In Odum’s model, the ratio of gross productivity to biomass:
The ratio of gross primary productivity to standing biomass, GPP/B, generally decreases during succession in Odum’s ecosystem-development model. Pioneer communities possess little biomass but consist largely of rapidly growing, metabolically active tissues, so annual carbon fixation can be large relative to the amount of living material present. As succession proceeds, woody stems, roots, supportive tissues, and long-lived organisms accumulate. Total gross production may rise or stabilize, but standing biomass increases faster, and a greater fraction of captured energy supports maintenance respiration rather than new growth. Consequently, each unit of biomass turns over or produces new carbon more slowly. This trend parallels a shift from rapid-growth, high-turnover strategies toward resource conservation and persistence. It is not an immutable rule: disturbance, climate, nutrient supply, and community composition can alter the ratio. Nor does a decreasing GPP/B ratio mean mature forests photosynthesize little in absolute terms. It means their large biomass produces less per unit biomass than a young community. That denominator effect is the key reason the ratio falls with succession.
Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 20