Notes S1 – extended methods description
Model differences with Limpens et al. 2011
We used the models for the response of production (PRODlnrr) and Sphagnum nitrogen
concentration (Nlnrr) outlined in Limpens et al. (2011) as a basis for our main model b and c
(Tables 2,3), adding interactions between the explanatory variables and experimental type to
investigate if glasshouse experiments behaved differently from field experiments. Not all
variables included in Limpens et al. (2011) for field experiments could be used in the
comparison with glasshouse experiments, however.
Precipitation was excluded because we lacked enough corresponding glasshouse data for
reliable analysis.
Temperature - the interaction between July temperature and Microhabitat (Limpens et al.,
2011) could not be tested reliably because the temperature ranges of glasshouse and field
experiments for species characteristic of dry microhabitats (hummocks) were not comparable.
Consequently, we included the Temperature × Experiment type interaction in the model
instead of the term Temperature × Microhabitat × Experiment type.
Microhabitat/position above the water table - Limpens et al., 2011 used microhabitat
(hummock-dry vs. lawn-wet) as a proxy for water table depth. As microhabitats may differ in
water table depth as well as dominant species, disentangling the effects of species and water
table depth in the same model was impossible for field experiments. For glasshouse
experiments the species and water table effect could be separated, but made a straight
forward comparison between field and glasshouse experiments difficult. Consequently we ran
two different models. We tested the interaction between experiment type and microhabitat,
keeping in mind that for field experiments microhabitat encompasses differences in water
table and species, while for glasshouse experiments microhabitat only reflects different
species (Tables 2, 3; model b). The interactions between water table depth (cm), species and
response to N-application were tested for a data subset containing glasshouse experiments
only (Table S2).
Explanatory variables
Experiment type: glasshouse or field experiment. Field experiments were defined as all
experiments where (i) Sphagnum was exposed to diurnal and seasonal changes in solar
irradiance and temperature, and (ii) where the control was subject to the same temperature
regime as the fertilization treatments. This choice entails that experiments where Sphagnum
was kept outside under a roof, or experiments using open top chambers, were classified as
field experiments.
Presence of vascular plants: presence vs. removed by the experimenter by clipping
aboveground parts. Removal of vascular plants was mainly restricted to short-time field
experiments and glasshouse experiments. For field fertilization studies, the effect of presence
of vascular plants was confounded by no. of seasons. After analyses on separate datasets
(Limpens et al., 2011), we used presence of vascular plants in our models testing the
Sphagnum production response and no. of seasons in the models testing the response of
Sphagnum N concentration. When tested for glasshouse experiments, no. of seasons did not
affect the production response (model coefficient estimate: -0.05, P = 0.68 but did affect the
response of Sphagnum N concentration similar to field experiments, supporting the model
choices.
No. of seasons: the number of growing seasons over which N was applied. Growing season
was defined as 6 months in the summer half-year for field experiments. For glasshouse
experiments 6 months were also counted as one growing season.
Background N deposition: the wet N deposition rate (g N m-2 yr-1) at the experimental site.
For those experiments where vegetation was moved to another site (i.e. all glasshouse
experiments), we used wet deposition rate at the collection site. If not provided by the
experimenters, wet N deposition rate was extracted from the EMEP website
http://webdab.emep.int/Unified_Model_Results/) for the year Sphagnum N concentration
data were collected. We selected wet deposition rather than total deposition because of its
smaller estimation error (Boring et al., 1988). The EMEP model has been shown to be very
accurate for Central and Northern Europe. Validations of EMEP models have given
correlation coefficients around ~0.7 for wet and dry deposition (e.g. EMEP Status Report
2007; August 8, 2007, Norwegian Meteorological Institute).
N application rate: amounts of N (g N m -2 yr-1) applied by the experimenters.
P-application: P (PO43-) applied vs. no P applied.
Temperature: mean July temperature at the study site in oC for field studies. For glasshouse
studies we used the glasshouse temperature averaged over the experimental period.
Microhabitat/ position above water table: species characteristic of hummock (dry) vs. lawn
(wet) microhabitats for glasshouse experiments and dry vs. wet microhabitats in field
experiments. For more information see previous caption.
Scale: the experimental unit, shoots, pot, plot or field used in the experiments. These four
groups represent different scales of the experiments; shoot being the smallest and field at the
biggest scale. We used this as a crude measurement in a separate model to investigate if there
is linear trend with scale.
Sphagnum species: the dominant Sphagnum species within the experimental units. Sample
size differed widely among species and we could therefore not include species as an
additional variable in the main models. Instead, we ran models for those species for which we
had a substantial amount of data covering a broad range of our explanatory variables for both
glasshouse and field experiments. We performed this analysis for S. fallax, S. magellanicum
and S. fuscum.
A potential problem in meta-analyses that can lead to biased results is covariates associated
with the experimental design. Covariates relevant to our study included the N concentration
(g l-1) of the fertilizer solution applied to the experimental unit (N dose concentration), the
form (NH4+, NO3-, NH4NO3) in which N was applied (N form) and the frequency (low,
medium, high) in which N fertilization was applied (N frequency). N frequency did not
affect the responses of Sphagnum in field experiments (Limpens et al., 2011) and could not
be analyzed reliably for glasshouse studies as most studies used high application frequencies
only (Table 1). Both N form and N dose concentration showed very small effects and were
strongly driven by a few data points. When included in models b and c (Table 2,3) they did
not affect the main results (Table S2).
Exploring N artefacts
We compared the relationships between non-standardised Sphagnum N concentration in the
upper 0-3 cm of the shoot and N influx between glasshouse and field experiments. We used
the same model as Bragazza et al. (2005) and Limpens et al. (2011) as a basis: N
concentration = intercept + β × loge(N influx). For field experiments N influx was defined as
the sum of background N deposition and N application rate; for glasshouse experiments N
influx equalled N application rate. The model was expanded by adding Experiment type and
an Experiment type × N influx interaction. The analysis was performed by fitting a
generalized least square regression (GLS) using mean N concentration in the treatments as
the response variable. A GLS model was applied to account for the within-study correlation
with a compound symmetry correlation structure, using the R package nlme (Pinheiro et al.,
2009). Weights were used to achieve homogeneity of the residuals applying an exponential
variance function structure (Zuur et al., 2009).
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