Forest Structure Drives Fuel Moisture Response across Alternative Forest States

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.

Indirect effects of climate change on forest structure alters fuel availability in wet Eucalypt forests

“…Our research suggests that the impact of vegetation-mediated indirect effects of climate change on potential fire activity across seven alternative forest states is substantial. Across 48 years of modelled climate data, the results were broadly consistent with single-season observations reported by Burton et al (2019) and Brown et al (2021), suggesting that these comparatively shorter studies captured an appropriate range of climatic conditions. Across the 48-years of modelled data, FMC variability within and between the alternative forest states was greater than modelled FMC variability at the open site, suggesting that the indirect effects of climate change (that have altered forest structure and composition) may be more important than the direct effects on FMC in this ecosystem.…”

“…Field studies across a range of alternative forest states and age classes to mature wet Eucalypt forests (Brown et al, 2021;Burton et al, 2019) report both positive and negative feedbacks. However, a key limitation to these studies is that they have a limited timeframe of observation.…”

Increasing fire frequency may trigger eco‐hydrologic divergence

“…However, there are also areas where both E. regnans and their replacement species, A. dealbata , have both been early extirpation of both E. regnans and A. dealbata (and other acacia species) from their native habitats. Brown et al (2021) investigated how forest structure influences fuel moisture in alternative forest states in southeast Australia. They suggested that Acacia forests would transition into the non‐eucalyptus forest over time.…”

Terrestrial Laser Scanning: An Operational Tool for Fuel Hazard Mapping?

“…Given the cost of multi-spectral sensors, difficulty in aligning sensor data and the assignation of moisture values to fine scale vegetation objects, it is unlikely that these sensors will rapidly become operationally viable for land and fire management agencies. Despite this, the structural information contained in single-wavelength scanner data can assist in fuel moisture modelling which has found strong relationships between vegetation structure and dead fine fuel moisture content beneath the forest canopy [63][64][65].…”