Target species identity is more important than neighbor species identity

This study investigates how the ability to overtop other saplings and the ability to capture most of the incident light together determine interspecific competition in young and dense self-thinning stands. To this end we measured crown positions and stratification, and quantified the effect of total foliage mass and foliage mass density of neighboring trees on the relative crown depth (RCD) of target trees. Field measurements were done on naturally established saplings in two large forest gaps of different age (5- and 15-year-old) in The Netherlands, and consisting of four species differing in shade tolerance. Results indicated a vertical stratification between species which was related to inherent species morphologies, and not growth rates. Foliage densities did not diverge much, but total foliage mass and crown size were bigger for more shade-tolerant species. The neighbor species had an effect on the RCD of a target tree, but the identity of the target tree species itself explained far more of the observed variation in RCD. On this basis we conclude that (1) the studied species do not stratify according to light demand, nor is foliage density related to light demand. And (2) a stronger competitive effect is not related to having a higher foliage density per unit crown volume, and competitive effects are more strongly related to the target species' identity while competitors are equivalent. In translating the results to stand development, we predict that based on inherent morphology and light demand, Pinus sylvestris will be competitively excluded from the studied communities, underlining the importance of morphology in competition