Effects of environmental drivers and irrigation on yields of macadamia orchards along an altitudinal gradient in South Africa

Abstract

Macadamia is a high value tree nut crop experiencing a global rise in demand. South Africa is the worldwide largest producer and macadamia orchards are rapidly expanding in the country. However, yields are highly variable across years and have been declining in recent years. Therefore, to sustainably increase the productivity and resilience of macadamia orchards to climate change, the impact of environmental factors on the trees’ vegetative and reproductive cycles needs to be better understood. To this end, the extent to which orchard characteristics, climatic and soil factors, as well as irrigation amounts drive macadamia yields in Levubu (Limpopo Province, South Africa) along an altitudinal gradient (600–950 m a.s.l.), was quantified by using mixed-effects models. Climatic variables were selected separately for the different macadamia phenological stages, in which they potentially affected yields. Furthermore, the effects of different yield determining factors were compared between irrigated and rainfed orchards.A pronounced interannual variability of macadamia yields was found (from 1.2 to 4.0 tons dry nut-in-shell ha−1), although a triennial bearing pattern was observed. Higher yields were found at elevations >700 m a.s.l. and in micro-sprinkler irrigated orchards. Orchard characteristics and environmental variables jointly explained 49% of yield variability. Cultivar, presence of irrigation, tree age and planting density were found to affect yield, while no significant effect was found for soil variables (i.e. soil texture, bulk density, pH and C:N ratio). High temperatures and low global radiation during the nut development stages, alongside poor rainfall amounts in the dry season, were the climatic factors more severely affecting yields. In particular, low irradiance was the main yield limiting factor in irrigated orchards, while extremely high temperatures and poor rainy seasons in rainfed orchards. Increased irrigation amounts, although beneficial, were not fully compensating the impact of climatic factors on productivity. Our findings suggest that irrigation alone cannot counteract adverse climatic effects on macadamia yields. To sustainably increase macadamia productivity and resilience to climate change, abiotic stress impacts will have to be reduced through a combination of genetic improvement and better orchard management practices.