Abstract
Fire is nearly ubiquitous in the terrestrial biosphere, with profound effects on earth surface carbon storage, climate, and forest functions. Fuel quality is an important parameter determining forest fire behavior, which differs among both tree species and organs. Fuel quality is not static: when dead plant material decomposes, its structural, chemical, and water dynamic properties change, with implications for fuel flammability. However, the interactions between decomposition and flammability are poorly understood. This study aimed to determine decomposition’s effects on fuel quality and how this directly and indirectly affects wood flammability. We did controlled experiments on water dynamics and fire using twigs of four temperate tree species. We found considerable direct and indirect effects of decomposition on twig flammability, particularly on ignitability and burning time, which are important variables for fire spread. More decomposed twigs ignite and burn faster at given water content. Moreover, decomposed twigs dry out faster than fresh twigs, which make them flammable sooner when drying out after rain. Decomposed fine woody litters may promote horizontal fire spread as ground fuels and act as a fuel ladder when staying attached to trees. Our results add an important, previously poorly studied dynamic to our understanding of forest fire spread.
Highlights
Fire has been a worldwide common phenomenon since the establishment of terrestrial plants and has profound effects on landscape, community composition, carbon and nutrient cycles, and climate [1,2,3,4]
Decomposed fine woody litters may promote horizontal fire spread as ground fuels and act as a fuel ladder when staying attached to trees
Wood chemical properties may change during decomposition [32]. Those changes may have considerable influences on wood flammability directly, via the changing structure and chemistry of the wood, or indirectly, through the effects of changing wood structure on moisture dynamics (Figure 1). This leads to the following research questions: (1) What are the effects of decomposition on twig structure and chemical properties within tree species? (2) How does decomposition directly and indirectly affect twig flammability of given tree species? How do the relationships in (1) and (2) vary among tree species? To disentangle the effects of fuel properties from effects of fuel amount and fuel bed structure, this study focuses on flammability of individual twigs
Summary
Fire has been a worldwide common phenomenon since the establishment of terrestrial plants and has profound effects on landscape, community composition, carbon and nutrient cycles, and climate [1,2,3,4]. Severe wildfires return large amounts of stored carbon into the atmosphere [5], representing the major substitute C release pathway, in addition to decomposition [6]. With increasing temperatures and duration of fire seasons due to climate change, the frequency and severity of wildfires is increasing [7,8,9]. In addition to abiotic factors such as temperature and moisture regimes, biotic drivers, such as amount and quality of living and dead plant material are very important [2]
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