Woody Stem Production in Alaskan Tundra Shrubs

Abstract

The contribution of woody stems to the aboveground net primary production (NPP) of several species of tundra shrubs was documented at four sites in northern and central Alaska. To measure stem secondary growth, a new method was developed based on the ability to measure the changes in mass per unit length of stem segments or known age. Net accumulation of N and P in woody stems was also estimated, from additional measurements of changes in element concentration with stem age. Results showed that stem secondary growth was an important component of NPP in all species and sites, ranging from 13 to 49% of total aboveground NPP for individual species. The most productive species and sites had the highest proportions of NPP accounted for by secondary growth. Accumulation of N and P in woody stems occurred at slower rates, presumably due to internal reuse of these elements, but still accounted for 5—19% of the annual net aboveground N accumulation and 6—24% of the P accumulation. One implication of these results is a faster rate of N and P turnover in the vegetation than previously believed, and a greater annual nutrient demand. The relative contribution of each age class of stem segments to the total woody biomass was due to an interaction between the number of stem segments surviving to a given age and the annual percentage increase in mass per unit length. Older stem segments also tended to be longer than younger segments.