Basal area in a New Zealand podocarp-broadleaved forest: Are coniferous and angiosperm components independent?

The implications of vertical structure for ecosystem properties in forests has received little attention. Several studies of stratified old-growth forests have shown the basal area of the canopy species to be independent of that of overtopping emergent trees. This phenomenon of "additive basal area" may provide insights into the mechanisms that limit growth and biomass accumulation in old stands. However, it is not clear how widely it occurs in stratified forests. Here, I test for the occurrence of additive basal area in a New Zealand old-growth podocarp-broadleaved forest, by comparing data from 19 plots from within a 10 km² area of highly uniform site characteristics and disturbance history. Total stand basal area (excluding tree ferns) ranged from 63 to 125 m² ha^-1. Basal area of the angiosperm canopy appeared to be little affected by that of the emergent conifers except in the two densest podocarp stands, where a compensatory depression of angiosperm basal area was observed. This angiosperm decline, resulting in a levelling-off of total stand basal area at high conifer densities, presumably results from greatly reduced light transmission beneath a continuous overstorey of podocarps, which have denser crowns than the other emergents for which additive basal area has been reported. Comparisons of results from several different studies suggest that crown traits and longevity of the emergent species are the principal influences on biomass accumulation in overtopped tiers. The apparent independence of limits to biomass accumulation by different strata in some forest types is difficult to reconcile with the hypothesis that nutrient sequestration is the main mechanism that checks growth of ageing stands.