Site selection and flooding data
To select sites with variable flooding regimes at small and large
spatial scales, we used previously collected data on inundation
frequency for the Rastmyran wetland (Åhlén, Jarsjö and Hambäck 2023). In
that study, inundation frequency (the proportion of days when each 2x2
m2 pixel was under water) was estimated across the
wetland during the normally unfrozen period from the end of March to the
end of October using a combination of water table measurements in the
field and extrapolation through a digital elevation model. Water tables
were estimated with a HOBO water table logger placed at the in- and
outlets of the fen and the digital elevation model was constructed based
on data from the Swedish Mapping, Cadastral and Land Registration
Authority (for details see Åhlén, Jarsjö and Hambäck 2023). For the
purpose of this study, we selected sites that included both
homogeneously dry, homogeneously wet and mixed wet/dry conditions. As a
measure of inundation heterogeneity during the selection process, we
contrasted the inundation frequency in the central pixel (2x2
m2) and peripheral pixels (within 6x6
m2) (Fig. 3a). Please note that wet and dry periods
are somewhat separated in time, with wetter conditions during spring and
drier conditions during summer and autumn.
Dry pixels included those with inundation frequency (IF)<0.45,
which corresponds to a dry situation during most of summer, whereas wet
pixels included those with inundation frequency>0.65. The
distribution of IF-values across the dry and wet classes was then per
definition bimodal in the central pixel (wet=high IF>0.65,
dry=low IF<0.45; Table 1, column 2), but this bimodality did not
always translate to peripherical pixels outside the central pixel, which
exhibited a more heterogeneous character (Table 1, columns 3-5). Thus,
the inundation frequency<0.45 in homogeneously dry sites both in
the central pixel and for most pixels within 6x6 m2,
and correspondingly >0.65 for homogeneously wet sites. In
dry-wet sites, the central pixel had an inundation
frequency<0.45 whereas most peripheral pixels had an inundation
frequency>0.65, and correspondingly for wet-dry sites.
Finally, we included dry-wet/dry and wet-wet/dry sites, meaning that
peripheral pixels had variable inundation frequencies with some wet and
some dry. Thus, we selected 6 categories with different dry-wet
conditions and 10 sites per category (Table 1).
Post selection, to further investigate scale effects, we estimated the
mean inundation frequency in successively larger areas (10x10
m2 … 26x26 m2, Fig. 3b)
surrounding the central pixels (not including the central pixel). We
also calculated the proportion of dry pixels within each scale to
estimate the occurrence of dry refuges in the vicinity. The inundation
frequencies and proportion of dry pixels (IF<0.45) were strongly
correlated (r>0.97) and we therefore dropped the inundation
frequency from analysis. Finally, a heterogeneity index was calculated
for each site from the proportion of dry to non-dry pixels using the
Shannon Diversity Index (\(H^{{}^{\prime}}=-\sum{p_{i}*ln(p_{i})}\),pi = proportion of dry pixels), where low values
indicate lower heterogeneity and large values indicate high
heterogeneity.
Environmental variables
In each site, we collected data on vegetation height (August) and soil
organic content (June). Vegetation was measured within each central
pixel by recording the maximum vegetation height at three randomly
selected points. Soil samples were collected using a soil core sampler,
placed in bags and transported to a laboratory freezer until further
processing. The soil organic content was determined using the loss on
ignition method. Samples were dried in an oven at 105°C for 24h. After
drying, the samples were sieved using a 2mm sieve and weighed on a
microscale to obtain the initial weight. The samples were then placed in
a high-performance oven at 550°C for 4h. After this, the samples were
reweighed, and the loss of ignition was calculated using the formula:
[(initial weight-after weight)/initial weight] x100. From this, the
soil organic content was derived.