Site, experimental design, methods, and data
The experiment was carried out at Ribeira do Botão, a 3rd order stream located near Coimbra, Central Portugal (40°18′23″N, 08°23′55″W; elevation 80 m) during 56 days from November to December 2011. The riparian vegetation was composed of mixed deciduous trees, mainly common alder. During the exposure period (data collected at a nearby meteorological station located at 40°12′33″N; 08°27′08″W; elevation 16 m), daily air temperature ranged 4.8–19.9°C, humidity ranged 54%–96%, and precipitation totalled 241 mm. In the stream, water was well-oxygenated (11.9-13.0 mg L-1), temperature ranged 5.9–13.4⁰C, pH was circumneutral (6.6-7.3), nitrate ranged 0.45-2.55 mg NO3--N L-1, ammonia ranged 21.5-94.8 µg NH4+-N L-1, phosphate ranged 79.7-109.4 µg PO43--P L-1, and current velocity ranged 0.239-0.458 m s-1.
Other characteristics of the study site and the litter decomposition study are described in detail in Abelho and Descals (2019). Briefly, decomposition of senescent leaf litter (2.80 g ± 0.52 SD ) of the N-fixing alder (Alnus glutinosa (L.) Gaertn.), hybrid black poplar (Populus × canadensis Moench) and a 1:1 mixture of both species was assessed on litter bags (5 mm mesh) exposed to both the terrestrial and the aquatic environment according to three exposure scenarios (Figure 1): 0:100 (0 days terrestrial followed by 56 days aquatic), 25:75 (14 days terrestrial followed by 42 days aquatic), and 50:50 (28 days terrestrial followed by 28 days aquatic). The riparian leaf bags were deployed on the floor next to the stream channel (approximately 1.5 m above the stream surface and 1 m away from the stream margin), and loosely covered with dead fallen leaves. The stream bags were tied to iron nails fixed at the bottom in a riffle area. Three additional similar portions of each litter type were used to determine the initial oven‐dry mass (60°C, 3 days) of the leaves used in the experiment. Sampling in stream (0:100) was carried out after 7, 14, 28 and 56 days of immersion. Sampling in the terrestrial environment occurred after 14 (25:75) and 28 days (50:50) of exposure; three replicates of each litter type were used to determine litter dry mass remaining and litter colonisation in the terrestrial environment, and the other groups were transferred to the riffle area of the stream and sampled after 7, 14, 28 and 42 days (25:75) or after 7, 14 and 28 days of immersion (50:50).
On each sampling occasion, mass loss of leaf litter (% dry mass), ergosterol as an estimate of fungal biomass (mg gAFDM–1), aquatic hyphomycete sporulation rates (number of conidia mgAFDM−1 day−1) and taxa richness (number of species sample–1), shredder biomass (mg bag–1), abundance (number of individuals bag–1), and taxa richness (number of taxa bag–1) were determined on three replicates as described in Abelho and Descals (2019). In the mixture, mass loss and the fungal variables were determined for each species; to obtain the value in the mixture, mass loss was summed, and the fungal variables were averaged.
The two leaf species differed significantly in their initial chemical characteristics, decomposition rates and colonisation patterns by decomposers and detritivores (Table 1), as determined by Abelho & Descals (2019).