Observations of fire regime and overstorey tree canopy decline in karri forests

Abstract Background Frequent mild fire regimes have been associated with healthy and bushfire-safe forests, particularly under traditional Aboriginal management in Australia and the New World. In this study, we tested the hypothesis that karri ( Eucalyptus diversicolor ) forest tree canopy health (TCH) declines with time since fire (TSF) due to changes in soil chemistry that adversely impacts tree roots. We visually assessed the TCH of dominant and co-dominant trees at 66 sites in mature karri forests based on the extent and density of foliage, the extent of epicormic growth, and the extent of branch death. TSF ranged from two to 94 years. Fire regimes can influence tree health via soil chemistry conditions, so we sampled topsoil at each site. Results A model based on TSF and soil exchangeable potassium content explained 63.9% of the variation in TCH with TSF explaining 56%. K content was an indicator of variation in soils and susceptibility to decline rather than a cause of decline. There was little evidence that changes in soil chemistry with TSF contributed to tree decline and no evidence that climate change was the primary driver of canopy decline. Conclusion Karri tree canopy decline is likely caused by the physical effects of the accumulation of forest floor litter (or mulch) on tree roots and on soil microbiota via changed soil conditions such as temperature, moisture, aeration, insolation and penetrability. Other than gravel content, we did not measure soil physical factors, so this proposal requires further investigation. If the healthy functioning of tree roots is compromised by long periods of fire exclusion, then declines in tree canopy health may be exacerbated by drought events associated with climate change.