Tree-crop interactions in vineyards in the Western Cape

How windbreaks can reduce crop evapotranspiration and mitigate climate change in agricultural landscapes

South Africa faces greater challenges in the coming decades regarding climate change and its negative impact on agriculture and viticulture and fruit growing. Agroforestry systems can contribute to the adaptation of agriculture to climate change. The research project ‘Agroforestry in Southern Africa – new pathways of innovative land-use systems under a changing climate (ASAP)’ targets the application of trees in agricultural landscapes as a suitable response to the impacts of climate change. The project contributes to the research program SPACES II – Science Partnerships for the Adaptation to Complex Earth System Processes in Southern Africa establishes collaborative research projects between German, Namibian, and South African research institutions, and universities. The program intends to contribute to the formulation of science-based recommendations for Earth System management and to ensure the sustainable use of the multiple ecosystem services of the region. Spaces II is funded by the German Ministry of Education and Research (BMBF).

Dr. Maik Veste from CEBra – Centre for Energy Technology Brandenburg in Cottbus (Germany) and Dr. Roger Funk from the Leibniz Centre for Agricultural Landscape Research (ZALF) in Müncheberg in cooperation with Kerry-Anne Grey and Guy Midgley from the Department for Botany and Zoology at Stellenbosch University, is investigating the tree-crop interactions in citrus orchards and vineyards in the Stellenbosch area. The objective is to use the discovered environmental benefits to support the use of agroforestry systems as an innovative, multi-purpose land-use management practice, which needs to be integrated into agricultural landscapes. Thus, typical agroforestry systems will be systematically investigated to determine the effect of the trees on the radiation and heat balance of the adjacent annual and permanent crops. In this context, individual trees, windbreaks, or even smaller forests can have a favorable effect on the surface temperatures of the surroundings, which has a particularly positive influence on the heat and energy balance of the adjacent crops. For this purpose, Campbell Scientific weather stations, equipped with Apogee- Net Radiometers, carry out microclimatic measurements to determine the effects of the trees on crop evapotranspiration and to monitor the intensive spatial and seasonal variances of the radiation balances.

These investigations are coupled with ecophysiological measurements from a LI-600 Porometer/ Fluorometer system to determine the stress state of the plants. Additional leaf gas exchange and photosynthesis measurements are carried out with a Li-6400. Additionally, the temporal and spatial monitoring of the ecophysiological capacity and its development is measured using NDVI (Normalized Vegetation Index) and PRI (Photosynthetic Reflectance Index) spectral sensors. This records the photosynthetic capacity and the stress state of the plants in the stands during the respective vegetation period. Previous measurements in a vineyard have already shown positive effects on the reduction of wind speed and evapotranspiration. With reference to a control station in the open field – the mean wind speed (about 18 m from the hedgerow at canopy level) was reduced by 39% over the summer growing season. When applying empirical crop-specific Kc values for well-irrigated grapes, the reduction of evapotranspiration is 18.8% over the summer growth period. The current investigations will focus on the interaction between wind speed and radiation balances and heat stress. More details regarding the effects of wind on crop physiology and heat stress will be collected.

The use of tree shelterbelts is a suitable eco-engineering approach to reduce water consumption and to enhance water saving in vineyards, orchards and crop fields. Meanwhile, similar experiments are being installed in Müncheberg, Germany. This will allow a unique cross-continental research approach to understand the tree-crop interactions under different climatic conditions and facilitate scientific exchange on climate change in agricultural systems on both continents.