Leaf litter decomposition
To measure leaf litter decomposition rates, we enclosed 4 ± 0.05 g and 2
± 0.05 g of dried Alder (Alnus glutinosa) leaves in 1 cm mesh
(coarse-mesh) and 250 μm mesh (fine-mesh) bags, respectively. Where
coarse-mesh bags allowed invertebrate to enter and consume leaves,
fine-mesh bags only permitted microorganism-driven (i.e. by fungi and
bacteria) decomposition (e.g. Datry et al. 2011, Foulquier et al. 2015).
Alder leaves were collected in autumn 2020 from a single site, near
Chambery, France (approx. 80 km from the Albarine), air-dried inside the
laboratory and then oven-dried for 48h at 70°C prior to being weighed
and enclosed into the bags. Six coarse- and six fine- mesh bags were
incubated for 29 \(\pm\) 4 days (mean\(\ \pm\) SD) in the riparian (n=3
of each mesh-type) and instream (n=3) habitats at each site and
campaign.
At the end of the incubation period, bags were taken to the laboratory
where remaining leaves were cleaned from sediments and other organic
matter (e.g. invertebrates, twigs, other leaves). Remaining leaves were
then oven-dried and ashed as described in the “Leaf litter
quantity” section. Decomposition rates were calculated for each litter
bags based on the negative exponential decay model (K) corrected for the
duration of incubation and cumulative temperatures (i.e. degree days;
Swan et al. 2021). The decomposition rates were then averaged across the
three bags exposed in each habitat (instream vs. riparian) at each site
and in each season. Of the total 720 mesh bags incubated in the field
nine and four fine-mesh and six and two coarse-mesh bags were lost in
the instream and riparian habitats, respectively, due to floods mainly.