The effects of long-term, partial shading on growth and photosynthesis in Pinus radiata D. Don trees

Abstract

Two 8-year-old Pinus radiata trees growing in a dry-land plantation forest were subjected to a partial shading treatment for 2 years in order to investigate the responses of photosynthesis and tree growth to a long-term reduction in illuminated leaf area. The lower 60% of the crown of shaded trees was surrounded by 50% neutral density shade cloth. This reduced radiation absorption by 13% for the trees as a whole compared to the control trees during the first year, while modelled annual whole-canopy photosynthesis was 9% lower, and was strongly regulated by seasonal soil water availability, temperature and irradiance. The shaded trees responded by increasing carbon partitioning to the branches, at the expense of the stem. As a result, basal area growth of the shaded trees was almost 50% lower than that for control trees. In the second year, the effects of the shade cloth on absorbed radiation, photosynthesis and growth were reduced. This occurred because of needle abscission below the shade cloth and rapid branch and leaf area growth above the shade cloth, while the lower foliage on the control trees became increasingly self-shaded. Allocation to stem growth increased in the shaded trees, resulting in similar relative basal area increment between shaded and control trees. The difference in modelled whole-canopy photosynthesis was smaller in the second year. This was partly due to the reduced effect of the shade cloth, but also to a significant summer drought, which severely reduced stomatal conductance, photosynthesis and stem growth in both shaded and control trees. Seasonal root-zone water deficits at this site reduce annual carbon assimilation by around 40%, and increase the proportions of carbon assimilation occurring in spring, autumn and winter.