Net primary production (NPP) of Sphagnum fuscum
is the product of linear growth of stems (cm
yr-1) and stem mass density (SMD), with SMD
defined as the mass of 1-cm lengths of Sphagnum
stems beneath the capitula per m2. We
measured linear growth using the cranked wire method (Clymo 1970). We
set 30 cranked wires in each of the 21 study plots, with all wires
placed in hummocks dominated by S. fuscum. Each
year, from 2011 through 2015, we set cranked wires in May after the
surface peat had thawed, and remeasured them at the end of the growing
season (in late September or early October).
To measure SMD, we collected surface cores (6.5 cm diam.) from
S. fuscum hummocks in each plot between July 7th
and 10th of each year. From each core, we removed and counted all
capitula to determine S. fuscum plant density
(individual plants m-2). We separated 70
stems of S. fuscum, cut them into 2-cm lengths,
and weighed them after drying at 55 C for 5 days. These weights
divided by 2 and again by 70 represent the average mass of a 1-cm stem
of an individual Sphagnum plant; this value was
multiplied by plant density to yield SMD.
From cores collected in early July and early October from 2013 through
2015, we calculated capitulum mass density (CMD) (g
m-2) as the dry mass of all capitula from a
core, scaled up to a m2 basis. Capitula and
stems were dried, homogenized in a Thomas Wiley Mini-Mill, and
analyzed for N concentration on a Flash EA 1112 Series CN Soil
Analyzer.
We calculated the amount of N annually utilized by S.
fuscum to satisfy NPP requirements (g N
m-2 yr-1) as the
product of S. fuscum NPP and the N concentration
in the 0-2 cm stem sections. For both summer and fall collections, we
calculated the quantity of N stored in S. fuscum
capitula (g N m-2) as the product of CMD
and N concentration in the capitula.