Natural selection on a carbon cycling trait drives ecosystem engineering by Sphagnum peat mosses

Bryan T. Piatkowski and A. Jonathan Shaw, Biology, Duke University, Durham, NC, Joseph B. Yavitt, Natural Resources, Cornell University, Ithaca, NY, Merritt R. Turetsky, Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO

Background/Question/Methods
Sphagnum peat mosses have extraordinary impact on the global carbon cycle as they control long-term carbon sequestration in boreal peatland ecosystems. Sphagnum species engineer these peatlands, which harbor roughly a quarter of all terrestrial carbon, through peat accumulation by constructing their own niche that allows them to outcompete other plants. Variation in peat production, largely resulting from differences in tissue decomposability, is hypothesized to drive niche differentiation along microhabitat gradients thereby alleviating competitive pressure among co-occurring species. However, little empirical evidence exists for the role of selection in the creation and maintenance of such gradients. In order to document how niche construction and differentiation evolved in Sphagnum, we quantified decomposability for 54 species under natural conditions and used phylogenetic comparative methods to model the evolution of this carbon cycling trait.
Results/Conclusions
We show that tissue decomposability tracks the phylogenetic diversification of peat mosses, that natural selection favors different levels of decomposability corresponding to optimum niche, and that divergence in this ecologically important trait occurred early in the evolution of the genus prior to the divergence of most extant species. Our results demonstrate evolution of ecosystem engineering via natural selection on an extended phenotype, of a fundamental ecosystem process, and one of the Earth’s largest soil carbon pools.