Results and Discussion
After 60 days on the forest floor, we found that just over half of the
bags of both tea types had been chewed through by macrofauna: 31
[52%] of the rooibos tea and 30 bags [50%] of the green tea.
In our model the only significant terms were the type of tea (rooibos or
green) and the interaction between the state of the bags (opened or
unopened) and the tea type (see Table 1 for model outputs). There was no
significant effect of time on the decomposition rate. This meant that
generally green tea decomposed more rapidly than the rooibos tea.
However, where the bags were opened, rooibos tea showed a greater
response than in green tea, where the open and closed bags had very
similar decay slopes (Table 1, Fig 1). This serves to illustrate that
the major problem with the TBI in the tropics is with the rooibos tea
bags, which were commonly attacked by macrofauna (probably predominantly
termites) and the contents often completely removed. In our study, more
than half of the rooibos tea bags were chewed open, and more than 10%
were emptied. A similar proportion of green tea bags were chewed open,
but none of them were emptied. It seems that macrofauna, at least in
this context, prefer rooibos tea to green tea.
It is possible to estimate the two tea bag parameters in a subset of
bags without holes (i.e. estimate from the undamaged bags, estimating
microbial and mesofaunal decomposition only); the values here were S =
0.242 and k = 0.053. This k value from Bornean rainforst
is higher than any values in the original teabag paper, including the
tropical rain forest site in Panama (S = 0.06, k = 0.0392; Keuskamp et
al. 2013 site 13, Table S1). This is, however, a measure of microbial
decay only, as the macrofauna were specifically excluded. Keuskamp et
al. 2013 could not calculate the standard deviation for their Panama
site, ‘due to overdispersion’ of data. It is possible that holded
teabags, empty of tea, may have contributed to this overdispersion.
However, it is difficult to assess projects that have used the TBI in
the exact way it was recommended, as the method specifically excludes
bags that have been chewed. Therefore they are self-selected for
microbial and mesofauna decay and explicitly exlude any macrofauna
decay.
If the aim is to quantify microbial decomposition it is therefore
possible if only undamaged bags are used. However, sufficient bags need
to be buried so that one can afford to ignore damaged ones. We suggest
that given half of the bags were damaged at least twice as many are
needed (>40 in this case for every period, giving a total
of 120). Furthermore, the incubation time (ie the time the tea bags are
left out) should be reduced to 60 days, as by 90 days many of the
teabags would be likely to be chewed by termites (note we recorded
50%).
The TBI is a valuable idea but our results show it can only give an
accurate idea of total decomposition rates in the few locations
where invertebrate decomposition is negligible and there is
overwhelmingly microbial decomposition (e.g. boreal areas). It works as
a measure of microbial decomposition alone in all locations provided the
bags are not damaged. Unfortunately, the TBI does not incorporate a
measure for including the effects of larger decomposers.
We see no easy way to incorporate the contribution of macrofauna
(particularly termites) to decomposition processes using the TBI,
without also putting tea bags into macroinvertebrate litter bags made
with mesh strong enough to withstand termite attack (such as stainless
steel mesh or nylon; (Yates & Grace 1999; Lenz et al. 2012) .
Other papers that use small and large mesh sizes to exclude or include
macrofauna may suffer the same issue (eg (Handa et al. 2014)) if
the small sized mesh material is not strong enough, then some samples
may (are likely) to be attacked by termites.
This is because of the very high stochasticity that termites, in
particular, contribute to the process. The problem becomes one of
modelling two different simultaneous processes: one an exponential decay
process and the other potentially a binomial process (either the bags
are opened or they are not). There are other more costly methods, in
both time and expertise, that produce robust total decomposition results
and assess the contribution of all decomposer organisms, such as using
leaf litter and wood decomposition assays and mesh to include/exclude
invertebrates (e.g. (Smith et al. 2009; Davies et al.2013). However, these methods are not in the spirit of the TBI (see Fig
1), which was designed to produce a quick, simple estimate of two
meaningful parameters.
In summary, the TBI can work well to assess the microbial contribution
to decomposition where no (or few) bags are opened by macrofauna, such
as in boreal locations, but the method risks seriously underestimating
total (microbial + macrofauna) decomposition in most habitats. (Wallet al. 2008) present a global map showing the areas where
macrofauna (not always termites) are important in dead plant
decomposition processes, althoughthis figure significantly downplays the
effect of macrofauna (especially termites) in savannas, grasslands and
other drier areas (Bignell & Eggleton 2000). Wall et al. 2008 used moth
balls (active ingredient naphthalene) to exclude invertebrates. As with
any insecticidal active ingredient, it has variable effects on different
species of insect…
Termites appear to manufacture naphthalene (possibly secreted by
themselves, but more likely via microbes(Chen et al. 1998)). So
the treatement would have a reduced effect on termites.
While Keuskamp et al. (2013) do specifically state that macrofauna are
excluded they make no other comment about the errors that this might
introduce. We believe that any method that ignores the contribution of
macrofauna to plant decomposition cannot be recommended as a global
method. It may still be useful for estimating purely-microbial effects
but it cannot yield measures that include all decay agents.
Data accessibility: the data for this paper has been deposited in DRYAD,
doi: TBA
Table 1