Estimation of the primary productivity of Spartina alterniflora using a canopy model

A comprehensive canopy productivity model was built to study the productivity of a primary salt marsh grass, Spartina alterniflora, in Georgia, USA. The canopy model was unique in employing plant demographic data to reconstruct canopy profiles and dynamics, which showed many growth processes that are otherwise difficult to discern in the held. By linking canopy dynamics and leaf photosynthesis, the net total primary productivity of S. alterniflora in a Georgia salt marsh was estimated to be 1421, 749, and 1441 g C m(-2) yr(-1) for the tall, short, and N-fertilized short populations respectively. These estimates are reasonable in terms of the physiological capacity of S. alterniflora and well below the range of 3000-4200 g C m(-2) yr(-1) as reported by some recent harvest studies, Our detailed analysis suggested the net total productivity of S. alterniflora might be greatly overestimated in the past. This is mainly because of 1) failure to consider the translocation of photosynthate between aboveground and belowground pans, and 2) possible overestimates of belowground production. We estimated the net belowground production to be 872, 397, and 762 g C m(-2) yr(-1) for the tall, short, and N-fertilized populations respectively. After receiving nitrogen fertilizer, the net leaf carbon fixation in the short population increased from 1489 to 2487 g C m(-2) yr(-1); and our simulation showed the contribution of elevated leaf N to this increase was small, 21%, compared with that of increased leaf area, 79%. Both tall and short populations allocated ca 48-49% of their annual gross leaf carbon fixation to belowground structures. Nitrogen enrichment caused more allocation to aboveground parts in the short population, mainly for increasing leaf area. The canopy model assumed that there was no leaf photosynthesis under tidal submergence, but if this assumption was relaxed, then leaf carbon fixation might increase 7-13% for different S. alterniflora populations. Although this research focused only on a salt marsh species, our general approaches, especially the coupling of leaf physiology with the reconstructed canopies, should be applicable to the study of production processes of many other plant populations.