Terrestrial-aquatic interactions: Implications for aquatic leaf litter decomposition

Energy transfer from riparian forests to streams through leaf litterfall represents one of the most important interactions between streams and adjacent riparian forest ecosystems. Different species of leaf litter have been shown to decompose at different rates in streams and provide energy for aquatic invertebrates, fungi and bacteria. This dissertation makes five major predictions and conclusions about this interaction between forests and streams: (1) Tree species diversity of riparian forests affects stream function and communities through leaf litter decomposition. Litterbags with experimentally manipulated leaf litter species diversity in three streams showed distinct rates of decomposition and distinct aquatic invertebrate communities. (2) Two cottonwood species and their hybrids decompose at different rates and host different macroinvertebrate communities. This study shows significant variation in decomposition rate and macroinvertebrate community colonization within a hybridizing complex. (3) Distinct cottonwood leaf litter genotypes in a stream show significant variation within species and hybrid types in terms of initial litter chemistry, rates of decomposition, and aquatic fungal biomass accumulation. Percent condensed tannin in litter explained 71% of the variation in decomposition. (4) Terrestrial arthropod herbivores affect rates of in-stream decomposition through induction of foliar defense compounds. This study shows that a leaf-galling aphid affects leaf chemistry and subsequently influences decomposition rates in streams, aquatic invertebrate communities and aquatic fungal biomass accumulation. (5) Finally, taxonomic classification at the levels of division, class, order, family, genus and species explain the variation in decomposition rates across the globe to a larger extent than climatic variables such as mean annual air temperature, mean daily precipitation and latitude. This study shows increasingly specific levels of taxonomy (from division to species) explain considerably more variation in global decomposition rates than climatic variables. Expanding traditional one or two species decomposition studies and incorporating genetic diversity and insect herbivores (as significant influences on decomposition) advances the field of aquatic ecology with insights into complex interactions between terrestrial and aquatic ecosystems.