Evidence that trees do not shift carbon allocation from stem support to foliage under competition

Do trees adjust support costs to the stem in order to maintain or boost allocation to foliage to compete for light? This question was addressed with data collected from three, long-term studies investigating the growth effects of controlled levels of competition for Alnus rubra, Pseudotsuga menziesii, and Pinus resinosa. Costs and benefits of marginal height were defined as the stem volume and foliage added when trees grew 1 m in height, respectively. Effects of competition effects on the distribution of these variables within tree populations were evaluated with growth dominance indexes (Gdv/dh and GdAl/dv, respectively). Competition effects on the distribution of relative height growth (Gdg) were also evaluated to verify mode of competition. The hypothesized simple tradeoff between marginal height cost and marginal height benefit was not supported with these data. Instead, Gdv/dh and GdAl/dh were linearly related for all three species. The values of Gdh were generally positive for all species supporting asymmetric competition within these plots. The strength of support for this hypothesis varied by species, however, with the shade intolerant A. rubra providing the strongest support and the more shade tolerant P. menziesii providing the least support. The age-adjusted values of P. menziesii of Gdh were only significantly greater than zero in the unthinned, control plots. The results suggest that competition effects on marginal height benefit translate to marginal height cost through biomechanical relationships between crown size and structure and stem dimensions. Competitive effects on stand dynamics can therefore be linked to a basic tree morphology that scales with height. Realized additions of stem volume and foliage per growing season are subsequently determined by the distribution of relative height growth.