Vertical structure and basal area development in second-growth Nothofagus stands in Chile.
Some studies have shown that mixed forest stands can develop more biomass or basal area than pure stands when the component species occupy different strata. It has been predicted that such additive effects are most likely in moist habitat sites where size-asymmetric competition controls stand development. In south-central Chile, secondary succession often gives rise to vertically stratified mixed forest stands, with a Nothofagus overstorey overtopping lower tiers dominated by more shade-tolerant taxa. We examined the relationship between the degree of understorey development and total basal area accumulated by 83 second-growth Nothofagus stands aged 40-70 years. Basal area is defined as the cross-sectional area at breast height of all trees in a stand, per unit land area. When age, overstorey leaf habit and geographical location were controlled by ANCOVA, stands with a heavy understorey (>6 m2 ha-1) yielded >25% more total basal area on average than stands with little or no understorey. The average basal area of the Nothofagus component did not differ significantly between stand structure categories. The total basal area of stands dominated by evergreen N. dombeyi was nearly 50% higher on average than that of stands of similar age dominated by deciduous N. obliqua and/or N. nervosa, reflecting denser stockings in the former as a result of allometric differences. Although the percentage of total stand basal area contributed by the understorey was higher on deciduous-dominant plots, mean absolute basal area of understorey associates was similar beneath evergreen and deciduous Nothofagus overstoreys. Synthesis and applications. The results broaden the evidence for additive effects in mixtures of species with complementary light use, suggesting that no significant advantage to overstorey wood production will be gained by eliminating a woody understorey in Nothofagus second-growth stands on mesic sites. Despite the minor contribution of the understorey to merchantable timber volume, other benefits and ecosystem services associated with subcanopy species in these stands can be regarded as additive to benefits obtained from the Nothofagus overstorey. Our findings are also consistent with the proposal that size-asymmetric competition controls stand development after canopy closure on mesic sites.