ESTIMATION OF THE PRIMARY PRODUCTIVITY OF SPARTINA-ALTERNIFLORA USINGA CANOPY MODEL

A comprehensive canopy productivity model was built to study the produ ctivity of a primary salt marsh grass, Spartina alterniflora, in Georg ia, USA. The canopy model was unique in employing plant demographic da ta to reconstruct canopy profiles and dynamics, which showed many grow th processes that are otherwise difficult to discern in the held. By l inking 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 reasona ble 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 rec ent harvest studies, Our detailed analysis suggested the net total pro ductivity of S. alterniflora might be greatly overestimated in the pas t. This is mainly because of 1) failure to consider the translocation of photosynthate between aboveground and belowground pans, and 2) poss ible overestimates of belowground production. We estimated the net bel owground production to be 872, 397, and 762 g C m(-2) yr(-1) for the t all, short, and N-fertilized populations respectively. After receiving nitrogen fertilizer, the net leaf carbon fixation in the short popula tion 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 sh ort populations allocated ca 48-49% of their annual gross leaf carbon fixation to belowground structures. Nitrogen enrichment caused more al location to aboveground parts in the short population, mainly for incr easing leaf area. The canopy model assumed that there was no leaf phot osynthesis under tidal submergence, but if this assumption was relaxed , then leaf carbon fixation might increase 7-13% for different S. alte rniflora populations. Although this research focused only on a salt ma rsh species, our general approaches, especially the coupling of leaf p hysiology with the reconstructed canopies, should be applicable to the study of production processes of many other plant populations.