Linking fire and decomposition in gymnosperm trees: the role of biotic disturbances

Forests play a crucial role in providing key ecosystem services, including climate regulation through carbon absorption and storage. However, climate warming poses a threat to this function by increasing the frequency and severity of disturbances such as droughts, fires, and insect outbreaks. These disturbances can directly and indirectly affect forest functions and carbon stocks. Wildfires, in particular, are significant ecological processes influenced by factors like ignition sources, fuel types, and climates, with drought being a primary driver under climate change. Understanding the interactions between abiotic disturbances like wildfires and biotic disturbances such as tree attacks by plant pathogens and invertebrates is essential. However, the causes and consequences of these interactions of biotic and abiotic disturbances for tree and forest carbon dynamics, whether or not via plant traits, are still poorly understood. Therefore, in this thesis, I put focus on surface fires driven by fine plant litter and coarse wood of gymnosperm tree species, and investigate how fire and decomposition, and their mutual interactions, are driven by important forest biotic disturbance agents (parasitic fungi and beetles) for coarse deadwood and by plant-centered biotic factors, i.e. the ‘legacy effects’ of variations in leaf litter SSS (the size and shape spectrum) and PES (the plant economics spectrum) traits. From Chapters 2 to 5, this thesis addresses four main research questions: (a) Are leaf litter decomposability and flammability decoupled across gymnosperm species based on their trait spectra? (b) Will the opportunistic parasitic fungus Armillaria affect the flammability of coarse deadwood in exotic pine plantations? (c) Does the legacy of this opportunistic parasitic fungus speed up the coarse wood decomposition of exotic pine plantations? (d) Do wood-boring beetles influence the flammability of deadwood across decomposition stages? In summary, the findings of this thesis demonstrate that both plant traits and biotic disturbances are important drivers of decomposition rates and fire properties, and their interactions. First, different (mostly decoupled) plant trait spectra drive the decomposability and flammability of fine litters. Second, specifically parasitic fungal attacks and wood-boring beetle infestations, have a notable impact on coarse deadwood. These disturbances lower the wildfire threshold or extend the smoldering phase, promoting decay processes. These dynamics, from fine litters to deadwood, have significant environmental implications, as together they expedite carbon release and contribute to climate warming. However, it is important to acknowledge that this thesis primarily focused on model species, and future research should encompass a broader range of species, also in mixture, to enhance the generalizability and practicality of the findings.