The effect of the mixture differs among exposure scenarios
Under the fully aquatic exposure, the mixture had a synergistic effect
on mass loss and decomposers (sporulation rates and fungal richness),
while the effect on detritivores was additive. The synergistic effect on
mass loss of the mixture was due to a species-specific effect on mass
loss of alder in the presence of poplar (16% more, on average). Fungi
(abundance, activity and/or diversity) may demonstrate evidence for
interactive effects of mixtures in streams (López-Rojo et al., 2020;
Santonja et al., 2020; but see Sanpera-Calbet et al. (2009) and Abril et
al. (2021) for contrasting results). Fungi colonizing the mixture were
more diverse, probably due to a better substrate for conidia attachment
and germination (Dang et al., 2007; Kearns & Bärlocher, 2008) and
invested more in reproduction than in biomass when compared to the
average of the single species. Aquatic hyphomycetes invest on average
50% of their biomass in sporulation, and the conidia produced may
correspond to 10% of the daily litter mass loss (Gessner, 1997). Thus,
the increased sporulation rates of the mixture and possibly also
positive complementarity among fungal species may explain the increased
mass loss (López-Rojo et al., 2020). Additionally, shredders prefer
high-quality in detriment of poor-quality leaves (Swan & Palmer, 2006)
and may strongly increase the decomposition rates of alder (Bruder et
al., 2011). Thus, their feeding activity probably also contributed to
the increased mass loss of alder in the presence of poplar and to the
synergistic effect of the mixture.
When the period of terrestrial exposure was shorter than the period of
aquatic exposure, the mixture had a synergistic effect on detritivores,
stronger for shredder biomass and abundance, but not on mass loss or
decomposers, although fungal richness was also higher-than-expected. On
average, the mixture had 63% more fungal species than the average of
the monocultures, but the increased richness did not translate into
higher mass loss, possibly due to functional redundancy among aquatic
hyphomycete species (Bärlocher, 2005; López-Rojo et al., 2020). On the
other hand, invertebrate detritivores have been shown to play a
fundamental role on the synergistic effects of mixtures on mass loss in
terrestrial and aquatic habitats (Hättenschwiler & Gasser, 2005;
Schädler & Brandl, 2005, Vos et al., 2013; Jabiol & Chauvet, 2015, Liu
et al., 2020; Santonja et al., 2020). But increased colonisation may
also be a consequence of the mixture effect, as shredders may benefit
from the structured habitat created by recalcitrant species which can
also provide shelter against predators or stream flow (Hättenschwiler et
al., 2005; Sanpera-Calbet et al., 2009; Jabiol et al., 2014). Although
the mixture had on average 29% more biomass, 16% more abundance and
10% more shredder taxa than the average of the monocultures, mass loss
of the mixture was not higher-than-expected. Functional redundancy among
shredder taxa may be due to dominance by food generalists
(Hättenschwiler et al., 2005). Furthermore, the more complex habitat may
have also attracted more predators and shredders might have reduced
consumption rates as a behavioural response to their presence (Jabiol et
al., 2014). However, it is more plausible that there was a similar
consumption of both species during decomposition. When given a choice,
shredders consume preferentially the highest quality leaves available
(Swan & Palmer, 2006). Given that the availability of each species in
the mixture changed during decomposition due to the faster breakdown
rates of alder, and that N-concentrations of the two species converged
during decomposition (Abelho & Descals, 2019), shredders probably
started by consuming alder, but when mass remaining of alder was already
low and the quality of poplar had increased, shifted consumption from
alder to poplar.
When the period of terrestrial and aquatic exposures was equal, the
process of decomposition of the mixture was enhanced but there was a
´dilution’ of the effects on each compartment: the effect of the mixture
was additive on mass loss, decomposers, and detritivores. However, the
partitioning of the contributions of each species within the mixture
showed a synergistic effect of the presence of alder on mass loss of
poplar. While exposed in the terrestrial habitat, the water buffering
effect of the mixture might have increased moisture persistence in the
litter layer (Wardle et al., 2003) thus favoring fungal colonisation. In
fact, although the effect was nonsignificant (but barely), fungal
biomass of poplar was elevated in the presence of alder. The conditions
in terrestrial habitats are more likely to favor N-transfer than in
streams (Gessner et al., 2010) and it is possible that there was a
transference of nitrogen, either passively or actively by fungal hyphae,
from alder to poplar (Schimel & Hättenschwiler, 2007; Lummer et al.
2012). But the effect could also have occurred during stream immersion,
given that the increase in decomposition rates of nutrient-poor species
by the presence of N-fixing species has also been shown in aquatic
ecosystems (Migliorini & Romero, 2020). However, this species-specific
effect was small and did not translate in a synergistic effect on mass
loss of the mixture.