Effects of Nitrogen Additions on the Vertical Structure of a Constructed Cordgrass Marsh

Because tall vegetation can enhance habitat quality in intertidal wetlands, we examined the effects of N fertilization on the height growth of a constructed cordgrass (Spartina foliosa) marsh in San Diego Bay, where plants are short and soil N low. We varied the duration (therefore also the quantity; 30 g N/m2 every month for 1, 2, 4, or 6 mo) and timing (30 g N/m2 in March, April, June, or August) of N additions during 1993. By fertilizing a second set of replicates for the 6-mo treatment during 1994, we were able to examine the persistence of effects from 1993 vs. the effects of amendments in both years. Traditional canopy response measures (total stem length, maximum height, foliar N) increased most with the greatest duration/quantity of additions in 1993. All duration and timing treatments produced canopies with >100 stems/m2 and ≥30 stems/m2 taller than 90 cm, considered suitable for nesting by the endangered Light-footed Clapper Rail (Rallus longirostris levipes). However, without additional fertilizer in 1994, the previous year’s canopy responses were not retained, suggesting insufficient storage of N in belowground tissues. Only plots fertilized in both years continued to produce ≥30 tall stems/m2. The coarse dredge-spoil sediments of the constructed marsh had only 1/4 to 1/3 the total N found in the fine-textured soils of a nearby natural marsh, where cordgrass grows tall and clapper rails nest. We recommend that future marsh restoration projects begin with fine soils that promote N retention and accumulation to sustain tall canopies, rather than suggesting long-term fertilization.