Old-growth forests with large emergent conifers can achieve high levels of basal area. For some species, e.g. Agathis australis in New Zealand, this basal area accumulation has been observed without compensatory reduction in the basal area of the associated angiosperm-dominated forest matrix suggesting overyielding. Such reduction in the basal area of other species would be expected if basal area is assumed to be a measure of biomass, and if there is a limit to the amount of biomass supported on any forest site. The observations of basal area accumulation of emergent conifers not affecting other forest components has been termed ‘additive basal area’, and used to hypothesise that large conifers avoid competition with lower-statured neighbours by vertical stratification and more efficient spatial packing. An alternative explanation is that only sapwood basal area of emergent conifers varies in proportion to basal area of other forest components. We present results of a study in which we determine whether additive basal area can be detected in old-growth Pinus strobus stands in New England. We also measure sapwood basal area and assess whether this is a sufficient explanation for the variation observed.
Results/Conclusions
Stands with P. strobus attained higher total basal areas than stands without this species in environmentally similar forests, and total basal area was strongly related to the presence of P. strobus. Total basal area was also positively related to greater mean size difference between P. strobus and other species. Stands varied markedly in terms of the basal area of other species, but overall there was a limited reduction in the basal area of other species as P. strobus basal area increased. When these calculations were repeated on the basis of sapwood basal area only, however, sapwood basal area was similar on most stands and not related to P. strobus presence or abundance. We suggest that P. strobus in US northeastern forests is another long-lived emergent pioneer that promotes the ‘additive basal area’ effect in old-growth forests. Size difference between the two forest components reducing light competition may operate here, but a more parsimonious explanation is that much of this accumulated basal area in giant trees is heartwood that does not conduct water, does not support living biomass, and does not contribute to limiting basal area. This suggests the importance of sapwood basal area as an indicator of stand biomass.