Fuel dynamics following fire hazard reduction treatments in blue gum (Eucalyptus globulus) plantations in Portugal

Abstract

Eucalypts planted for pulp production are highly valued forest resources often at risk in fire-prone environments. Fuel management programs can diminish fire hazard, yet little is known about how to treat, and how often, to maximize treatment effect and persistence. This study describes the temporal dynamics of fuel structure attributes and fire behaviour following the application of one of four alternative treatments in blue gum (Eucalyptus globulus) plantations in Portugal. We quantified fuel characteristics using non-destructive methods and attributed visual fuel hazard scores in plots (n=256) that had been treated between zero and 111months earlier. Fire behaviour simulation was based on the Dry Eucalypt Forest Fire Model. We used generalized linear modelling to relate post-treatment fuel hazard metrics and fire behaviour characteristics to treatment type and time since treatment (T), plus site and stand conditions for selected dependent variables. Fuel hazard increased with T, but the proportion of explained variation in fuel metrics was moderate at best, presumably because of environmental variation among sites. Fuel dynamics in relation to T varied with treatment type and fuel metric. Immediate pots-treatment fire hazard was higher for low-impact treatments (herbicide, manual cutting) than for high-impact treatments (disk harrowing, slashing). Subsequently, low-impact treatments resulted in declining fire hazard and the opposite trend was observed for high-impact treatments; fire hazard convergence between the two treatment groups occurred at T=3–4years. Low-impact treatments differ from mechanical treatments in that their effects are not immediate and are limited to near-surface and elevated fuels, and thus are not recommended as a stand-alone fuel treatment practice. Independently of treatment, fire hazard increased with stand age, stand density, and productivity-related variables. Variation in those influences and in slope determines that fire suppression in blue gum stands is effective for 2–6years after disk harrowing, currently the prevailing treatment option. Because the impact of mechanized treatments is restricted to inter-rows, improved fuel control would be achieved through low-intensity prescribed burning. Effective fuel treatment programs in industrial eucalypt plantations are constrained by a mix of natural and economic factors, which implies that adequate protection from wildfire cannot be achieved without due consideration of the plantation and fire management contexts.