Mid-term effects of a thin-only treatment on fuel complex, potential fire behaviour and severity and post-fire soil erosion protection in fast-growing pine plantations

Abstract

Fuel hazard reduction is an essential component of pine plantation management. In NW Spain, plantation managers assume that thin-only reduces the potential fire behavior and effects by decreasing the quantity and continuity of canopy fuels. Nevertheless, thin-only can alter additional variables than control fire behavior and effects such as surface fuel availability and microclimate conditions. Moreover, the duration of thinning effects could vary and be species-specific. We present an approach linking field-measured fuel variables to well-established models for assessing the mid-term effects (six years) of thin-only on potential fire behaviour and severity and ability of burned stands to protect soil against erosion. Field variables were obtained from 41 thinning trials installed in stands of P. pinaster (22 locations) and P. radiata (19 locations). Canopy fuel load and canopy bulk density were still significantly lower in treated sample plots for both species six years after treatments, whereas total and fine understory fuel loads were slightly but significantly larger in thinned plots in P. pinaster. In a simulation process, different fire danger scenarios were considered through two fire-related weather variables: the 10-m open wind speed and the fine dead fuel moisture content. Results showed that thin-only could significantly reduce the likelihood of active crown fire and its rate of spread for both species, particularly in P. pinaster, while passive crown fire probability increased. The potential fire severity, in terms of crown scorch height, scorch volume and tree mortality, hardly worsened by thinning. For fireline intensities lower than 4000 kW m−1, dead fuel moisture content effect on fire severity surpassed the wind speed influence. Moreover, protection of soil against erosion improved only slightly when thinning was previously carried out. The results of the study reveal that, in the mid-term, it is unlikely that thin-only, without intervention in the surface fuels, may have a marked influence on the reduction of the severity of a subsequent fire. The results also highlight the need of a comprehensive assessment of the effects of thin-only on both the whole fuel complex and microclimatic variables for obtaining realistic results.