Old‐Growth Pseudotsuga menziesii Communities of a Western Oregon Watershed: Biomass Distribution and Production Budgets

Living biomass, organic matter distribution, and organic matter production budgets were determined for plant communities of a small watershed dominated by 450—yr—old Pseudotsuga menziesii (Mirb.) Franco forests. Dominant trees in the communities were large, up to 175 cm diam and 80 m tall. Aboveground tree biomass of the various communities ranged from 491.8—975.8 tonnes/hectare, total aboveground living biomass ranged from 500.4—982.5 t/ha, total leaf biomass ranged from 10.4—16.3 t/ha and total organic matter accumulations ranged from 1,008.3—1,513.7 t/ha. Total tree biomass in the various communities was more related to past mortality than habitat differences. Biomass of standing dead trees and fallen logs was generally inversely related to aboveground tree biomass. Amounts of woody detritus were large, ranging from 59.0—650.6 t/ha or 4.3%—43.0% of total community organic accumulation. Aboveground tree biomass increment was negative in all communities, ranging from —2.9 to —6.2 t/ha. Positive increment by shrubs and trees <15 cm dbh, produced overall aboveground biomass increment —2.5 to —5.0 t/ha. Mortality averaged 1% of standing biomass. Aboveground net primary production in the various communities ranged from 6.3 to 10.1 t°ha — 1 ° — 1 and was roughly proportional to standing biomass. Net primary production consisted entirely of detritus. Total community autotrophic respiration ranged from 102.9—203.7 t°ha — 1 °yr — 1 of which °70% was by foliage. Gross primary production ranged from 111.2—216.8 t°ha — 1 °yr — 1 of which only 6.0%—7.9% was net primary production. Net ecosystem production ranged from 0.12—5.6 t°ha — 1 °yr — 1 , entirely as an accumulation of woody detritus on the soil surface. Available evidence indicates larger peak biomass in seral P. menziesii than in climax Tsuga heterophylla forests. These communities may be in the process of declining from seral peak to steady—state climax biomass.