Limits on aboveground net primary production, leaf area, and biomass in vegetational zones of the Pacific Northwest

Samples of mature vegetation from 8 of 12 major vegetational zonesin Oregon and Washington, representing about 80% of the area of the twostates, were studied along a latitudinal transect from the Pacific Coastto the east slopes of the Cascade Mountains. Six stands were in forestzones, one in woodland and one in the shrub-steppe. Aboveground netprimary production (NPP, estimated as the sum of stem, branch, and foliageproduction) ranged from 0.3 to 14.7 t ha-1 yr-1, aboveground biomass from3 to 1500 t ha-1, and area of all sides of leaves from 1 to 40 ha ha-1,with minimums in the shrub-steppe zone and maximums in the coastal forestzone. Average day air temperatures were less than -2°C only 2% of thewinter at the coast, contrasted to 87% in the shrub-steppe. Althoughannual precipitation ranged from 20 cm in the shrub-steppe to 260 cm atthe coast, it was a relatively poor predictor of stand structure andproduction.
Maximum leaf areas were closely related to a simple growing seasonwater balance in seven vegetational zones. In the subalpine conifer
zone, leaf area appeared limited by temperature. Of the water balancecomponents, evaporative demand alone accounted for 95% of the variancein leaf area. Biomass and NPP increased linearly with leaf area up toa leaf area of 20 ha ha-1. Biomass continued to increase with
increasing leaf areas. NPP was also linearly related to minimum Januarytemperatures. Except in the coastal forest zones, NPP for any givenleaf area was less than maximum values reported for other mature systemselsewhere in the world.