BIOMASS AND NUTRIENT ALLOCATION IN DOUGLAS-FIR AND AMABILIS FIR SEEDLINGS - INFLUENCE OF GROWTH-RATE AND NUTRITION

Allocation of biomass and nutrients to shoots and roots was followed f or three years in fast and slow growing populations of Douglas-fir (Ps eudotsuga menziesii (Mirb.) France), a fast growing pioneer species, a nd amabilis fir (Abies amabilis Dougl. ex J. Forbes), a slow growing s hade-tolerant species. Seedlings were grown for three seasons in five nutrient treatments containing varying proportions of nitrogen and pho sphorus (N:P). In both species, growth was greatest in the 250:20 N:P treatment followed by the 100:60 and 100:20 treatments. Vector analysi s showed that, in both species, relative to the 100:20 treatment, seed lings in the 20:20 treatment were N deficient and seedlings in the 100 :4 treatment were P deficient, i.e., where deficiency is defined to me an that an increase in nutrient supply increases nutrient content, nut rient concentration and plant dry weight. Seedlings in the 100:60 trea tment had a higher P content than seedlings in the 100:20 treatment bu t the same dry weight, indicative of what Timmer and Armstrong (1987) termed luxury consumption. No nutrient retranslocation was observed in either species until the third growing season. In Douglas-fir, the gr eatest percentage of nutrients was exported from one-year-old shoots b etween May and July of the third growing season to support new growth. The total amount and percent of nutrients retranslocated was higher i n Douglas-fir than in amabilis fir. Amabilis fir seedlings also export ed N and P from older shoots, but this was later partially replenished . In both species, P retranslocation was greatest in treatments with a high N:P ratio. Nitrogen retranslocation was greatest in amabilis fir seedlings in treatments with a low N:P ratio, and greatest in Douglas -fir seedlings in the 250:20 and 100:60 treatments. Potassium retransl ocation was correlated with seedling size. Douglas-fir retranslocated more of its shoot N reserves into new growth at the expense of older n eedles when soil fertility was high and sinks were strong. Otherwise, both species preferentially translocated the elements in short supply. Thus, retranslocation varied with the ecological characteristics of s pecies, the relative availability of soil nutrients and sink strength.