Discussion
Our study showed phylogenetic isolation (PI) of a focal oak might significantly affect and mostly reduce the decomposition of its own litter. Phylogenetic isolation operates via both aboveground (PIA) and belowground (PIB): aboveground effects often being replaceable by low phytophagy, and belowground effects by microclimate-drivers such as tree vegetation period, budburst or decreasing soil moisture. These patterns are consistent with predictions from different mechanisms presented in Table 1. Accounting for multiple trajectories by includingdecomposer*PI interaction terms was important and permitted to identify significant direct or indirect effects of PI on decomposition in 7 out of 9 analyses on 5 out of the 6 dependent variables, compared to 3 out of 6 models that did not account for multiple trajectories. The biotic trajectories T1 and T2 might be important aboveground, representing 3 out of 6 observed relationships of decomposition to PIA, mainly involving fungal diversities and later Collembola abundance. The biotic trajectories T1 and T2 are even dominant belowground, representing 5 out of 6 relationships of decomposition to PIB, involving first Collembola and Acari abundance, but later microbial biomass. Moreover, we find a decrease in 13C, first with PIA, then with PIB, suggesting that PI reduces the role of biotic trajectories of decomposition besides decreasing overall rates of decomposition.
We experimentally separated effects of above- and belowground PI, but did not manipulate PI as such, nor the presence of particular decomposer species or particular abiotic conditions. Manipulating PI would be impossible given the lifespan of trees. Replication and blocking - PI and non-PI trees at less than 150 m - limit the problem of correlational evidence, but do not entirely resolve it. Also, manipulating particular decomposer species is difficult, but would be needed to assess whether a significantly positive PI * decomposer interaction reflects rather a shift towards more efficient decomposer species, a shift within each decomposer species towards increased decomposition performance, or a support of decomposers by nutrients from dung and urine of the herbivores above (Cherif & Loreau 2013). Finally, manipulating abiotic conditions might be more doable and would represent a direct prove for abiotically-mediated effects of PI, i.e. our trajectory T3. Overall, we profit from a quasi-experiment of PI established by foresters, and independently manipulate above- and belowground. However, we only use correlational approaches to explain observed effects of PI by different mechanistic pathways. Hence, conclusion about mechanisms remain interpretations of patterns, causal terminology refers to statistical relationships rather than direct proof of mechanisms.
For simplicity we will from now focus on explaining the relationships of PIA and PIB alone on decomposition in analyses only accounting for combined effects of PIA and PIB (Fig. 2). To do so, we will use trajectory analyses (Fig. 1) and the replacement of variables, verifying the predictions from Table 1.