Results

The effect of salinity gradient on plant growth and productivity.

Survivorship and Morphology

There were measurable changes in all three species that occurred as salinity increased. All plants in the 60 ppt treatment had died prior to collection and were removed from the experiment. Percent survival was based on pot counts with live plants at the end compared to the initial number of pots. Adjusted to the three salinity treatments, percent survival was 100% for all three species in freshwater; 100% for Spartina and Juncus in brackish conditions, but only 70% for Schoenoplectus. No Schoenoplectus survived at salinity >10ppt. No salinities >10ppt were observed for Spartina, so no survival data is available in marine salinity. In contrast, the Juncus did survive at 20 and 40 ppt (marine salinity) with 73.75% of pots remaining at the end of the experiment (all mortalities occurred in 40 ppt).

Plant morphology included results on mean number of stems, mean canopy height from the leaf length measurements, and mean leaf width. For Spartina, there was no difference in stem density among between fresh and brackish treatments, but the mean canopy height and the mean leaf width was great in brackish than freshwater. For Juncus, the stem density increased to a maximum in brackish, and then declined with in both fresh and marine (not significant), whereas the canopy height and mean stem width declined from fresh to marine treatments. For Schoenoplectus, the stem density increased slightly in brackish compared to fresh, whereas canopy height and leaf width declined significantly from fresh to brackish. Generally higher salinities tended to result in longer and narrower leaves and fewer stems.

From the stem density and leaf morphology measurements, a rough estimate of mean canopy area (a function of stem density x mean number leaves per stem x leaf length x leaf width) was calculated. The assumption used was that Spartina typically has a mean of 6 leaves per stem, Juncus has 2 full length leaves per stem, and Schoenoplectus has a single full length leaf per stem. From this estimate, there is a clear decline in mean canopy area from Spartina, to Juncus, to Schoenoplectus. Within the species there were different patterns in response to salinity stress. Spartina had an increase in canopy area from fresh to brackish, similarly Juncus also showed an increase from fresh to brackish followed by a decline in the marine treatment. Schoenoplectus also showed a decline from fresh to brackish in mean canopy area. The latter two species these responses were driven mainly by the shorter canopy height and reduced leaf width observed.

Biomass

Total biomass was largely dominated by the below ground (BG) fraction (see below). For Spartina the total biomass in the control (fresh) was greater than in the brackish treatment. For Juncus there was a clearer unimodal biomass response with greatest biomass at brackish than either fresh or marine treatments. Finally, for Schoenoplectus biomass at brackish was greater than at fresh (control). The below ground biomass data mirror these same patterns

In contrast to the belowground and total biomass patterns, the above live (AL) and above dead (AD) fractions showed different responses by species and salinity. For Spartina, the AL and AD fractions were greater in the brackish treatment compared to the control. There was more AD biomass than AL biomass in this species. For Juncus, the AL biomass declined from the fresh to the marine treatments. In contrast, the AD biomass was greater in the brackish treatment than either fresh or marine. At these mid salinities, there was more AD biomass than AL biomass. For Schoenoplectus, the AL biomass was greater than the AD biomass in both fresh and brackish treatments. For both AL and AD fractions, there was more biomass in the control (fresh) than in the brackish treatment, reflecting the lower salinity tolerance of this species.

Chlorophyll index and fluorescence

Measurements of the chlorophyll index and fluorescence indicated that for the more variable light-adapted actual quantum yield measurements, neither Spartina nor Schoenoplectus exhibited strong change due to the salinity between 0 and 10 ppt. Juncus, however, decline at 40 ppt compared to 0 to 20 ppt. In the dark-adapted potential quantum yield measurements, there was more variability due to treatment by species, but this was occurring over a much reduced range (0.74 – 0.81) of values. For Spartina, the F_v/F_mincreased from 0 to 10 ppt. For Juncus F_v/F_m was similar for 0 and 10 ppt, increased at 20 ppt and then decreased at 40 ppt to below the values observed at 0 ppt. For Schoenoplectus the F_v/F_m values were the highest of the three species but did not change much between 0 and 10 ppt levels. Chlorophyll fluorescence data did not show strong depression from elevated salinities in any of the three species, with exception of Juncus at 40 ppt. Lack of observations for Spartina between 20 and 50 ppt makes it harder to know if a similar pattern would be observed in this species. The final metric is the mean chlorophyll content index as measured by the CCM-300. CCI did not appear to change as a function of the salinity treatment in Juncus, indicating mean chl levels were consistent within and among leaves across the three salinity conditions tested. In contrast, the CCI values for Spartina and Schoenoplectus declined from fresh (control) to brackish, similar to the results seen previously in the chl yield data. This implies that the fluorescence data are picking up variation due to stress imposed on leaf/plant function that is independent of chlorophyll content. CCI was lowest in Spartina, intermediate in Schoenoplectus, and highest in Juncus.