Dead wood is a source of life : Stem traits, fungal communities, and stem decomposition of temperate tree species

Summary

Forests play a key role for terrestrial biodiversity. Trees are large and long-living organisms that define forest structure and form a key element in nutrient and carbon cycling. Tree species differ in ecological strategies and related functional traits, which may allow them to partition resources and coexist. It is well know that tree species differ in stem traits, but the afterlife effects of such traits for the diversity of decomposers (e.g., fungi, insects) within dead stem logs and their dynamics with ongoing decay are poorly known. To fill this knowledge gap, I studied the effects of stem traits on the infestation, composition and diversity of fungi during different phases of decay. I benefited from an existent long-term common garden decomposition experiment, called “LOGLIFE”. I showed how stem traits and related stem functions co-vary across species, and how these traits affect afterlife effects on the succession of diversity and composition of wood and bark inhabiting fungi over time, and the implications for wood decomposition. In this study, I included eight conifer and six broadleaf tree species in order to cover a wide variety of stem traits and thus explore the afterlife implications of such variation.

In chapter 2 I investigated how stem traits vary across wood and bark of 14 temperate tree species, and what trait trade-offs and plant strategies are found. Stem trait variation was largely explained by major taxa and stem compartments, i.e., inner, outer wood and bark. A continuous plant strategy gradient was found across and within taxa, running from hydraulic safe Gymnosperms to conductive Angiosperms. Gymnosperms strongly converged in their trait strategies because of their uniform tracheids, whereas Angiosperms strongly diverged because of different vessel arrangement and tissue types. Bark had higher concentrations of nutrients and phenolics, whereas wood had stronger physical defence. This indicates that stem compartments fulfill different strategies; bark serves as storage organ and a first physical and chemical defence layer, while wood is physically well defended by having stronger tissues.

In chapter 3 I investigated the effects of stem traits for the succession of fungi fruiting bodies over 8 years of decay. Fungal communities diverged early in the decay process because tree species largely differed in substrate conditions. However, these fungal communities converged later, likely because this substrate became more similar with ongoing decay. Dead wood quality, as determined by species stem traits and decay stage, is therefore an important driver of fungal diversity and fungi community composition. These results imply that forest with larger variation in stem traits and stem logs in different phases of decay will promote fungal and other microbial diversity and, thereby, forest biodiversity.

In chapter 4, Internal Transcribe Spacer (ITS) region amplicon next generation sequencing was used to assess the effects of stem traits on the entire fungal community within different stem compartments (inner wood, sapwood and bark) after one and four years of decay. Bark contained higher fungal diversity than wood. Fungal community composition differed between inner wood versus outer wood and bark. Stem traits regulate therefore the fungal composition via their effects on compartment accessibility, fungal nutrition, and physical or chemical defence against fungi. Traits associated with accessibility were important in the initial decay stage whereas traits related to nutrition (e.g., lignin : cellulose) became most important after four years of decay. Hence, stem trait differences across tree species and their stem compartments have significant afterlife effects in regulating fungal diversity and composition, and contribute to forest biodiversity.

In chapter 5, I integrated the results from chapters 2, 3 and 4 by assessing how stem traits and fungal communities related to differences in stem decomposition rate amongst tree species. The molecular technique is able to detect microscopely invisible fungal communities within the decaying logs and provided therefore a better approach than fungal fruiting body surveys to assess fungal diversity while fungal fruiting body surveys can better indicate the most active fungi that form fruiting bodies when inactive mycelia hidden inside the wood. Wood decomposition rate varied markedly across tree species (from 0.019 to 0.166). It was mainly determined by stem traits and composition of saprotrophic fungal community, but not by fungal diversity. Early in decay, stem traits are the strongest determinants of decomposition rate whereas later in decay the fungal composition becomes more important.

I conclude that stem traits from an important component of tree strategies, with potentially strong effects on species performance, coexistence, and ecosystem functioning. Moreover, these stem traits have profound afterlife effects by interacting with fungal decomposers. Hence, stem traits and their fungal decomposers affect stem decomposition and carbon and nutrient cycling, and contribute to forest biodiversity.