Nutrient status, canopy size, crop load and fruit quality of a fire impacted ‘Royal Gala’ orchard: implications for orchard management

The increased frequency and intensity of extreme bushfires has had a wide-ranging impact on primary industries and particularly perennial horticultural productions. However, not much is known about bushfire effects on vegetative and reproductive abilities of fruit trees. This study aimed at providing a clearer understanding of the immediate effects of extreme bushfires on apple trees affected by spot-fires during the 2019-20 Australian bushfires. The evaluation was carried out on ‘Royal Gala’, apple trees on M26 rootstocks, with four identified fire-impact levels. The nutrient concentrations variability of leaves, in time and space, and important vegetative and reproductive attributes including tree canopy size, crop load and fruit quality at maturity, were assessed in the growing season following the bushfires in the previous summer. Mean comparison of macro- and micronutrients in leaves, across the four impact levels revealed lowest but non-significantly different values in leaf nutrient concentrations of severely impacted trees except for manganese with a significantly lower concentration (20.7 mg kg^‑1). Summer nutrient values were generally lower than the standard nutrient range for macro- and micronutrients. The only nutrients with concentrations within the standard nutrient range and across most impact levels were P, B and Cu. Potassium nutrient concentrations were higher than the standard nutrient range except for severely impacted trees (1.45%). In addition, canopy recovery assessed through leaf area index (LAI) measurement, in summer (0.57±0.13) was lowest for severely impacted trees. Similarly, fruit quality parameters including average weight, firmness, soluble solid contents, pH and malic acid content indicated reduced quality of fruits from severely impacted trees compared to other impact levels. In terms of yield, total yield of severely impacted trees was significantly higher than the other damage levels, although with much smaller fruits. These findings confirm the potential negative effect of bushfires on fruit tree nutrition, vegetative/reproductive performances and fruit quality at harvest. Enhanced management strategies to reduce the detrimental effects of bushfire must be pursued in order to ensure optimal orchard productivity.