Assessing fire impacts on the carbon stability of fire‐tolerant forests

Abstract

The carbon stability of fire-tolerant forests is often assumed but less frequently assessed, limiting the potential to anticipate threats to forest carbon posed by predicted increases in forest fire activity. Assessing the carbon stability of fire-tolerant forests requires multi-indicator approaches that recognize the myriad ways that fires influence the carbon balance, including combustion, deposition of pyrogenic material, and tree death, post-fire decomposition, recruitment, and growth. Five years after a large-scale wildfire in southeastern Australia, we assessed the impacts of low- and high-severity wildfire, with and without prescribed fire (≤10 yr before), on carbon stocks in multiple pools, and on carbon stability indicators (carbon stock percentages in live trees and in small trees, and carbon stocks in char and fuels) in fire-tolerant eucalypt forests. Relative to unburned forest, high-severity wildfire decreased short-term (five-year) carbon stability by significantly decreasing live tree carbon stocks and percentage stocks in live standing trees (reflecting elevated tree mortality), by increasing the percentage of live tree carbon in small trees (those vulnerable to the next fire), and by potentially increasing the probability of another fire through increased elevated fine fuel loads. In contrast, low-severity wildfire enhanced carbon stability by having negligible effects on aboveground stocks and indicators, and by significantly increasing carbon stocks in char and, in particular, soils, indicating pyrogenic carbon accumulation. Overall, recent preceding prescribed fire did not markedly influence wildfire effects on short-term carbon stability at stand scales. Despite wide confidence intervals around mean stock differences, indicating uncertainty about the magnitude of fire effects in these natural forests, our assessment highlights the need for active management of carbon assets in fire-tolerant eucalypt forests under contemporary fire regimes. Decreased live tree carbon and increased reliance on younger cohorts for carbon recovery after high-severity wildfire could increase vulnerabilities to imminent fires, leading to decisions about interventions to maintain the productivity of some stands. Our multi-indicator assessment also highlights the importance of considering all carbon pools, particularly pyrogenic reservoirs like soils, when evaluating the potential for prescribed fire regimes to mitigate the carbon costs of wildfires in fire-prone landscapes.

Structural diversity underpins carbon storage in Australian temperate forests

“…For example, successional pathways may lead to similar or greater complexity at early‐ (Donato, Campbell, & Franklin, ) or mid‐seral stages (Brassard et al, ). Also, disturbances like non‐stand replacing fire may lead to mixed‐aged structures comprised of post‐fire regeneration and older resprouting cohorts (Bennett et al, ).…”

Wildfire refugia in forests: Severe fire weather and drought mute the influence of topography and fuel age

“…Fire severity and the spatial patterns of severity classes (e.g. patch size, configuration) have important implications for ecosystem response (Bennett et al, ; Chia et al, ; Doerr et al, ; Smucker, Hutto, & Steele, ). Unburnt patches within large wildfires, here termed ‘fire refugia’, facilitate the persistence of fire‐sensitive plants and animals in forest ecosystems globally (Meddens, Kolden, Lutz, Smith, et al, ; Robinson et al, ).…”

Aboveground forest carbon shows different responses to fire frequency in harvested and unharvested forests

“…Evidence from southern Australia suggests that crown fires may destabilize carbon stocks in resprouting eucalypt forests (Bennett et al. ). While prescribed burning can reduce canopy fire occurrence in eucalypt forests, the window of effectiveness is typically short (i.e., up to 5 yr after a prescribed burn; Price and Bradstock , Collins et al.…”