Regulation of litter decomposition in forest ecosystems of Sweden and New Zealand

Litter decomposition is a core ecosystem process critical for carbon and nutrient cycling, and the functioning of terrestrial ecosystems. This thesis explores two contrasting ways by which plants can influence litter decomposition in forested ecosystems. The first half of the thesis examined the role of feather mosses in mediating the environmental conditions on the soil surface in boreal forests and the impact this has on litter decomposition. The feather moss layer intercepts much of the litterfall in boreal forests as well as retaining significant quantities of rainfall. The results showed that the pattern of moisture inputs to the moss layer was a major driver of litter decomposition and moss-associated cyanobacterial N2-fixation. Under conditions of limited moisture availability the mosses promoted the decomposition of leaf litter within the moss layer by preventing the desiccation of the litter, maintaining decomposer activity. In this way the effect of the moss layer was dependent on the prevailing moisture conditions. Further, the results showed that the moss layer promoted the decomposition of intercepted leaf litter to a similar extent across all stages of a 360-year successional gradient, despite a large increase in the depth of the moss layer and changing ecosystem properties. The second half of the thesis investigated two poorly explored aspects of the relationships between plant functional traits, litter quality and litter decomposition in temperate rain forests in New Zealand. The results showed that the decomposition of leaf, twig and wood litter of 27 co-occurring tree species was uncoordinated. This lack of coordination occurred because wood decomposition was driven by differences in the wood density of trees, which itself was unrelated to the chemical traits driving the decomposition of leaf and twig litter. The results also revealed a high degree of intraspecific variation in the leaf functional traits and leaf litter decomposition of 16 co-occurring plant species. Within species, leaf traits varied predictably in response to a soil nutrient gradient but were not related to the variations in decomposition. Overall, the results of this thesis illustrate the important and contrasting impacts that plants can have on litter decomposition, and contributes to the understanding of the complex interactions between multiple factors that regulate litter decomposition in forest ecosystems