Ecophysiological characterization and modelling of macadamia orchards in Limpopo, South Africa

Macadamia is a high-value tree nut crop experiencing a global rise in demand. South Africa is the world’s largest producer, and macadamia orchards are rapidly expanding across the country. However, this expansion is linked to increased irrigation water use in a water-scarce region, already facing challenges from unsustainable freshwater withdrawals, partly driven by population growth, and exacerbated by the impact of climate change. Therefore, enhancing irrigation water use efficiency (IWUE - here defined as crop yield per unit of irrigation water used) is critical for sustainable macadamia production. This can be achieved through two complementary strategies: (i) sustainably increasing yields by targeting and reducing yield-limiting factors, and (ii) reducing water consumption by optimizing irrigation management and minimizing unproductive water loss. This thesis explores both approaches to enhance IWUE of macadamia orchards employing a combination of methods and techniques, including literature reviews, expert interviews, analyses of historical climatic and production data, a comprehensive field experimental and environmental monitoring campaign, and ecophysiological modelling. The field experiments and monitoring systems were established and conducted over two years on two farms (in total four macadamia orchards) in Levubu, South Africa. This included continuous data collection on tree transpiration, microclimate, and soil water content, alongside periodic monitoring of tree morphology and phenology. These data were analyzed to gain deeper understanding of macadamia water use behavior and were used to calibrate and validate a novel macadamia transpiration model, enabling the upscaling of findings from field-level to broader cultivation regions and diverse climatic conditions. Chapter I provides a general introduction to the topic, objectives, methodology, and the structure of this thesis. Chapter II reviews sustainable management practices to mitigate the impacts of agricultural intensification under climate change. These include adopting water-saving irrigation systems, reducing irrigation to precisely meet the trees’ specific water requirements, and integrating technological innovations (e.g. soil moisture probes and remote sensing) along with water-saving practices (e.g. mulching) to reduce the water footprint of macadamia orchards. Chapter III investigates strategies to sustainably increase the productivity (i.e. nut yield) of macadamia orchards. In particular, it analyzes the effects of orchard characteristics and environmental factors on historical yields (2010-2021) from 247 orchards along an altitudinal gradient (600 to 950 m a.s.l.) in Levubu, using mixed-effects models. The study also evaluates the role of irrigation in counteracting abiotic stresses (i.e. heat and drought) on productivity. Results show highly variable yields (from 1.2 to 4.0 tons dry nut-in-shell per hectare) and a declining trend in productivity, driven by alternating bearing patterns and increasingly adverse climatic conditions. Key climatic factors affecting yields included high temperatures and low global radiation during nut development, as well as insufficient rainfall during the dry season. Orchard characteristics such as cultivar, elevation, planting density, and tree age also significantly influenced productivity. While irrigation increased yields, it did not fully counteract climatic stress, emphasizing the need for integrated solutions combining improved orchard management and genetic advancements to sustainably enhance macadamia productivity and climate resilience. Chapter IV examines macadamia water use behavior, focusing on daily water use and transpiration patterns across seasons, cultivars, and tree age classes, while identifying the key drivers of tree transpiration. Full-bearing trees transpired between 0.7 and 1.1 mm per day during dry and rainy seasons, respectively – which is lower than assumed by current industry standards (on average 2.0 mm per day). This suggests that orchards are very likely over-irrigated. Macadamia trees in our study orchards exhibited a predominantly conservative (isohydric) water use strategy, characterized by strict stomatal control. Different cultivars displayed varying water use behavior. Such plasticity in water response behavior has implications for both macadamia tree productivity and climate resilience. Transpiration was driven by microclimatic factors, with a vapor pressure deficit threshold of 1.5–2.0 kPa limiting stomatal conductance, regardless of soil water availability. Thus, increasing irrigation under high evaporative demand does likely not enhance tree transpiration but instead risks leading to over-irrigation and reduced IWUE. Ecophysiological modelling approaches provide valuable tools for complementing and upscaling the field-level results from Chapter IV and other similar studies on macadamia tree water requirements. In Chapter V, a simple, process-based model designed to simulate tree transpiration under non-limiting water conditions with minimal data requirements was adopted and evaluated for macadamia trees. The model was further developed to simulate transpiration under water deficit conditions, accounting for the variability in water availability commonly found in southern Africa. The model’s satisfactory performance suggests that it could serve as a valuable tool for optimizing irrigation practices and conserving water resources in macadamia orchards. The final chapter VI discusses and evaluates the two proposed pathways for enhancing IWUE in macadamia orchards, with a particular focus on the implications of climate change, and addressing socioeconomic aspects related to water costs and allocations, as well as nut prices and global trends of the macadamia industry. Based on the findings of this thesis, a major recommendation to practitioners and policy makers is to prioritize the reduction of irrigation water use, potentially through the adoption of promising strategies such as regulated deficit irrigation.