Fragmentation of tropical forests can promote edge effects that lead to accelerated increases in plant recruitment and mortality [1] [2] . Small forest fragments showed changes in their structure, increased density, and reduced biomass [3] [4] , resulting in both local and regional extinctions of tree species [5] [6] and impoverished edge communities [12] . Some species, however, such as pioneer trees [7] and lianas [8] , can be favored by the higher solar radiation levels found in disturbed sites, and show increases in recruitment and growth rates [9] . The structural and functional changes caused by edge effects are currently viewed as responsible for many of the major alterations seen in tropical forests [10] .

The Atlantic Forest is the second largest forest type in Brazil, originally covering an area of 1.1 million km² and extending from the states of Rio Grande do Norte to Rio Grande do Sul [11] [12] although it is now reduced to only 11% of its original size [13] . This biome is counted among the 25 primary global “hotspots”―areas of high diversity and high concentrations of endemic species that are threatened by rapid habitat losses [14] . The ecological relationships between species have been severely altered in many tropical forests and their biological communities are at risk of extinction [15] due to deforestation, selective cutting, highway construction, and urban expansion [16] , all leading to habitat loss and fragmentation [14] [17] [18] .

These anthropogenic processes create mosaics of small forest remnants within urban or agricultural matrices [19] , and increases in forest edges are followed by structural and functional changes in communities that arise from alterations of abiotic and biotic conditions [20] [21] . Increased light and wind flux near edges [22] can increase air temperatures and vapor pressure deficits, and decrease soil moisture levels [3] , generating an edge- interior microclimatic gradient. These changes invariably lead to negative biotic responses, including increased mortality, recruitment, damage, and tree falls [7] [8] [22] .

While oil pipeline canopy openings represent a threat to tropical forests, they can also be viewed as potential large-scale field experiments [23] , and we used such openings (created by two oil pipelines inside a protected area) to examine whether they create edge effects that impact the structures and dynamics of Brazilian montane Atlantic Forests.

The tree densities per hectare (n. ha⁻¹) (2004/2005) were 1853/1857 in the forest interior (INT) and 2136/2173 and 2353/2380 along the edges of the Old Pipeline (OldP) and New Pipeline (NewP) respectively (Table 1). Thus, trees densities per plot were higher along the disturbance edges, with a significant difference mainly between the New Pipeline and the Interior in the years 2004 and 2005 (F = 4.25; p < 0.02 and F = 5.05; p < 0.01 respectively) (Figure 1). In 2005, the edges also showed more standing dead trees, with 5.5% (OldP) and 6.6%

(NewP) as opposed to 4% in the Interior (Figure 1). Surprisingly, no differences were found in terms of basal area between treatments during the study period.

Size structures considering all trees (DBH ≥ 5 cm) did not differ between the treatments (Figure 2), although considering small trees (5 ≤ DBH < 10 cm) and large trees (DBH ≥ 10 cm) separately showed distinct patterns. Small tree densities and basal areas per plot no showed differences between the treatments during the study years (p < 0.05). Large tree densities were higher along the New Pipeline as compared to the Interior in the years 2004 and 2005 (F = 5.23, p < 0.01 and F = 4.83, p < 0.02 respectively) (Table 2). Differences were observed between the Old Pipeline and the Interior in terms of the DBH values of small trees (5 ≤ DBH < 10 cm) in 2003 and 2004 (F = 3.52, p < 0.03 and F = 4.33, p < 0.01), and in 2005 both edges were different from the Interior (F = 4.93, p < 0.007), (Table 2) with the latter having higher values. The same pattern was observed for the size structures of the large trees, with the interior site having higher values than the New Pipeline in the years 2004 and 2005 (F = 3.96, p < 0.02 and F = 3.42, p < 0.03) (Table 2). Tree densities along the New Pipeline were higher, however, mainly in terms of the size classes 10 - 20, 20 - 30, and 30 - 40 cm, while the densities of very large trees (diameters > 40 cm) were always higher in the Interior (Figure 2).

Vegetation dynamics along the edges were more accelerated as compared to the interior, principally along the New Pipeline. Despite the general pattern of recruitment of new individuals between the years 2003-2005 (Old Pipe- line: 1.5%; New Pipeline: 7.28%, and Interior: 3.23%), the edges showed reduced basal areas (Old Pipeline: −0.59%; New Pipeline: −3.97%) while Interior gained less basal area (3.12%). Furthermore, New Pipeline mortality rates were higher and differed significantly from the interior site during the period from 2004-2005 (Figure 3).

Edge effects can also be detected by observing the occurrence of individuals with multiple trunks, and higher frequencies of these individuals were seen along the edges (Figure 4). On the other hand, the palm tree Euterpe edulis exhibited strong reductions in numbers along forest edges.

Edge effects due to disturbance regimes can cause profound changes in species interactions and resource

availability [4] [5] [7] [8] often leading to increases in mortality, recruitment, biotic damage, and tree falls [7] [8] [22] , and tropical forests subjected to edge effects often remain in early-successional stages [15] [26] [27] . Tree structures and their dynamics indicated that the forest communities in the Tinguá Biological Reserve were subject to edge effects, especially along the edge of the most recently installed pipeline.

Tree densities increased over the years, and were highest along the pipeline, due to greater light availability, which generally favors recruitment [28] . Canopy openness near edges (with increased wind and light flux) can therefore favor recruitment, mortality [7] [8] [28] and the growth of small plants and/or juveniles [29] . Their observed mortality and recruitment rates were similar, however, to other Tropical Forests [12] [30] [31] . Some tropical forests lose from 15% to 35% of their biomass due to increased mortality in the first 5 - 10 years after fragmentation [32] . In the case of Tinguá Biological Reserve, the expected decrease in basal area was observed only along pipeline edges during the period from 2003-2005 (Old Pipeline: −0.59%, New Pipeline: −3.97%, Interior: +3.12%, Table 1).

Therefore, increases in the numbers of small pioneer trees [28] [33] [34] and changes in successional patterns due to edge effects [5] [35] resulted in higher densities of small trees along the forest edges. As small plants are generally more fragile and may exhibit higher mortality rates [36] [37] vegetation dynamics will often be accelerated along edges. Additionally, very large trees may be excluded depending on the disturbance scale and intensity [8] [15] , which would explain why very large trees were only found in the Interior.

Edge effects may be attenuated over time [5] , however, by the buffering effects of new vegetation originating at the edge [19] , or by the establishment of certain vegetation communities such as vines and bamboos [38] along those disturbed areas [7] [12] [39] [40] . The oldest edge (Old Pipeline) was found to be more similar to the interior. The maintenance of abrupt edges and the proximity of shrubby or herbaceous matrices, however, can favor continuing anthropogenic impacts [40] [41] . Linear canopy openings may be less harmful, than fire-prone sites and agricultural fields, although they can facilitate the entry of hunters and collectors. The lower edge densities of the heart-palm Euterpe edulis (pers. observation) is related to illegal extraction and numerous cut trunks were observed during the course of the fieldwork.

Tree densities were higher along forest edges while tree basal areas decreased along edges due to higher mortality rates. On the other hand, edges showed higher densities of small trees, while the interior had more very large trees. Furthermore, pipelines can also favor the anthropogenic damage by providing access to hunters and collectors. Thus, these linear canopy openings promote edge effects, although they seem to be less detrimental than edges on fire-prone sites and agricultural fields.

We thank Haroldo Lima, Sebastião Neto, Rejan Guedes-Bruni, Flavio Ramos and Andrea Costa for comments made on an earlier version of this work; Walter da Silva and Adilson Pintor for their help during the fieldwork; two anonymous referees by their corrections; the Fundação Flora and PETROBRAS for financial support (research grant number 6000.0023998.06.02 to Programa Mata Atlântica); the Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio) and Colonia de Férias dos Aeroviários for providing logistical support; Mr. Roy Funch for linguistic advice.

Pablo J. F. Pena Rodrigues,Leticia R. Melo,Rodolfo C. R. de Abreu,Mariana A. Iguatemy, (2016) Edge Effects of Oil Pipeline Canopy Openings on Tree Community Structure and Dynamics in a Montane Atlantic Forest. Journal of Geoscience and Environment Protection,04,132-140. doi: 10.4236/gep.2016.47014