Abstract
Climate warming is expected to increase fire frequency in many productive obligate seeder forests, where repeated high-intensity fire can initiate stand conversion to alternative states with contrasting structure. These vegetation–fire interactions may modify the direct effects of climate warming on the microclimatic conditions that control dead fuel moisture content (FMC), which regulates fire activity in these high-productivity systems. However, despite the well-established role of forest canopies in buffering microclimate, the interaction of FMC, alternative forest states and their role in vegetation–fire feedbacks remain poorly understood. We tested the hypothesis that FMC dynamics across alternative states would vary to an extent meaningful for fire and that FMC differences would be attributable to forest structural variability, with important implications for fire-vegetation feedbacks. FMC was monitored at seven alternative state forested sites that were similar in all aspects except forest type and structure, and two proximate open-weather stations across the Central Highlands in Victoria, Australia. We developed two generalised additive mixed models (GAMMs) using daily independent and autoregressive (i.e., lagged) input data to test the importance of site properties, including lidar-derived forest structure, in predicting FMC from open weather. There were distinct differences in fuel availability (days when FMC < 16%, dry enough to sustain fire) leading to positive and negative fire–vegetation feedbacks across alternative forest states. Both the independent (r2 = 0.551) and autoregressive (r2 = 0.936) models ably predicted FMC from open weather. However, substantial improvement between models when lagged inputs were included demonstrates nonindependence of the automated fuel sticks at the daily level and that understanding the effects of temporal buffering in wet forests is critical to estimating FMC. We observed significant random effects (an analogue for forest structure effects) in both models (p < 0.001), which correlated with forest density metrics such as light penetration index (LPI). This study demonstrates the importance of forest structure in estimating FMC and that across alternative forest states, differences in fuel availability drive vegetation–fire feedbacks with important implications for forest flammability.
Highlights
Fire is a critical process in many ecosystems globally that influences the distribution, composition and successional stage of vegetation communities [1]
The fuel availability rankings for end-member sites were mirrored in the Powelltown–Maroondah timeseries (Acacia10, relative proportion of fuel availability (RFA) = 1.32; Non-eucalypt80, RFA = 0.24), the difference in relative fuel availability increased between the two timeseries presented
Cawson et al [97] reported that stand age is not a key driver in fuel moisture content (FMC) for forests older than 33 years, and given limited difference in fuel availability between forests regenerated in 2009 (E. regnans10, RFA = 0.68) and 1759 (E. regnans260, RFA = 0.60), our research suggests that dead fuel moisture insensitivity to time since disturbance may begin earlier than 33 years
Summary
Fire is a critical process in many ecosystems globally that influences the distribution, composition and successional stage of vegetation communities [1]. Repeated high-intensity fires can alter the successional pathways of forest communities by overwhelming the utility of fire adaptive traits [8], which can lead to abrupt shifts in ecosystem composition and forest structural properties [9,10,11]. While resprouting forests can persist in maturity through fire and are considered fire tolerant [13], obligate seeding forests are generally killed by high-intensity fire and persist through mass regeneration from seed [14] They are considered fire sensitive because if regenerating juveniles are burnt before reproductive age, local extinction can occur [13] and the dominant vegetation community may transition to one more adapted to short-interval fires [11,15]. The potential for climate-induced changes in fire frequency to drive forest conversion is a global challenge and has been recognised in high-altitude forests in Patagonia [16], tropical forests in the Amazon [17], boreal forests of North America [10,18,19,20] and Australian eucalypt forests [21,22,23,24]
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