Does site quality affect the additive basal area phenomenon? Results from Chilean old-growth temperate rainforests

Complex old-growth forests with large emergent trees may support more basal area than those without these trees. The so-called additive basal area phenomenon occurs when the presence of these emergent trees does not affect the basal area of the canopy trees, i.e., the basal areas of emergent and canopy trees add up. We studied two old-growth forests in south–central Chile with similar species composition and structure but contrasting soil quality. We sampled fifty-seven 1000 m2 plots in a relatively high-quality site (Quillaipe) and sixty-five 1000 m2 plots in a relatively low-quality site (Yaldad) and evaluated total stand basal area, basal area of emergent trees, basal area of canopy trees, and the influence of the basal area of emergent trees on total and canopy basal areas. On average, Quillaipe accumulated 50% higher basal area than Yaldad. The additive basal area effect only occurred on the high-quality site, Quillaipe. A more detailed analysis showed that canopy associates were differentially affected in terms of basal area depression with increasing basal area of emergent trees in each site. The relative competitive ability of canopy species, as affected by their ecological traits and geographical distribution, presumably explain if the basal area of individual species or functional groups are depressed by the basal area of emergents. Above- and below-ground resource partitioning may be decisive mechanisms affecting the occurrence of additive basal area accumulation in forest stands.