Do secondary compounds inhibit microbial- and insect-mediated leaf breakdown in a tropical rainforest stream, Costa Rica?
Abstract:
We examined the
hypothesis that high concentrations of secondary compounds in leaf
litter of some tropical riparian tree species decrease leaf breakdown
by inhibiting microbial and insect colonization. We measured leaf
breakdown rates, chemical changes, bacterial, fungal, and insect
biomass on litterbags of eight species of common riparian trees
incubated in a lowland stream in Costa Rica. The eight species spanned
a wide range of litter quality due to varying concentrations of
nutrients, structural and secondary compounds. Leaf breakdown rates
were fast, ranging from 0.198 d−1 (Trema
integerrima) to 0.011 d−1
(Zygia longifolia). Processing of individual
chemical constituents was also rapid: cellulose was processed threefold
faster and hemicellulose was processed fourfold faster compared to
similar studies in temperate streams. Leaf toughness (r = −0.86,
P = 0.01) and cellulose (r = −0.78,
P = 0.02)
were the physicochemical parameters most strongly correlated with
breakdown rate. Contrary to our initial hypothesis, secondary compounds
were rapidly leached (threefold faster than in temperate studies), with
all species losing all secondary compounds within the first week of
incubation. Cellulose was more important than secondary compounds in
inhibiting breakdown. Levels of fungal and bacterial biomass were
strongly correlated with breakdown rate (fungi r = 0.64,
P = 0.05; bacteria r = 0.93,
P < 0.001) and changes in
structural compounds (lignin r = −0.55,
P = 0.01).
Collector−gatherers were the dominant functional group of
insects
colonizing litterbags, in contrast to temperate studies where insect
shredders dominate. Insect biomass was negatively correlated with
breakdown rate (r = −0.70,
P = 0.02),
suggesting that insects did not play an important role in breakdown.
Despite a wide range of initial concentrations of secondary compounds
among the eight species used, we found that secondary compounds were
rapidly leached and were less important than structural compounds in
determining breakdown rates.
Ardon, M., and C. M. Pringle. 2008. Do secondary compounds inhibit microbial- and insect-mediated leaf breakdown in a tropical rainforest stream, Costa Rica? Oecologia 155: 311-323.
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