Do light and alfalfa responses to cloth and slatted shade represent those measured under an agroforestry system?

Shade cloth is commonly used in agroforestry research. It produces a continuous, uniform reduced light environment. Shade cloth and a slatted structure were compared in relation to the inability to represent the light regime and plant responses of an agroforestry system. The split-split-plot randomised block experiment had main plots as covering status (with or without radiata pine trees), subplots as artificial shade (none, shade cloth or wooden slats) and sub-subplots as growth rotation, over sown alfalfa, in three replicates. The quantity of light transmittance was 49% under trees, 41% under cloth and 44% under slats. Temporal changes and spectral composition under trees were more accurately reproduced under the slats than shade cloth. The red to far red ratio was 0.64 under tree shade and 0.74 during the shaded period under slats. This compared with 1.31 in open pasture, 1.28 under shade cloth in open and 1.26 under slats during sunny periods. To compensate for low light quantity and quality, alfalfa had elongated stems and internodes. In open pasture and under cloth in the open, it produced short stems. The mean dry matter yield under trees was 68% of the 30.3 t ha(-1) in open pasture, 56% under cloth and 57% under slats. The slats induced similar morphological responses in alfalfa to those in the agroforestry system. The magnitude of changes had little effect on growth and yield responses. The artificial slatted structure approximated the intermittent light environment and consequent plant responses observed in an agroforestry system.