The agriculture, forestry, and land use sectors contributed approximately 5.9 ± 4.1 Gt CO2 of global greenhouse gas (GHG) emissions per year between 2010 and 2019. Some scholars opine that GHG emissions, through improved carbon removals, methane avoidance, and emissions mitigation, could diminish by up to −12.5 ± 3.2 Gt CO2 per yr. (1−3) While many studies have been undertaken with the aspiration of developing practices, skills, and/or technologies for reducing GHG emissions, most have focused on the livestock, cropping, and/or land use change sectors; (4,5) few have examined the mitigation potential associated with the horticultural sector. (6)
Enterprises that cultivate fruit and nut trees conceivably have significant advantages over other agricultural sectors in terms of GHG emission mitigation due to the ability of perennial trees to sequester carbon in woody vegetation. (7) Indeed, a meta-analysis showed that the global average carbon footprint of beef was 26.6 kg carbon dioxide equivalent (CO2eq) per kilogram (kg) of bone-free meat, whereas that of tree nuts was as low as 1.20 kg CO2eq per kg of raw product. (8) This suggests a greater propensity for tree-nut enterprises to achieve low- and negative-carbon products relative to other agricultural industries.

Macadamia nut trees (Macadamia integrifolia) were first discovered in Australia in the late 1850s (9) and have since gained prominence, with contemporary Australian macadamia farm gate value worth more than $AU286 M per yr. (10) Despite having existed for more than a century, little is known about why tree nut emission intensities vary so widely across production systems and agroecological regions. For example, the emissions intensity (EI) of almonds, hazelnuts, and pistachios grown in Italy has been quantified at 2.3, 1.2, and 2.5 t per tonne of CO2eq, respectively. (11) After accounting for carbon sequestered in nut shells and in tree vegetation, however, net emissions have decreased to 1.9, 0.5, and 1.7 t CO2eq per tonne of raw product, respectively. Similar results have been found in Spain, where GHG emissions from almond production systems varied 4-fold across management interventions. (12) Taken together, these studies imply significant scope for mitigation via practice change.

Key factors influencing enterprise GHG emissions of tree production systems are thought to arise from fertilizer, fuel, and electricity use. (13,14) Practice changes aimed at reducing such inputs have been shown to reduce gross GHG emissions, but, again, the extent of mitigation has been context-specific, depending on enterprise, location, environment, and current and previous management history. (15−17) Studies exploring approaches for emissions reduction and removals from fruit or nut trees have shown that shifting from conventional to organic farming reduced gross emissions by 56% in Mediterranean contexts, (18) while transitioning from intensive tillage to no tillage reduced net emissions in almond production under semiarid Mediterranean conditions by more than 60%. (12)

Given that GHG emissions vary as a function of agroecological region and production system, we contend that neither the carbon footprints nor the GHG emissions mitigation potential of macadamia orchards can be inferred from previous studies...