Environmental limits on aboveground net primary production, leaf area, and biomass in vegetation zones of the Pacific Northwest

Mature vegetation from eight of the 12 major vegetation zones in Oregon and Wash-ington was sampled along a transect from the Pacific Coast to the east slopes of the Cascade Moun-tains. Six stands were in forests. one in woodland, and one in the shrub-steppe. Aboveground-over-story net primary production NPP. estimated as the sum of annual stem. branch, and foliageproduction) ranged from Maximum leaf area index. biomass. and NPP were all strongly related both to a simple index ofgrowing season water balance and to mean minimum air temperatures in January. In the subalpineconifer zone. though. cold winter temperatures apparently have a stronger influence than summerwater availability. Of the water balance components. evaporative demand alone could account for>90% of the variation in leaf area index. Although annual precipitation ranged from 20 cm in theshrub-steppe to 260 cm at the coast. it was a relatively poor predictor of stand structure and produc-tion. Biomass and NPP increased linearly up to a leaf area of ~30 ha/ha: above this point. biomasscontinued to increase while NPP decreased. Except in the coastal forest zones. NPP was less thanmaximum values reported for other mature systems elsewhere in the world for the same range in leafarea indices. Compared to other forested regions of the temperate zone with the same NPP. thesesystems receive more annual precipitation. and average twice the basal area and biomass.
Key words: biomass; evaporation: forest: leaf area index: net primary production: Oregon: pre-cipitation: shrub-steppe: temperature; vegetation zone: water balance.