On the Ecological and Evolutionary Significance of Branch and Leaf Morphology in Aquatic Sphagnum (Sphagnaceae)

Abstract

In aquatic environments difiusivity is low and CO2 availability can limit plant growth. The hypothesis that natural selection has favored morphological features that reduce resistance to diffusion of CO2 was tested using three phylogenetically independent species pairs from the genus Sphagnum (S. macrophyllum and 5. strictum; S. portoricense and S. papillosum; and S. trinitense and S. recurvum). The aquatic (former) and the nonaquatic (latter) species were grown submerged and emerged in a common garden and used for studies of form and function. Aquatic taxa all had similar morphological features that included larger, thinner branch leaves arranged at lower densities and photosynthetic cells more greatly exposed at the leaf surface. The relationship between observed branch and leaf morphology and boundary layer resistance in the S. trinitense–S. recurvum species pair was assessed by measuring diffusion and convection of ions onto nickel-plated models in a variable-speed electrochemical fluid tunnel. For all flow speeds and orientations, the aquatic S. trinitense model had thinner boundary layers than the nonaquatic S. recurvum model. Analysis of stable isotopes of carbon from the growth experiment corroborated results from the fluid-tunnel experiments. The aquatic taxa all had lower δ 13C values when grown submerged compared to their nonaquatic pair with the exception of the nonaquatic S. strictum, which was removed due to low growth rates. These results indicate that aquatic species did experience lower overall resistance to CO2 uptake than nonaquatic taxa. Our observations suggest that aquatic habitats do select for morphological features that lower resistance to gas exchange.