Relationships of decomposition to phylogenetic isolation aboveground, PIA
First, PIA shows no effect on mass loss after 8 months, and a negative effect after 14 months (in analyses only accounting for PIA and PIB, Fig. 2). Analysis of trajectories fails to reproduce this negative effect and yields among others a negative effect of PIA on the role of fungal diversity on mass loss after 8 months. Possibly, the negative effect of PIA on 13C after 8 months suggests shortage of easily decomposable compounds (Benner et al . 1987), reducing the mass loss at 14 months (consistent with hypothesis PIA5). Second, PIA shows a positive effect on C loss after 8 months (in analyses only accounting for PIA and PIB, Fig. 2). This effect is explicable by a direct, abiotic effect of PIA (trajectory T3), which in turn, is replaceable by low herbivory (hypothesis PIA2): less attacked leaves might have kept more labile and soluble carbon rendering these leaves abiotically more degradable. In contrast, PIA shows a negative effect on C loss after 14 months. Analyses of trajectories suggests that this relationship is mediated by a decline in the relationship between Collembola and C loss, the latter being replaceable and possibly explicable by reduced phytophagy. These results are consistent with hypothesis PIA3: low phytophagy damage might reduce the access to leaves for decomposers (Ritchie et al . 1998). Finally, PIA shows no overall effect on N loss, despite a statistical marginally significant, positive effect on the role of fungal abundance for N loss after 8 months. This interaction effect is replaceable by high litter phenolics consistent with hypothesis PIA5, i.e. an increased competition for resources and/or allelopathic interactions between oaks and distantly related gymnosperms increasing phenolics and decreasing litter quality for decomposers (Fernandez et al . 2016).
Overall, litter that is produced in a neighbourhood of closely related lineages becomes more decomposable in the long run. This can be considered as a form of a phylogenetic home-field advantage, albeit one that is non-standard for two reasons. First this positive effect of neighbourhood results from aboveground processes, which so far have not been accounted for in the home-field research on decomposition (Ayreset al . 2009; Freschet et al . 2012; Veen et al . 2015). Second, home-field advantage among plants as proposed in the paleontological literature (DiMichele & Bateman 1996), has been explained by competitive superiority “at home”. In the present case, however, evidence suggests that home-field advantage can be explained by the increased support from decomposers (see also Bardgett & van der Putten 2014).