Nutrient effects on stand structure, resorption efficiency, and secondary compounds in Everglades sawgrass

Long-term studies along a 30-yr nutrient-enrichment gradient in the norther n part of the subtropical Everglades fen allowed us to assess the effects o f nitrogen (N) and phosphorus (P) additions on plant community structure an d chemical qualities of wetland plants. Areas in the highest P-enriched zon es (>1000 mgP/kg), once dominated by open-water sloughs and surrounding mon ocultures of sawgrass (Cladium jamaicense, a stress-tolerant low-nutrient-s tatus species), are now dominated by cattail (Typha domingensis, a competit ive, high-nutrient-status species). Areas of moderate (750-500 mg/kg) and l ow (<500 mg/kg) P soil concentrations have maintained their original plant composition. Analysis of nutrient-use efficiency indicates that sawgrass is highly efficient in nutrient resorption and nutrient proficiency, but this efficiency decreases at high soil P concentrations. Both indices indicate that suboptimal concentrations of P, which limit growth and optimize retent ion of P within the plant, exist in the Everglades. This information, when coupled with the low levels of P found in the plants, soil, and pore water of the unenriched portions of the Everglades, indicates that this ecosystem is extremely P limited. High molar N:P leaf ratios suggested P limitations for sawgrass at unenriched sites, but not in highly enriched areas. Interm ediate P loadings resulted in either N or P limitations. Increased leaf P c oncentrations correspond to decreased phenolic content of sawgrass leaves, suggesting that this species' metabolic growth response follows the carbon- nutrient balance hypothesis of decreased production of phenolics with incre ased nutrient supply. Concomitant with decreased foliar phenolic content at high P enrichment was an increase in herbivory and fungal infections.