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Intercropping, the cultivation of two or more crop species simultaneously in the same field, has been widely practiced by smallholder farmers in developing countries and is gaining increasing interest in developed countries. Intercropping can increase the yield per unit land compared to sole cropping. The yield advantage of intercropping is often assessed using the land equivalent ratio (LER). LER may be interpreted as the relative area required by sole crops to produce the same yields as achieved in a unit area of intercrop. LER>1 means intercropping is more efficient in land use than sole cropping. A large variation of LER has been found in the literature. However, few studies attempted to investigate reasons for this variation in LER. This thesis aims to reveal how temporal niche difference, crop type combination, and agronomic practices affect LER, productivity and interspecific interactions in annual intercrops. LER increased with temporal niche differentiation according to our meta-analysis of literature data. This positive relationship was valid in mixtures of C3 and C4 species but not in C3/C3 mixtures. Application of N fertilizer in intercropping decreased LER when the intercropped species were sown and harvested simultaneously. However, reducing overlap in growing periods of the intercropped species mitigated the negative effect of N fertilizer on LER. A functional-structural plant (FSP) model was developed to investigate the interplay between temporal and spatial complementarity and plant traits in mixed plant systems. The results showed that complementarity of light use in time and space likely determine productivity of species mixtures. The early-sown plants benefited from later sowing of the late-sown plants due to the relaxed competition for light from the late-sown plants until a plateau when the growth durations of the intercropped species overlapped less than 50% of the total growth period of the intercrop. By contrast, the late-sown plants suffered a great reduction in biomass due to the competition for light from the early-sown plants especially at moderately delayed sowing time and when spatial arrangement of the intercrop allowed strong interspecific competition. The shading effect from the early-sown plants on the growth and productivity of the late-sown plants was smaller if the late-sown plants had the potential to grow tall and if it had a high maximum CO2 assimilation rate. A meta-analysis of relative yields in cereal/legume intercrops was conducted to investigate the relationship between performance of intercropped species and management. Earlier sowing of one species increased its competitiveness towards the other species while later sowing decreased it. Application of N fertilizer enhanced the competitiveness of a cereal towards a legume, resulting in overall low productivity of legumes in intercrops. However, sowing legumes earlier than cereals mitigated the negative effect of N on productivity of legumes. Overall, this thesis shows that the complementary resource use resulting from plant traits diversity and temporal and spatial arrangements of plant mixtures is one of the key factors for high productivity of intercropping. This finding strengthens the basis for further research on the possible contribution of species diversity in agricultural systems to meeting the demand for food and other agricultural products while mitigating the environmental impacts of modern agriculture. |
