Environmental drivers of Sphagnum growth in peatlands across the Holarctic region

The relative importance of global versus local environmental factors for
growth and thus carbon uptake of the bryophyte genus Sphagnum – the main
peat-former and ecosystem engineer in northern peatlands – remains
unclear. 2) We measured length growth and net primary production (NPP) of
two abundant Sphagnum species across 99 Holarctic peatlands. We tested the
importance of previously proposed abiotic and biotic drivers for peatland
carbon uptake (climate, N deposition, water table depth, and vascular
plant cover) on these two responses. Employing structural equation models,
we explored both indirect and direct effects of drivers on Sphagnum
growth. 3) Variation in growth was large, but similar within and between
peatlands. Length growth showed a stronger response to predictors than
NPP. Moreover, the smaller and denser Sphagnum fuscum growing on hummocks
had weaker responses to climatic variation than the larger and looser S.
magellanicum growing in the wetter conditions. Growth decreased with
increasing vascular plant cover within a site. Between sites,
precipitation and temperature increased growth for S. magellanicum. The
structural equation models indicated that indirect effects are important.
For example, vascular plant cover increased with a deeper water table,
increased nitrogen deposition, precipitation and temperature. These
factors also influenced Sphagnum growth indirectly by affecting moss shoot
density. 4) Synthesis Our results imply that in a warmer climate, S.
magellanicum will increase length growth as long as precipitation is not
reduced, while S. fuscum is more resistant to decreased precipitation, but
also less able to take advantage of increased precipitation and
temperature. Such species-specific sensitivity to climate may affect
competitive outcomes in a changing environment, and potentially the future
carbon sink function of peatlands.