The impact of elevated atmospheric CO2 on self-thinning and management of loblolly pine

Abstract

The self-thinning rule for single species, even-aged forest stands dictates that an increase in average tree size (e.g., mean stem volume) must be met with a corresponding decrease in stand density (e.g., trees per hectare) due to competition for light. It is widely accepted that, when plotted on a log-log scale, the relationship between average tree biomass and stand density is linear with a slope of -3/2, regardless of species. The research that first depicted the self-thinning line using average stem volume for loblolly pine in the Piedmont of North Carolina took place in the 1980s, when the atmospheric concentration of CO2 was 20% lower than it is today. The Duke Free-Air CO2 Enrichment (FACE) experiment demonstrated that elevated atmospheric CO2 (200 ppm above current CO2 levels) increased shade tolerance of loblolly pine as foliage became photosynthetically more efficient at lower levels of light. This suggests a potential shift in loblolly pine’s self-thinning line: the same stand density as before may now support a larger average tree size. This project seeks to determine whether the self-thinning line of loblolly pine has shifted upwards and explores potential management implications.