Photon flux partitioning among species along a productivity gradient of an herbaceous plant community

Abstract

Summary We studied light partitioning among species along the natural productivity gradient of herbaceous vegetation with an above‐ground dry mass of 150–490 g m−2. The aim was to investigate how the light capturing ability per above‐ground biomass and leaf nitrogen changes in an entire community and to reveal whether different species respond similarly to changes in soil conditions and competition. Species becoming dominant at high soil resources have intrinsically low leaf area ratios (LAR) and lower tissue nitrogen concentration, and hence relatively high nitrogen use efficiency. These traits lead to dominance when soil resources allow rapid growth so that benefits arising from the ability to locate leaves above neighbours and thereby increasing asymmetry of competition, become more crucial. In contrast to our expectations, above‐ground efficiency of nitrogen use on the community level (aNUE) increased along the productivity gradient. Species level nitrogen use efficiency was unaffected by variation in site productivity; the increase in community aNUE was solely as a consequence of changes in species composition. Light absorption per unit of above‐ground mass, ΦM, declined significantly at the community level and also in most species, indicating that light use efficiency increased with increased site productivity and LAI. Light absorption per unit of leaf nitrogen, ΦN, as an indicator of the ratio NUE/LUE showed no clear pattern on the community level because both NUE and LUE tend to increase with increased productivity. At the species level, ΦN tends to decrease because NUE did not change with stand productivity. Some subordinate species responded by enlarging their LAR to increased competition. Additionally, these species were the most responsive in their leaf chlorophyll/nitrogen ratio to changes in light conditions, which shows that physiological plasticity is important for species that are unable to compete for light with the ability to position their leaves above those of other species. This study shows how plasticity in above‐ground growth pattern and nitrogen allocation differs between species with respect to increased soil fertility and competition, leading to distinctive strategies of survival. Light partitioning analysis reveals that increased competition for light, resulting in changes in species composition, is the key factor that leads to decoupling of species and community level acclimation.