; p = 0.079, NE ₄ = 3.389; p = 0.495) did not differ significantly within habitats.

3.2. Allometry

The relationship between tree height and diameter was significant in the three habitats (Figure 2, AE r²₃₄₅ = 0.545; FI r²₃₈₀ = 0.586; NE r²₃₄₅ = 0.705; p<0.001 for each), and the allometric coefficients were low for the three habitats. Nevertheless, the trees in the NE showed less variation around the regression line and a greater allometric coefficient (b = 0.54) than trees in the AE and FI (b = 0.46 for both, F_2,1070 = 6.21, p = 0.003).

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