Assessment of climate sensitive indices over various reproductive phenological stages for Robusta coffee in Uganda

Abstract

The study aimed at developing and assessing climate-sensitive indices over the reproductive stage of Robusta coffee for both the historical (1990-2021) and future (2025-2040) periods in the Robusta Coffee Growing Region (RCGR) of Uganda. The reproductive phenology of Robusta coffee was determined using data from site experiments monitored over the crop-year running August 2022- July 2023. Later, climate sensitive indices such as the Modified Thornthwaite Climate Indices (MTCI) and Water Requirement Satisfaction Index (WRSI) were computed using crop phenology data and a combination of climate variables with direct impact on crop growth. Phenology data included length of the growing season, Growing degree days and Crop coefficients (Kc) during the reproductive stage. Whereas, the climate variables included Vapor Pressure Deficit (VPD), Fraction of Absorbed Photosynthetically Active Radiation (FAPAR), Actual Evapotranspiration (AET), Potential Evapotranspiration (PET) and Precipitation. For the future, downscaled Coupled Model Intercomparison Phase 6 (CMIP6) data on Precipitation and Air Temperature, Relative Humidity, Windspeed and downwelling shortwave radiation was used to simulate future PET based on two Shared Socioeconomic Pathways (SSP) SSP245 and SSP585. Results show that selected Robusta coffee varieties have comparable growth and phasic transitional patterns with relatively small but crucial stage-specific variations in terms of bud, flower and berry morphologies, timing and duration of phasic transition. Thus, there were no significant differences in phasic heat requirements of the reproductive phase of Robusta coffee based on Kruskal Wallis statistic. Conversely, there were significant differences in water demand (Kc) based on One-way ANOVA. Trend test results showed both significantly increasing and decreasing trends in monthly VPD and PET and significantly decreasing trends in FAPAR for most parts of the RCGR. Significant changes in Mbale’s seasonal AET, abrupt changes in Kituza AET and Amolatar VPD were also revealed. Both climate-sensitive indices exhibited stability for most of the RCGR except, Arua and Kabale which were characterized by significantly decreasing trends for the historical period. Finally, except for Kitgum and Mbale, significant differences in MTCI were observed between the historical and future periods. Based on SSP585, significantly increasing trends in MTCI were projected for Kabale and Masindi. Possible interventions recommended to address issues faced by farmers and improve productivity include but aren’t limited to; (i) diverse area-specific breeding programs to fit the varying moisture and heat distribution observed and projected in the RCGR, (ii) bundling and distribution of only varieties with matching phenology especially during the flowering phase to farmers and, (iii) expansion and intensification of Robusta coffee production both out and in the RCGR, respectively, as previously neglected regions such as Abim and Amolatar exhibited potential for higher Robusta coffee productivity with little adjustments in water and heat management in the crop canopy during sensitive stages such as the reproductive stage.