Branch growth and biomass allocation in Abies amabilis saplings in contrasting light environments

Aboveground biomass allocation, and height andbranch growth were studied in saplings of the shade-tolerantconifer, Abies amabilis Dougl. ex Forbes growing in largeopenings and in the understory of an old-growth forest inwestern Oregon. The presence of annual overwinteringbudscale scars was used to infer extension growth histories;annual growth rings in branches and stems were used in com-bination with extension histories to compute partitioning ofnew biomass among leaves, branches and stems. Saplingsgrowing in large gaps had conical crowns, whereas understorysaplings had umbrella shaped crowns as a result of muchgreater rates of branch extension than stem extension. Under-story saplings grew slowly in height because of low rates ofbiomass production and low allocation of biomass to stemextension. About 40% of new biomass was allocated to foliagein both groups, but understory saplings allocated more of theremaining growth increment to branches and less to stem thandid saplings growing in large gaps.
These results differ from the patterns observed in shade-tol-erant saplings of tropical forests, where allocation to foliageincreases with shading and branch allocation is much lowerthan observed here. This difference in allocation may reflectmechanical constraints imposed by snow loads on the ever-green A. amabilis crowns, particularly on flat-crowned under-story saplings.
Keywords: allometrv, branch growth, conifer, height growth.