Environmental Limits on Aboveground Net Primary Production, Leaf Area, and Biomass in Vegetation Zones of the Pacific Northwest

Abstract

Mature vegetation from eight of the 12 major vegetation zones in Oregon and Washington was sampled along a transect from the Pacific Coast to the east slopes of the Cascade Mountains. Six stands were in forest, one in woodland, and one in the shrub—steppe. Aboveground—over—story net primary production) (NPP, estimated as the sum of annul stem, branch, and foliage production) ranged from <1 to 15 Mg°ha—1°yr—1, aboveground biomass from 3 to 1500 Mg/ha, and area of all sides of leaves from 1 to 47 ha/ha; minima were in the shrub—steppe zone and maxima in the coastal forest zone. Maximum leaf area index, biomass, and NPP were all strongly related both to a simple index of growing season water balance and to mean minimum air temperatures in January. In the subalpine conifer zone, though, cold winter temperatures apparently have a stronger influence than summer water 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 the shrub—steppe to 260 cm at the coast, it was a relatively poor predictor of stand structure and production. Biomass and NPP increased linearly up to a leaf area of °30 ha/ha; above this point, biomass continued to increase while NPP decreased. Except in the coastal forest zones, NPP was less than maximum values reported for other mature systems elsewhere in the world for the same range in leaf area indices. Compared to other forested regions of the temperate zone with the same NPP, these systems receive more annual precipitation, and average twice the basal area and biomass.