The Ecology of Tijuana Estuary, California: An Estuarine Profile

Import
Export
/ l I , # I I I ~ k
MAY JUN JUL AUG SEP OCT NOV DEC JAN FEBMAR APR
1971-p '-1978-'
A
Import
Export
L *.A-.-I-/
MAY JUN JUL AUG SEP OCT NOV DEC JAN FEBMAR APR
,___-I_____ 97 7--.- .. . - 1 -- 1978-I
Figure 54. Net flux of total inorganic nitrogen
dissolved in the tidal waters for (a) the succulentdominated
study site and (b) the mixed cordgrasssucculent
study site of Winfield (1 980). Reprinted with
permission from Winfield (1 980).
Before Covin's (1984) study at Tijuana Estuary,
the influence of nitrogen on salt marsh vegetation
was in question. D. Turner (SDSU; unpubl. data)
had fertilized pickleweed-dominated vegetation at
San Diego River Marsh and found large increases
in vascular plant productivity. However, when
Nordby added the same concentrations of urea to
cordgrass transplants at Tijuana Estuary, he failed
to see a growth response (Nordby et al. 1980). The
latter experiment took place during 1980, when
major floodwater may have enriched the marsh
soils with nitrogen or stimulated local nitrogen
recycling. These conflicting results stimulated
Covin to develop a detailed investigation of
nitrogen-plant interactions.
Covin's first step was a broad survey of soil
nlrrogen in 1981, using 67 of the 102 tower marsh
monitoring stations at Tijuana Estuary (Chapter 51
Soil nitrogen proved to be varrable within stations,
among stations, and among transects. There was
only a hint that cordgrass growth was related to
soil n~trogen concentrations; the transect w~th
highest soil nitrogen had the highest total stem
length (a biomass estimate) of cordgrass.
Reasoning that nutrient uptake by cordgrass
might be enhanced by nitrogen concentrations or
reduced by competitive uptake on the part of
pickleweed, Covin set up two-way experiments with
both nitrogen (rt urea) and presence of
competitors (rt pickleweed) as variables. In
addition, he repeated the + urea experiment in an
area of pure cordgrass. The field experiments were
carried out in 1983 near transects TJE-28 and
TJE-30 (Chapter 5). Urea was broadcast onto the
marsh soils biweekly through the growing season
in + urea plots. Biomass was measured in August
1983 and significant treatment effects were found.
Urea stimulated only the growth of pickleweed;
competitor removal stimulated only the growth of
cordgrass (Table 22). Pickleweed was the superior
competitor for nitrogen uptake; its biomass
increased significantly with urea additions, with no
significant effect from the presence of cordgrass.
Nttrogen added to pure stands of cordgrass was
readily taken up by the plants - so thoroughly that
soil nitrogen concentration did not increase.
Instead, the nitrogen went directly into the leaves,
and could be measured only as increased nitrogen
concentrations in plant tissues. Insects seemed
able to locate plants with enhanced tissue nitrogen,
and it appeared that local outbreaks did serious
damage to cordgrass. Thus, the net effect of urea
fertilization on pure stands of cordgrass may be
beneficial, by stimulating growth, or detrimental by
causing insect damage. This cause-effect
relationship is under further study by Covrn (in
prog.) at the Pacific Estuarine Research Laboratory.
The exper~mental results (Table 22) were
complex, and they led to a new model of nitrogenmarsh
dynamics (Covin 1984; Covin and Zedler,
unpubl. ms.): Nitrogen is certainly limiting to
vascular plant growth. Additions will stimulate
cordgrass in pure stands, but herbivores may gain
the ultimate benefit from the increased nutritional
quality of the plants. In mixed stands, additions
stimulate pickleweed, which then outcompetes
cordgrass
Some ~nteresting questions remain: Why didn't
nutrient additions stimulate herbivory on
pickleweed? Was it chance that precluded an
outbreak in the fertilized plots? Perhaps, but a
1984 experiment performed by Beare and Beezley
(unpubl. data) suggests otherwise. Plots with urea
added had less herbivory than plots fertrlized with
Miiorganite In replicate plots of pure pickleweed,
urea was added biweekly along with fresh water to
cylinders of 0.33 rn2 area that penetrated 10 cm of
soil and protruded 30 crn aboveground. Milorganite
was added in low, medium, and high concentrations
to add~tionai cylinders, with the high treatment