there must be appropriate salinity, nutrients, 

 and soil moisture; and second, there must be 

 reduced competition from pickleweed. The 

 latter condition appears to be associated with 

 reduced drainage, that is, increased 

 inundation. Two experimental studies were 

 designed to test these ideas. 



5.4.1 Freshwater Addition in a Field 

 Experiment 



An experimental field study was initiated 

 in 1984 to determine how coastal wetlands 

 would respond to increased streamflow caused 

 by treated wastewater. The data provide a test 

 of the above hypotheses concerning cordgrass 

 responses to salinity reductions at different 

 times of the year. A brief summary and 

 pertinent findings follow from the work of 

 Beezley and Beare (SDSU, unpubl. data). 



The experiment compared year-long, 

 winter, and summer irrigation with fresh 

 water, all compared to unwatered control 

 plots. A block experimental design was set up 

 with replication in both cordgrass and 

 pickleweed habitats. City water was piped to 

 the marsh and used to fill meter-square 

 cylinders that surrounded salt marsh 

 vegetation. Watering began early in January 

 1984 and continued approximately biweekly 

 for 7 months until the drought disturbed the 

 experiment. Late in the experiment, watering 

 changed soil moisture as well as the soil 

 salinity. The drying of marsh soils, following 

 estuary closure and high rates of evaporation 

 created deep cracks throughout the 

 experimental area, and the cylinders would no 

 longer hold water. In late May, the winter- 

 watering treatment ended and the summer- 

 watering treatment began. Cordgrass 

 responses were measured as in the monitoring 

 program; pickleweed growth was measured as 

 increased length of tagged stems. 



Four hypotheses were tested with results 

 from the irrigation experiment. For the most 

 part, the predictions were upheld. 



• We predicted that cordgrass growth 

 would increase wherever we added freshwater 

 and that maximum biomass would occur with 

 continuous watering. Plots had 394-761 

 cm/m 2 of cordgrass stems prior to watering 

 (Table 5.4). Unwatered controls increased 

 the least amount (mean = 834 cm/m 2 ) by 

 late June, and then decreased with drought- 

 caused mortality. Year-long and summer- 

 watered plots continued to increase, on the 

 average, throughout the experiment, while 

 winter-watered plots declined along with 

 controls. 



• We predicted that winter watering 

 should result primarily in increased height, 

 as had occurred following the 1980 flood. 

 Average height of cordgrass was 40 cm prior 

 to watering in January, and controls increased 

 10 cm by late June. The height response was 

 greater with watering, but the results were 

 not quite as predicted. By August, year-long 

 watering had increased plants 56 cm and 

 summer watering, 52 cm, compared to 30 cm 

 in winter-watered plots and 18 cm in 

 controls. Winter-watered plots should have 

 matched the year-long plots, if timing of 

 freshwater influence were the only 

 controlling factor. The growth response is 

 complex and possibly modified by the unusual 

 summer drought. 



• We predicted that summer watering 

 would increase plant density, based on the 

 increased density of cordgrass with reduced 

 salinities during summer 1983 (Figure 5.9). 

 Increases in cordgrass density were greatest 

 in the summer-watered plot (22 

 stems/quadrat/6 months), as predicted, while 

 year-long and winter-watered plots were 

 similar to controls. 



• We predicted that cordgrass mortality 

 should be lowest where soil moisture dropped 

 gradually, based on the pattern of high 

 cordgrass mortality in 1984 (higher around 

 creek banks than further inland). Thus, 

 winter- and summer-watered plants were 

 expected to have higher mortality after plot 

 irrigation ended than plants in unwatered 

 control plots. All treated plots experienced 



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