Response of emergent macrophytes to experimental nutrient and salinity additions

1. Nutrient additions often result in species dominance/compositional changes in wetland ecosystems, but the impact of nutrients may be constrained by different salinity levels. Wetlands of northern Belize, distributed along a salinity gradient, are strongly phosphorus-limited and dominated largely by three species of emergent macrophytes: Eleocharis cellulosa Torr., Cladium jamaicense Crantz and Typha domingensis Pers. 2. We conducted a mesocosm experiment to assess changes in growth characteristics [biomass allocation, plant height, relative growth rate (RGR), rhizome length] and nutrient uptake of these three species in response to simultaneous changes in levels of nutrients (nitrogen and phosphorus) and salinity. 3. The growth characteristics of Typha and Eleocharis responded positively to N and especially P addition, whereas the growth response of Cladium was largely insignificant. The RGR of Typha increased under P addition, while RGR of Eleocharis increased with N and decreased with salinity addition. Nutrient addition increased the rhizome number of both Typha and Eleocharis. However, plasticity in rhizome length was observed only in Typha, which showed increased rhizome length at medium and high P. 4. Salinity decreased plant height and shoot and root biomass of Cladium and Eleocharis, while in Typha it reduced only height. Rhizome number and length were decreased only in Eleocharis. 5. Both medium and high P additions increased tissue P content in all three species, but Eleocharis accumulated significantly more P than Cladium and Typha. Nitrogen additions increased tissue N content in Cladium and Eleocharis, but not in Typha. 6. Cladium exhibited strong morphological constraint and behaved as a stress-tolerator that was well adapted to low nutrients. Typha - characterized by its plastic, opportunistic guerrilla growth strategy, fast and efficient space occupancy, and rather wasteful nutrient management - behaved as a typical competitor. Eleocharis responded rapidly to nutrients but displayed limited rhizome plasticity, and its growth was affected at higher salinity. 7. According to recorded traits, we hypothesize that P input into wetlands will result in expansion of Typha, leading to competitive exclusion of both co-occurring species. The only conditions allowing coexistence of all three species are those limiting vertical and horizontal growth of Typha: low P and higher salinity. To ensure the stability of Belizean wetlands, the maintenance of oligotrophic status is therefore crucial.