Oregon tidal wetlands and climate change (pt. 3)

Our survey work in Oregon's coastal wetlands showed patterns of species distribution suggesting how and which species might be vulnerable to climate change on the Pacific Northwest coast.  In previous posts, I discussed some of our findings relative to wetland algae and plants. However it was important to extend our knowledge of climate impacts by conducting controlled experiments too. During the summer of 2012 I was fortunate to work with an enthusiastic summer intern who participated in an EPA program that gives undergraduate students from smaller liberal arts colleges opportunities to work in federal research labs.

Seedlings.

The project we conducted was a short-term experiment to assess the dual effects of salinity and flooding on wetland plant productivity. We used seven species that were easily grown from seed; most are commonly found in Oregon tidal marshes. The species we used also differed in apparent tolerance to salinity and flooding based on their field distributions. We grew several hundred seedlings at the lab starting in the spring.

Some of the experimental pots placed at three tidal
elevations at one of our sites. A blue salinity sensor
can be seen at bottom center; a tall white stilling well
with a water level sensor is at the right.

To conduct the experiment with a range of salinities, we planned to work at three sites in the Yaquina estuary - one near the mouth of the estuary and two farther inland. We placed salinity/temperature sensors in the field track conditions over the course of the experiment and the data time confirmed that our sites had quite different salinity profiles.

To create differences in flooding intensity among treatments, we planned to set out plants at three tidal elevations at each site. We used mean higher high water (MHHW) as our baseline, which is about a mid-marsh elevation on the Oregon coast. We placed other plants at 25 and 50 cm below this elevation. We synchronized positions in the tidal frame across the three sites (to within about 5 cm) with high accuracy GPS. Our approach followed efforts in marshes in other parts of the US that have used "organ pipe" arrays to vary flooding intensity. However, instead of actually building an apparatus to hold plants at different elevations, we simply used the sides of tidal channels in the marshes to provide the needed elevations for the study.

In mid June, with some welcome help from another summer intern, we excavated terraces from the channel banks to place several hundred potted seedlings into the field. It was two long and very muddy days of field work!

Our plants grew under different salinities and flooding levels for five weeks. This was a relatively short length of time - constrained by the limited period of the internship - but long enough to assess treatment effects on the seedlings. We checked on the plants about every week and some were lost to the vagaries of field experimentation. (I think a few were uprooted by birds.) During our checks we noted which plants browned, probably due to physiological intolerance of environmental conditions.

Above ground dry mass (means and SE) of Grindelia stricta seedlings grown at 3 tidal elevations in low, moderate and high salinity wetlands. Like Grindelia, most species we investigated had lower above- and below- ground growth with greater flooding and/or greater salinity.

By mid summer, the experimental results were pretty unequivocal. All species, including one that we expected to be most tolerant of flooding and salinity stress based on its field distribution, grew less with higher salinity and/or greater flooding. Species seemed to differ in terms of their sensitivity to one experimental factor or the other, but all showed the same trend.

One concept we explored in the study was idea that "salinity exposure" - the combined effects of both flooding and salinity - could account for differences in plant productivity. In other words, we tested whether plants exposed to low salinity for long periods of time might be stressed as much as plants exposed to higher salinity for shorter periods of time. We created a simple index to quantify this total exposure and found that it correlated reasonably well with plant biomass for a number of species in the study.

Change in shoot dry mass in Plantago maritima seedlings with increasing salinity exposure. Our salinity index combined the length of exposure to flooding with absolute levels of local salinity. The index would be 0 in wetlands that are never flooded by salt water and ~33 for at a site continuously submerged in full seawater. Intermediate values could be due to long exposure to low salinity water or brief flooding by higher salinity water.

In terms of future sea-level rise, the overall results seemed pretty clear: for the species we investigated, if vertical marsh growth cannot match sea-level rise, plant production is expected to decline. Of course any increase in flooding or salinity at a given site would occur over the scale of decades, not the short time scale of our study. Yet declines in plant production in future wetlands might result in less food for marsh consumers and less detritus for the formation of new wetland soils.

I really enjoyed conducting this study. I was nervous about whether our seedlings would grow in the lab or whether they would quickly get destroyed once transplanted in the field. But the plants were hardy enough (or we had enough luck) to give us a good data set. Conducting this research, I had an opportunity to think about plant physiology and environmental stress. I read about some basic ideas in plant ecology such as how plants may trade off allocation of resources to above (shoot) or below-ground (root) production.

Though exciting, manipulative field experiments are challenging! The goal is to isolate factors to determine cause and effect, but at the same time maintain conditions that are as realistic as possible. Also, ecologists often want to be able to derive broad conclusions about such experiments, but various constraints often mean it is necessary to work at a single site, with a limited number of species, or only for a limited period of time. After conducting experiments such as these, it is important to ask: would different species or locations or seasons give different results? Extracting generalities from such complex ecosystems is a rewarding, but heavy intellectual challenge.

Reference

Janousek CN, Mayo C. 2013. Plant responses to increased inundation and salt exposure: interactive effects on tidal marsh productivity. Plant Ecology 214:917-928.