Photosynthesis, growth, and decay traits in Sphagnum – a multispecies comparison

Abstract

Peat mosses (Sphagnum) largely govern carbon sequestration in Northern Hemisphere peatlands. We investigated functional traits related to growth and decomposition in Sphagnum species. We tested the importance of environment and phylogeny in driving species traits and investigated trade‐offs among them. We selected 15 globally important Sphagnum species, representing four sections (subgenera) and a range of peatland habitats. We measured rates of photosynthesis and decomposition in standard laboratory conditions as measures of innate growth and decay potential, and related this to realized growth, production, and decomposition in their natural habitats. In general, we found support for a trade‐off between measures of growth and decomposition. However, the relationships are not strong, with r ranging between 0.24 and 0.45 for different measures of growth versus decomposition. Using photosynthetic rate to predict decomposition in standard conditions yielded R 2 = 0.20. Habitat and section (phylogeny) affected the traits and the trade‐offs. In a wet year, species from sections Cuspidata and Sphagnum had the highest production, but in a dry year, differences among species, sections, and habitats evened out. Cuspidata species in general produced easily decomposable litter, but their decay in the field was hampered, probably due to near‐surface anoxia in their wet habitats. In a principal components analysis, PCA, photosynthetic capacity, production, and laboratory decomposition acted in the same direction. The species were imperfectly clustered according to vegetation type and phylogeny, so that some species clustered with others in the same section, whereas others clustered more clearly with others from similar vegetation types. Our study includes a wider range of species and habitats than previous trait analyses in Sphagnum and shows that while the previously described growth–decay trade‐off exists, it is far from perfect. We therefore suggest that our species‐specific trait measures offer opportunities for improvements of peatland ecosystem models. Innate qualities measured in laboratory conditions translate differently to field responses. Most dramatically, fast‐growing species could only realize their potential in a wet year. The same species decompose fast in laboratory, but their decomposition was more retarded in the field than that of other species. These relationships are crucial for understanding the long‐term dynamics of peatland communities.

Biochemical determinants of litter quality in 15 species of Sphagnum

“…This does not contradict the trade-off in resource partitioning between metabolic and structural carbohydrates (Turetsky et al 2008), which describes the partitioning between dead hyaline cells (cell walls) composed of decay-resistant polymers and living chlorophyllous cells that constitute an easily decomposable carbohydrate storage. Linked to this, our results support the suggested trade-off between growth and decay (Turetsky et al 2008;Hájek 2009;Laing et al 2014;Bengtsson et al 2016a); fast growing species decompose fast, while slow growing species invest in metabolites which retard their decomposition.…”

“…The study was conducted at two localities: Kulflyten and Glon, which were also the study sites of our earlier study about Sphagnum species' growth and decomposition traits (Bengtsson et al 2016a). The main site Kulflyten (59°54'N, 15°50'E) is a mire complex in central-southern Sweden (Västmanland province).…”

“…Anoxia is particularly linked to the lower peat layers (the permanently water-logged catotelm), while the acrotelm is intermittently oxic (Morris et al 2011;Rydin and Jeglum 2013). Decay resistance of dead plants differ for different growth forms (Dorrepaal 2005), but also between different species of Sphagnum (Turetsky et al 2008;Hájek 2009;Bengtsson et al 2016a). Consequently, knowledge about the litter quality of ecosystem engineers, such as Sphagnum spp., is crucial to the understanding of ecosystem functions in northern peatlands.…”

“…Such a trade-off could be linked to the niche differentiation in Sphagnum species along the hummock-hollow gradient (i.e., gradient of acrotelm thickness). A hollow has a thin acrotelm with nearsurface anoxia, consequently, decay is to a large extent ruled by habitat, whereas a hummock has a thick acrotelm where the aerobic decay is more strongly determined by litter quality (Bengtsson et al 2016a). Shoots of most species grow better in wet habitats, but only those able to avoid desiccation can survive high above the water table (summarized in Rydin et al 2006).…”

“…The desiccation avoidance of hummock sphagna (mediated by large, empty hyaline cells) and the structural persistence of Sphagnum-built hummocks are thus interlinked via resource allocation to structural, rather than photosynthesizing tissues (Rice et al 2008;Hájek 2014). Thus, the compounds providing either structural integrity or other protection against decomposers could plausibly be trade-offs for competitive growth (Turetsky et al 2008;Hájek 2009;Laing et al 2014;Bengtsson et al 2016a); in contrast to the fast-growing hollow species, the hummock species often have greater decay resistance (Clymo 1965;Johnson and Damman 1991;Belyea 1996;Limpens and Berendse 2003). Consequently, we expect higher amounts of compounds that slow down decomposition in hummock species.…”