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.