The effect of eucalypt slash burning and repeated disc harrowing on fuel load reduction, soil properties, and stand productivity

Abstract

In plantation forestry systems, disc harrowing is sometimes used as a fuel load reduction measure to minimise wildfire risk. This study reports the long-term effects of repeated disking on fuel loading, soil properties and tree growth in two eucalypt stands. During stand regeneration, harvesting residue of the previous crop was burnt, followed by repeated annual disking as one treatment, contrasted to a non-disked control treatment. Repeated disking significantly reduced the average oven dried fuel loading of the 1-hour fuel class by 29.0 t ha⁻¹ and that of the 10-hour fuels by 4.3 t ha⁻¹ when compared to the non-disked treatments. Disking significantly altered the forest floor structure, the humus layer in particular. By rotation end, disking did not change the levels of N, Bray-extractable P, organic carbon or pH of the topsoil (0–10 cm depth). However, repeated disking (thereby partially incorporating the forest floor) significantly increased topsoil exchangeable cation quantities, and reduced soil bulk density. Topsoil exchangeable K, Ca, Mg, Na, and sum of base cations increased by 0.04, 0.34, 0.12, 0.01 and 0.51 cmol_c kg⁻¹ respectively following repeated disking. Tree survival and root distribution were visibly (but not significantly) affected by the repeated disking treatment. Stand volume production was reduced by 5% (from 212.5 in the control to 202.6 m ³ ha⁻¹ in the disked treatment), but this difference was also not significant. Repeated disking between selected tree rows in eucalypt plantations is a cheap and effective technique to reduce fuel loading (and hence wildfire risk), and it clearly speeds up nutrient cycling. Soil tests do not show significantly negative effects on soil fertility or stand productivity emanating from annual disking over one rotation under the conditions tested. However, repeated disking did reduce the carbon stock stored in the topsoil plus forest floor layer from 50.1 to 30.4 t C ha⁻¹.