Differential responses of yield and shoot traits of five tropical grasses to nitrogen and distance to trees in silvopastoral systems

Abstract

Context Light intensity and nitrogen availability are important factors influencing the growth of C4 forage species. Trade-offs may occur in the adaptive responses of species to shading and nitrogen inputs, and functional shoot traits can help to explain the consequences of these responses for species performance. Aims Our objective was to gain understanding of the mechanisms involving shoot traits of grasses that determine above-ground dry matter yield (DMY) when resources, light and nitrogen all vary. Methods Five C4 perennial forage grasses were grown in six shading conditions (full sunlight vs five positions between Eucalyptus dunnii rows) with two nitrogen levels (0 vs 300 kg N ha−1 year−1) and clipped when the canopy reached 95% light interception. Path analysis was used to explore the relationship between DMY, shading levels, nitrogen nutrition index and shoot traits. Key results Yield increased between 126 and 569 g dry matter m−2 with nitrogen fertilisation. Plant nitrogen concentration was the most important predictor of DMY. Increased shading reduced DMY by 6.94–12.5 g dry matter m−2 for each 1% increase in shading. DMY was also modulated by shoot traits such as specific leaf area, sheath length and leaf area index (via leaf area and tiller density), but with different responses according to species. Conclusions The five species compared adopted different mechanisms involving shoot traits, revealing different strategies to cope with changes in light and nitrogen availability. Implications Agroforestry practitioners may want to choose forages that are more likely to maintain biomass yield as trees grow.