Question: Does overyielding of tree species mixtures in vertically stratified forests depend on complementary light use?

Location: Andes of south-central Chile.

Methods: Basal area data were obtained from 80 circular plots distributed regularly throughout old-growth stands with an emergent Nothofagus dombeyi tier over a canopy composed mainly of Laureliopsisphilippiana and Saxegothaea conspicua. Radial growth was measured from cores obtained from trees at the centre of each plot. The effects of competition on growth were evaluated through a competition index (CI) based on distances to and diameters of the two nearest neighbours.

Results: Overall, basal area of the canopy species was only weakly affected by the number of N. dombeyi per plot, and with basal area of N. dombeyi. However, the two main canopy species responded differently: whereas basal area of S. conspicua was negatively correlated with that of N. dombeyi, that of L. philippiana showed no response. Radial growth of S. conspicua was negatively correlated with CI calculated from canopy trees and more weakly so from emergent N. dombeyi. In contrast, radial growth of L. philippiana was not affected by competition with either canopy or emergent neighbours.

Conclusions: Results indicate that emergent N. dombeyi tend to depress growth and basal area of S. conspicua, but not of the more shade-tolerant L. philippiana. This supports the proposal that enhancement of wood production in stratified mixtures will be greatest when component species have strongly contrasting light use traits.