Fragmented forest landscapes with large proportions of edge area are common in the tropics, though little is known about functional responses of trees to edge effects. Foliar δ13C can increase our understanding of tree function, as these values reflect changes in ci/ca as trees respond to environmental gradients. We expected that foliar δ13C would be enriched, indicating a decline in ci/ca, in Pentaclethra macroloba trees growing at forest edges in north-eastern Costa Rica. We also anticipated this isotopic shift in δ13C values of soil carbon and soil respired CO2. Three transects perpendicular to forest edges were established at three study sites, and six plots per transect were located 0–300 m from edges. Within plots, foliage, soil and soil respired CO2 were collected for isotopic analyses. Foliar δ13C, thus ci/ca, and soil carbon δ13C did not vary along the edge to interior gradient. δ13C for canopy and understorey foliage averaged −29.7‰ and −32.5‰, respectively, while soil carbon δ13C averaged −28.0‰. Soil respired CO2 δ13C ranged from −29.2‰ to −28.6‰ and was significantly depleted within 50 m of edges. The predominant lack of functional responses at forest edges indicates that P. macroloba trees are robust and these forests are minimally influenced by edge effects.