Testing model assumptions in the field: peatland carbon cycling, vegetation, and water table depth

English Abstract:

Peatland biogeochemical models represent vegetation as plant functional types (PFTs), and carbon fluxes are calculated for each PFT as a unimodal function of water table depth. However, field data has rarely supported such unimodal relationships. Moreover, models assume single PFTs but field studies typically measure vegetation that contains a mixture of PFTs. It is uncertain whether it’s reasonable to characterize a vegetation group by its dominant PFT. In order to test these assumptions, we measured species abundance and CO₂ exchange in 27 sites from May – September in 2012 and 2013 along a gradient from ombrotrophic bog to beaver pond. Plant species and PFTs fell into six vegetation groups: hummock, hollow, bog moss, bog margin, pond moss and pond margin. For each vegetation group we found unimodal relationships between weekly water table depth and gross ecosystem photosynthesis (GEP). The water table optima and range that we found for each group generally corresponded to values previously reported for that group’s dominant PFT, implying that peatland vegetation groups can usually be characterized by their dominant PFT. We also found unimodal relationships between species abundance and annual water table depth. We compared the water table range from species abundance curves to the water table range of the corresponding GEP curves. For hollow, bog moss and bog margin groups, the abundance and GEP ranges were statistically indistinguishable, implying that environmental factors are the dominant controls on species abundance and carbon exchange for these vegetation groups. For hummock, pond moss and pond margin the abundance and GEP ranges were significantly different, implying that biotic factors such as physiological tradeoffs and competitive interactions are more dominant in these vegetation groups. Overall, our work helps to bridge the gap between model assumptions and field studies.