Practice question

Productivity changes differently depending on whether gross primary production, net primary production, or net ecosystem production is measured. During early succession, expanding leaf area and improving soil conditions raise gross production rapidly. Net production can peak in young or intermediate communities because photosynthesis is high while maintenance respiration is still relatively low. As biomass accumulates, respiration by plants, animals, and decomposers increases, and net ecosystem production declines toward approximate balance in a mature system. Gross production may stabilize rather than decline sharply. Thus, the keyed phrase “peaks and then stabilizes” is a simplified description and is most defensible for gross productivity or a broad productivity measure; net productivity more commonly peaks and then decreases toward a lower steady level. A steady increase is implausible because resources and maintenance costs eventually constrain growth, while a continual decrease ignores the early biomass-building phase. The mechanism is the changing balance between photosynthetic capture and respiration as community biomass, leaf area, and decomposer activity develop through succession.