Biotic Drivers of Leaf Litter Decomposition in Shaded Coffee Agro-ecosystems in Chiapas, Mexico

Abstract

Decomposition is a critical ecosystem process, essential for the cycling of nutrients in and through ecosystems. Abiotic factors, like temperature and precipitation, drive decomposition processes, but biotic factors can also play a key role via direct and indirect pathways. This dissertation interrogates biotic drivers of leaf litter decomposition in shaded coffee agro-ecosystems. Coffee agro-ecosystems provide a tractable model system for testing biotic drivers of decomposition, given the often-reduced diversity of agro-ecosystems. The global distribution of coffee and range of management styles also makes it important to understand the dynamics underlying ecosystem function.

In Chapter 2, I proposed that decomposer specialization may be acting to the advantage of litter types that are grown at a particular location. This ecological theory, known as home-field advantage, was tested using two commercially grown species of Coffea with a reciprocal transfer experiment that measured the decomposition of each species in its home environment, a con-generic away environment and a forested-away environment. Results revealed support for home-field advantage during a shorter six-week experiment, but this effect did not persist in a year-long litterbag study.

I assessed the role of two common shade trees, Inga micheliana and Alchornea latifolia, on the decomposition of Coffea arabica leaf litter in Chapter 3. Non-additive effects can occur when multiple species are decomposing in combination, though the direction and strength of such effects tends to be highly context dependent. I found that that C. arabica decay accelerated in mixture, and that being in mixture with I. micheliana provided the biggest boost in decay rate, likely due to the relatively high nitrogen content in its litter. Being in mixture with C. arabica led to mixed results for decay, which may be a result of nitrogen being tied up in the secondary defense compound, caffeine. Micro-topography—being uphill or downhill—of a non-focal species did not have a significant effect on decay rates, suggesting that decomposers are not dispersing via rainwater runoff.

Coffee flowers synchronously, representing a potentially important contribution of flower petals to the detrital pool. In Chapter 4, I quantified the magnitude of a C. arabica bloom and its effects of the decomposer community and leaf litter decay rates. Results indicated that the bloom represents an ecologically relevant quantity of nutrients. While the decomposer did not respond on the time scale of one week, decay accelerated with the petals after one and two months.

Finally, in Chapter 5, I examined the indirect effects of a keystone ant species, Azteca sericeasur, on the decomposition of Inga micheliana leaf litter by determining the community of ants and decomposers in a radius around trees that had nests and trees that did not have nests. I found no decrease in ant species richness, but a different community of ants near nests. The decomposer community was not changed in the presence of A. sericeasur nests. A litterbag study found no differences in decay rates of litter at or away from nests, suggesting that decomposition function is maintained around A. sericeasur nests.

This research increases our understanding of biotic drivers of leaf litter decomposition in coffee agro-ecosystems and underscores the role of biota in decomposition processes.