The Ecology of Tijuana Estuary, California: An Estuarine Profile

d High cordgrass mortalrty in 1984 (Figure 65)
suggested that watered plants, I e, those that did
not experience gradual decrease In soil morsture,
should be highly susceptible to drought Thus.
winter- and summer-watered plants should have
hlgher mortality after the irrrgation ended than
plants In unwatered control plots All treated plots
expertenced cordgrass mortalrty In the last 5
weeks of the expernment, when soils drred out
between waterings However, one control plot
increased In denstty While evrdence is scanty,
there is a! least a suggestran that infrequently
wetted areas wrthstand drought better, and
differentral rooting depths might explarn differential
survlval Studres of below-ground plant growth at
different intertrdal elevatrons are needed to explarn
the high creek-edge rnortalrty seen during the 1984
drought
The importance of seedlrng recrurtmerit was not
addressed in these year-long experiments Most of
the dynamrcs of cordgrass have resulted from
changes in vegetative growth Although we
expected to see some seedlings appear In plots
watered all year, such lnvastons were ~nsignificant,
perhaps because of the srnall srze of the quadrats
and short duration of the experrrnent
The hypotheses that grew out of the monitoring
program withstood experimental tasting, and we
provide 8 conceptual model of the control of
cordgrass growth as fallows Cordgrass ts
comtnonly under stress due to hypersaline soils
Freshwater influxos to the marsh stimulate
increased growth If the influx occurs in winter,
plants are able to respond by increasing their
height If the tnflux occurs in summer, during the
peak of the growing season, plants respond by
increasing vegetative reproductron From the
standpoint of an rndrvrdual plant, it is a matter of
carbon allocation the later that fresh water ts
applied, tho less likely the plants can respond by
Increasing tn height increased productron late In
the season wtl! be channeled primarily to new
shoots
5.4.2 Nutrient Addition Experiments
From the flrst year on, the annual censuslng of
cordgrass documented considerable spatial
variabll~ty from quadrat to quadrat and transect to
transect While strong year-to-year differences
have been explained, local var~atioris have only
recently been investigated It is now clear that
nutr~ents and insects add to both spatral and
temporal changes
As discussed 117 Chapter 4 Covrfi (1984) found
that sol! nrtrogen 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 rn stands mrxed with
p~ckleweed The rnfluence of urea on cordgrass
thus depended in part on the presence of its most
frequent co-occurnng species, with ptckleweed the
better competitor for urea
Nutrient-additron expertments also suggested
that rnsect grazing has signrficant impact on
cordgrass growth and densrtres Covin's ureaadditron
plot at TJE-28 produced cordgrass with
higher tissue nitrogen content than at TJE-37, and
a late-season dieback occurred on those plants
Patches of cordgrass d~ed along that same transect
after the 1980 flood Because prckteweed was
lacktng, we know that compet~lion was not the
cause If nrtrogen inputs are commonly high at
TJE-28, nrtrogen uptake should be h~gh, which
should increase the probabilrty of insect attack
We have yet to rdentlfy patterns of nrtrogen influx
to different transects or quadrats, and we need to
evaluate directly the retattonship between rnsect
grazing and nutilent additions to help explain
spatial heterogenerty In the salt marsh vegetation
Research w~th other wetland (Onuf et a!. 19773 and
grassland (McNeil and Southwood 1978)
vegetai~on has shown that insects are attracted to
plants w~th high nrtrogen concentrations and that
herbivory is greater on leaves with augmented
nltrogen Clearly, these secondary effects of added
nitrogen can have lmportanl effects In nature