Relationships between wood quality, termite activity and
decomposition over time
Multiple traits, notably wood N, P, cellulose and lignin content and
wood density, together explained 55.9% of all variance explained by
PC1, interpreted here as the WES (see Methods and Fig. S1). Wood litter
of more resource acquisitive strategy species (i.e., high WES values),
generally decompoesed faster than that of conservative strategy species;
in the treatment without termite access (Fig. S1, Fig. 2) there were
significant positive linear relationships between WES score and mass
loss in the periods 0-6 and 12-18 months (but non-significant ones at
6-12 months), and cumulatively over 18 months, in both sites.
Our termite access treatment revealed that termites not only
significantly accelerated wood mass loss rate overall, but there was a
clear time pattern within this acceleration (Fig. 3), as hypothesized
(Fig. 1). In the first year (periods 0-6, 6-12 months) decomposition
rates scaled positively and linearly with WES in both sites (Fig.
2a,b,c,d), indicating that termites preferentially consumed the species
of resource acquisitive strategy (high nutrient content, less lignin,
less dense structure). Thereby, the termites’ consumption amplified the
initial WES effect on decomposability, i.e., it increased its positive
linear regression slope.
In contrast, partly owing to termite activity, the wood of the
acquisitive species had been considerably depleted after 12 months. In
the subsequent period of 12-18 months, the initially medium quality
species were consumed more by the termites, which modulated the tree
species’ decomposition trajectory on the WES. Now, in contrast to the
positive linear relation in the treatment without termite access, there
was a hump-back relationship between WES and period mass loss in the
treatment with termite access, at least in the more termite-rich PT site
(Fig. 2e, Table 1). Termite abundance showed a corresponding humpback
pattern with mass loss (equation in Fig. 2e). In the less termite-rich
site, TT, such a humpback relation was not apparent, but here the
positive relationship between WES and mass loss was less steep in the
latter (Fig. 2d,f) than in the initial period (Fig. 2b). This pattern
also suggests negative termite feedback on the positive relation between
WES and mass loss. These changing patterns of WES over period-specific
mass loss were confirmed by a significant interaction among WES
(covariate), harvest time and termite presence/absence on period mass
loss in PT and TT, respectively (Table 2). Together, as hypothesized
(Fig. 1), these deviating relationships over time caused overall
convergence of cumulative mass loss along the WES between the termite
treatments in both sites (Fig. 2g,h), as indicated by a lack of
interaction of WES (covariate) x termite access treatment on cumulative
mass loss over 18 months (Table 2).
Termite abundance patterns in the wood samples were consistent with the
above changing patterns of period-specific wood mass loss (Fig. 3) and
the termite contribution to decomposition (Fig. S3) along the WES
through time. At 6 months, there was a significant, exponential increase
from the conservative end to the acquisitive end of the WES in both
sites (Fig. 3a,b). At 12 months, the termite abundance peak occurred at
slightly higher than medium initial wood quality (Fig. 3c,d). By 18
months in PT, the termite abundance peak had moved further towards the
conservative end of the WES (Fig. 3e), i.e., to the centre of the range,
matching the humpback for mass loss (Fig. 2e). In TT both the height and
the width of the peak (i.e., the range) increased at 12-18 months (Fig.
3f). All in all, the peaks of termite abundance and termite contribution
to mass loss shifted from high to medium initial wood quality during the
decomposition process, broadly matching that of mass loss itself.