5.4.4 Nutrient Addition Experiments 



From the first year on, the annual 

 censusing of cordgrass documented 

 considerable spatial variability from quadrat 

 to quadrat and transect to transect. While 

 strong year-to-year differences have been 

 explained, local variations have only recently 

 been investigated. It is now clear that 

 nutrients and insects add to both spatial and 

 temporal changes. 



As discussed in Chapter 4, Covin (1984) 

 found that soil nitrogen was important to 

 cordgrass growth. His experimental 

 manipulations showed that urea increased 

 cordgrass growth in pure stands (although in 

 different amounts for the two sets of plots), 

 but not in stands mixed with pickieweed. The 

 influence of urea on cordgrass thus depended 

 in part on the presence of its most frequent 

 co-occurring species, with pickieweed the 

 better competitor for urea. 



Nutrient-addition experiments also 

 suggested that insect grazing has significant 

 impact on cordgrass growth and densities. 

 Covin's urea-addition plot at TJE-28 

 produced cordgrass with higher tissue 

 nitrogen content than at TJE-31, and a late- 

 season dieback occurred on those plants. 

 Patches of cordgrass died along that same 

 transect after the 1980 flood. Because 

 pickieweed was lacking, we knew that 

 competition was not the cause. If nitrogen 

 inputs were high at TJE-28, nitrogen uptake 

 should have been higher, and with that, 

 increased probability of insect attack. 

 Research with other wetland (Onuf et al. 



1977) and grassland (McNeil and Southwood 



1978) vegetation has shown that insects are 

 attracted to plants with high nitrogen 

 concentrations and that herbivory is greater 

 on leaves with augmented nitrogen. Clearly, 

 these secondary effects of added nitrogen can 

 have important effects in nature. 



5.4.5 Conclusions from Experiments 



The hypotheses that grew out of the 

 monitoring program withstood experimental 

 testing, and we provide the following 



conceptual model of the control of cordgrass 

 growth. Cordgrass is commonly under stress 

 due to hypersaline soils and low nitrogen 

 supplies. Freshwater influxes to the marsh 

 stimulate increased growth by reducing soil 

 salinities. If the influx is from flooding, 

 nitrogen supplies are likely to be enriched. 



The timing of freshwater inflows appears 

 to be important. When a growth stimulus 

 occurs in winter, plants can respond by 

 increasing their height and also density. If the 

 influx occurs in summer, during the peak of 

 the growing season, plants respond primarily 

 by vegetative reproduction. From the 

 standpoint of an individual, it is a matter of 

 when carbon is allocated to above- versus 

 belowground growth. The later that fresh 

 water (and/or nitrogen) is applied, the less 

 likely the plants will respond by increasing in 

 height. Increased production late in the season 

 seems to be channeled primarily to new 

 shoots. 



5 . 5 THE REVISED MONITORING PROGRAM 



By 1989, the monitoring program had 

 accumulated data from several years with 

 catastrophic disturbances (winter flooding in 

 1980, late-winter flooding in 1983, El Nirio 

 storms in 1983, and closure of the tidal inlet 

 through most of 1984). Recovery from the 

 flood events and from inlet closure had been 

 documented. It was time to reevaluate the 

 objectives of monitoring and the methods of 

 sampling to determine whether the data being 

 gathered were still the most appropriate for 

 the management questions being asked. 



The review led us to recognize the 

 following shortcomings: First, the transects 

 across the elevation gradient undersampled 

 the lower and mid-marsh elevations and 

 appeared to oversample the marsh plain. The 

 time involved in finding and sampling 215 

 stations no longer seemed warranted by the 

 amount of information gained. 



Second, studies of cordgrass at San Diego 

 Bay (Langis et al. 1991, Zedler 1991, 

 Zedler, in review) indicated the need for 

 detailed data on interannual variability in 



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