PARTIAL-DERIVATIVE-C-13 AS AN INDICATOR OF EDGE EFFECTS IN TROPICAL RAIN-FOREST RESERVES

1. The effects of proximity to an exposed forest edge on plants in Ama zonian lowland rainforest were assessed by examining carbon isotopic c omposition (partial derivative C-13) of canopy leaves and of understor ey leaves and air. 2. Exposed canopy leaves of Licania heteromorpha an d Scleronema micranthum, were collected (a) in continuous forest, (b) within 100 m of a 5-year-old edge, and (c) in isolated 1-ha forest fra gments. Leaves of the understorey species, Duguetia aff. flagellaris a nd Astrocaryum sociale, were sampled at intervals along transects from a 5-year-old edge towards the forest interior and in control areas. L eaves of D. flagellaris were also collected from young gaps, gap edges , old gaps, undisturbed understorey and from secondary vegetation. Par tial derivative C-13 and CO2 concentration of understorey air were mea sured along the study transects. The proportion of the transect area o ccupied by gaps was estimated. 3. The isotopic composition of the carb on in canopy leaves did not differ among the three types of sampling a reas. However, D. flagellaris had significantly higher partial derivat ive C-13 within 60 m of the edge than in undisturbed control areas. Pa rtial derivative C-13 of understorey air was elevated up to 40 m from the forest edge (-9.3 parts per thousand cf. -10.5 parts per thousand in control areas). This difference accounted for 87% of th elevation o f foliar partial derivative C-13 in D. flagellaris near the edge. 4. G aps occupied almost 90% of the transect area within 60 m of the edge a s compared to c. 14% in the control areas. Foliar partial derivative C -13 of D. flagellaris growing in gaps was significantly less negative than in undisturbed understorey, old gaps and gap edges, which had sim ilar values. 5. The frequency of gaps near the edge is probably respon sible for the extent of the elevation in foliar partial derivative C-1 3. While the exact nature of edge effects in tropical forest remains c omplex, it is clear that at least the outer 60 m of a forest patch mus t be considered 'edge-affected'.