The carbon isotope ratio (Ξ΄^13^C) of vascular plant leaf tissue is determined by isotope discrimination, primarily mediated by stomatal and mesophyll diffusion resistances and by photosynthetic rate. These effects lead to predictable trends in leaf Ξ΄^13^C across natural gradients of elevation, irradiance and nutrient supply. Less is known about shifts in Ξ΄^13^C for bryophytes at landscape scale, as bryophytes lack stomata in the dominant gametophyte phase, and thus lack active control over CO~2~ diffusion. Twelve bryophyte species were sampled across a matrix of elevation and soil ages on Mauna Loa, Hawaii Island. We tested hypotheses based on previous findings for vascular plants, which tend to have less negative Ξ΄^13^C at higher elevations or irradiances, and for leaves with higher leaf mass per area (LMA). Across the matrix, bryophytes spanned the range of Ξ΄^13^C values typical of C~3~ vascular plants. Bryophytes were remarkably similar to vascular plants in exhibiting less negative Ξ΄^13^C with increasing elevation, and with lower overstory cover; additionally Ξ΄^13^C was related to bryophyte canopy projected mass per area, a trait analogous to LMA in vascular plants, also correlated negatively with overstory cover. The similarity of responses of Ξ΄^13^C in bryophytes and vascular plants to environmental factors, despite differing morphologies and diffusion pathways, points to a strong direct role of photosynthetic rate in determining Ξ΄^13^C variation at the landscape scale.
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