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Asymmetric competition for light varies across functional groups

Aims Asymmetric competition for light may depress the growth rates (GRs) to different extents for different-sized tree individuals. Various responses of different functional groups to light availability result that tree individuals of different functional groups may experience different competition intensities, e.g. canopy and deciduous species grow faster and demand more light than understory and evergreen species. In this study, we estimated the effects of asymmetric competition for light using individual GRs and explored the effects of asymmetric competition on growth among different functional groups (e.g. canopy vs. understory species and deciduous vs. evergreen species). Methods We measured growth in circumference to determine the radial increments of a total of 2233 stems with diameter at breast height ≥ 5.0 cm in a permanent plot (140 × 80 m2) located in a typical evergreen and deciduous broadleaved mixed forest on Mt Shennongjia, China. All of the measurements were carried out at ~6-month intervals every April and October from 2012 to 2014, and biomass of each individual was calculated based on its diameter and species-specific allometry. We then calculated GRs of annual biomass growth (growth between October and the next October). Considering the hypothesis that asymmetric competition for light among trees of different sizes may result in a steeper allometric growth curve with increasing tree size, we further divided the sampled trees into different subsets according to their height, at intervals of 1 m, and then fitted the scaling relationship between the logarithm of the biomass GR (logGR) and the logarithm of diameter (logD) for each height class using standardized major axis regression. Finally, we used simple linear regression to test whether the scaling exponent was related to tree height. The above analyses were conducted for the annual growth of all tree species, canopy species, understory vs. treelets species and deciduous vs. evergreen species. Important findings We observed a concave curve for the relationship between logGR and logD with an increase in the scaling exponent between logGR and logD with increasing tree height. This pattern held for the annual growth of canopy species and deciduous species but not for the annual growth of understory species, treelets or evergreen species. These results suggest that asymmetric competition for light is more important in regulating the GRs of the fast-growing species, such as canopy species and deciduous species, than those of shadetolerant species, such as understory species, treelets and evergreen species.