Short-term effects of firebreaks on seedling growth, nutrient concentrations and soil strength in southern Australian wet eucalypt forests

Abstract

Wet eucalypt forests in south-eastern Australia are regenerated following harvest using broadcast burning followed by aerial sowing. Before burning, a mineral-earth firebreak is established around the perimeter of the harvested area to protect the standing forest edge. This results in a strip of disturbed soil and a visibly compacted track, which are bordered by a windrow of displaced debris. Firebreaks cover a significant proportion of the harvested area in recent coupes, but their effect on seedling growth has not been examined previously. This study quantified the short-term effects of firebreak construction on seedling size, foliar nutrients, soil strength (penetration resistance) and soil chemistry. These variables were measured on two disturbance types associated with firebreaks (disturbed soil and visibly compacted soil), as well as in the adjacent burnt windrow and in the general harvest area, at eight sites in Tasmania, Australia. Although there was considerable variation between sites, treatment effects were substantial and consistent. Seedlings growing on the compacted track and on disturbed soil on the firebreaks were 40% and 60% the size of those growing in the general harvest area, respectively. Seedlings on the firebreaks also had lower concentrations of foliar N, P and K, reflecting reduced nutrient levels in the two firebreak treatments. Reduced seedling growth on the compacted portion of the firebreaks can also be attributed to increased penetration resistance, which was up to 52% higher on the firebreak track compared to other treatments. Seedlings growing on the burnt windrow were similar in size to those growing in the general harvest area, despite higher nutrient concentrations in the windrow. Further research is needed to quantify the persistence of these effects, and to determine implications for site-level productivity.