Modeling light availability for crop strips planted within apple orchard

Rational design and management of the crop strips structure within fruit orchards are essential to make use of the economic and ecological advantages of intercropping but avoid the negative effects of resource competition on the fruit production. A better understanding of the light transmission processes could provide basis for quantifying resource competition and growth dynamics in the agroforestry system. Modeling work was combined with field experiment in this study to investigate light transmission in an apple tree (Males pumila M.) and cocksfoot (Dactylis glomerata L.) intercropping system. The experiment was conducted in 2016 and 2017 in a plantation of apple at a spacing of 4 m x 4 m on the Loess Plateau of China. Three floor managing patterns were applied: clean tillage (CT), 2.4-meter-wide cocksfoot strips set up between tree rows that harvested more frequently to maintain a low coverage (LC), and cocksfoot strips with a greater coverage (GC). The light model simulates light transmission in the tree crown with a geometrical model that considering the canopy spatial heterogeneity, and describes light extinction in the crop strips with a strip-path radiation transmission model which is suitable for strip-path canopy structures. Results showed that the model well predicted fraction of incident light over and transmitted through the cocksfoot strips with determination coefficient of 0.707 and 0.892 respectively and root mean square error of 0.096 and 0.037. Averaged over the two seasons, light interception by cocksfoot were 439.1 and 504.7 MJ m(-2) in LC and GC respectively, accounted for 23.1% and 26.5% of the total incoming light. Light use efficiency of cocksfoot in LC was far greater than that in the GC in both seasons. Planting cocksfoot showed no significant adverse effects on apple production, 6.32 and 5.67 t ha(-1) additional dry forage were produced in the LC and GC plots respectively in 2016, the production were only 3.02 and 2.07 t ha(-1) in 2017 as limited by water availability. Simulations with information on apple tree orchards of different age showed that seasonal mean fraction of light interception by cocksfoot under 3-10 years old apple trees varied from 0.41 to 0.29 for GC and from 0.36 to 0.25 for LC treatments, but the fraction became < 0.15 for 15 years and older orchards. This study provides basics for quantifying light and other resource competition in tree and crop intercropping systems and gives insights into floor management for rain-fed apple orchards on the Loess Plateau area of China.