Oregon tidal wetlands and climate change (pt. 4)

In the previous post, I discussed our attempt to understand how salinity and flooding affect tidal wetland plant growth. A final question I’ll describe in this series of posts was the effect of salinity on seed germination and the first days of seedling growth. Climate effects, whether manifest as higher temperatures, increasing salinity, or greater flooding could impact young seedlings, not just adult plants. Such effects on seed germination could impact the population sizes of species in wetland habitats.

Plantago maritima seeds in a germination experiment.

To address this question, we planned a simple series of experiments in the lab. Using one to several species per experiment, we put seeds collected from the field into lab dishes moistened at a series of salinity levels. Our treatments ranged from freshwater conditions (0 ppt) to 20 ppt, a level about 2/3 the salt strength of full seawater. From our measurements of soil salinities in the region’s tidal wetlands, we found that wet season salinities really didn't exceed about 20 ppt, even in the saltiest marshes. 

It is very well established that salt is physiologically stressful for vascular plants. It presents a challenge for intracellular osmotic balance, requiring plants to expend energy to maintain acceptable levels of ions in their tissues. Species living in salt marshes have various mechanisms for handling salt much better than most other plants. These "salt-lovers" are known as halophytes. Most plants cannot live in salty environments and are known as glaucophytes. To cope, halophytes may extrude salt from their leaves, store salt internally, or have other means of dealing with these unneeded ions.

We collected seeds in the field from perhaps some two dozen species and ultimately worked with 13 species that showed promise of germination under lab conditions. The species in our tests included grasses, some annual and perennial forbs, a rush, and a shrub (twinberry, Lonicera involucrata) that forms scrub-shrub wetland in some parts of Oregon estuaries. Unfortunately, we found that sedges did not germinate well under our basic lab conditions, so we were unable to examine salinity effects in this important group of wetland plants. With species from a variety of tidal marsh habitats and taxonomic groups, we were able to see which plants were more or less tolerant of high salinity at their earliest life history stage.

The lab tests we conducted generally confirmed what is already known about many estuarine species: though often tolerant of elevated salinity, most species germinated most readily in freshwater. These species are thus not really true "salt-lovers", but rather salt-tolerators. Also, unsurprisingly, we found that species varied in their tolerance of higher salinity conditions. Two species - pickleweed (Sarcocornia perennis) and Douglas' aster (Symphyotrichum subspicatum) - appeared to act the most like true halophytes. These results alone did not shed any profound light on seed germination biology, but did provide valuable data for plants found in the Pacific Northwest.

Germination responses (means and SE) for three Oregon tidal wetland species across a range of salinities.

 In the final part of our study, however, we tried to take our work one step further. We asked how salinity effects on germination matched, or failed to match, patterns of plant distribution in the field. To explore this, we returned to the data set described in parts 1 and 2 of this series of blog posts. We looked at the full range of summer soil salinities found in our research (~1 to 44 ppt) and assessed how each species was distributed along this gradient.

For about half of the species we looked at, the answer seemed to be that adult distributions didn't match predictions based on seed tolerance. In this group of species, seed germination was usually greatly reduced at salinities of 10 or 20 ppt, but they were commonly found in soils with salinities of 30 (even up to 44 ppt) as adults in the field. Because we didn't conduct additional experiments, we could not account for the reasons underlying this mismatch in these species, but one idea is that their seeds may be adapted to germinate under conditions of low salinity. These periods of low salinity are most likely to occur in late winter or spring when the Pacific Northwest is very rainy. As plants continue growing into the summer, they presumably become more tolerant of elevated salinities during dryer summer months in the Pacific Northwest.

Our germination study was relatively simple, but it highlighted the fact that we still have much to learn about even common species in our coastal habitats. Each species might be affected by salinity, temperature, changing carbon dioxide concentrations and other environmental factors differently. Moreover, each life stage of each species could have different responses to these factors. Add in the fact that species interact with other species, and the complexity of community ecology grows exponentially. With dozens of plant species in coastal marshes and swamps, there is much to learn about species relationships with the coastal environment and how these may be altered with climate change.

Reference

Janousek CN, Folger CL. 2013. Inter-specific variation in salinity effects on germination in Pacific Northwest tidal wetland plants. Aquatic Botany 111:104-111.

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.  

Oregon tidal wetlands and climate change (pt. 2)

Which factors affect the distribution of tidal wetland
plants? Will these gradients shift with climate change?

In the last post, I began to describe some of the work I conducted as an ecologist with the EPA. Our initial investigation was a regional survey to quantify plants, algae and environmental characteristics in wetlands scattered throughout four estuaries along the Oregon coast. After working through the algae, a second goal of the regional survey was to relate vascular plant species abundance, composition and diversity to the major environmental gradients present in these wetlands (link to the paper). Salinity and elevation relative to tides (this determines when and how long plants are flooded) were of particular interest. With future sea-level rise, both flooding and salinity exposure are expected to increase.

It is already well known that factors like elevation and salinity impact wetland vegetation in a general sense (e.g., Watson and Byrne 2009), but the specific relationships between plant communities and their local environment are less well known in the Pacific Northwest. Additionally, it is useful to know which environmental factors have the greatest effect on plant communities.

A first step was to look at different environmental factors as relative predictors of plant occurrence. From the plant surveys we had a simple dataset showing whether each species was present or absent at each location we sampled. We also had quantitative data at each location for five gradients of potential importance to plant distribution: tidal elevation, soil salinity, soil nitrogen content, soil grain size, and a hydrologic index that quantified the degree of marine versus river dominance for the estuary from which the data were collected. The plant and environmental data were put into logistic regression models for many of the common species. The exciting next step - which I learned about after encountering a study on butterfly habitat use - was to apply a technique called hierarchical partitioning. This statistical technique enables a researcher to assess the relative strengths of effects (independently and jointly with other factors) of different variables in a statistical model. Like all statistical methods, it has its limitations (for instance, it doesn’t perform well with non-linear relationships between dependent and independent variables), but it seemed like a promising technique to quantify the relative importance of selected environmental factors on plant distribution.

I ran the analyses for 20 of the more commonly-occurring species and obtained some interesting results. First, for quite a few species, soil salinity was the most important variable in explaining the presence or absence of the species in the wetlands. In the figure below, for instance, salinity was positively related to the presence of perennial pickleweed (Sarcocornia perennis). Pickleweed occurrence was also positively correlated with tidal elevation, estuarine river-dominance, and soil clay content, though less strongly. (The positive correlation with river flow seems somewhat counter-intuitive for this salt-loving species, but may be due to its high frequency of occurrence from low marsh at our most river-dominated site.)

Relative strength of abiotic factor effects on the occurrence of pickleweed (Sarcocornia perennis) in Oregon tidal wetlands. All factors had statistically significant effects, but soil salinity appeared to have the greatest effect in the statistical model.

Elevation turned out to be the most important variable predicting the presence or absence of some other species. And, more rarely, soil nitrogen stood out as a key environmental gradient. Grain size (percent clay) of the soils generally only weakly correlated with species presence and absence.

The logistic regression models enabled a species-by-species look at environmental correlates of plant occurrence, but wetland plant communities in the Pacific Northwest are very diverse and species associations occur in complex patterns. We used another exciting statistical technique, non-metric multidimensional scaling (NMDS), to investigate overall plant composition in our dataset. In a nutshell, NMDS is a computational technique that aims to represent all of the differences between pairs of samples in a simple 2 or 3 dimensional graphical display. Its value lies in its ability to take a complex multi-dimensional dataset and summarize that information in a visually-intuitive manner from which patterns can be deduced.

The plot below shows the results of our NMDS analysis based on the abundance of 20 common plant species. Points closer to each other are more similar in terms of species composition. In the figure, the samples are colored based on their tidal elevation. Brownish points are plots from lower wetlands (e.g., below mean higher high water, MHHW) and greenish points are plant assemblages from high tidal marsh that is less frequently flooded. The analysis shows that plant communities separate out on an elevation gradient, similar to the patterns of vertical zonation one would see with invertebrates and algae on a typical rocky shoreline.

Non-metric multidimensional scaling plot of vascular plant communities in Oregon tidal wetlands. Plots are colored according to their height above or below local mean higher high water (m). Plot stress = 0.11.

Below I’ve shown the same NMDS plot, but with points coded by summer-time soil salinity. Plant composition differs between more saline and fresher wetlands, but there is a gradual gradient as with elevation.

An observational study like this is valuable for generating hypotheses about which environmental factors affect the distribution of different wetland species. However, we know that species are affected my more than environment itself – interactions with other species matter too. To more clearly determine causality, and not just patterns in the data, controlled experiments are needed. With dozens of species in the tidal wetland flora of Oregon and many potential abiotic and biological factors of importance, comprehensive study of this question would be a massive undertaking! In the next two posts, I’ll discuss some limited experimentation we performed to assess the effects of a few abiotic factors on plant growth and germination.

References

Janousek CN and Folger CL. 2014. Variation in tidal wetland plant diversity and composition within and among coastal estuaries: assessing the relative importance of environmental gradients. J. Vegetation Science 25:534-545.

Watson EB and Byrne R. 2009. Abundance and diversity of tidal marsh plants along the salinity gradient of the San Francisco Estuary: implications for global change ecology. Plant Ecology 205:113-128.

Oregon tidal wetlands and climate change (pt. 1)

Marsh and scrub-shrub wetlands, Poole Slough, Yaquina estuary.

This summer I finished up three and a half years as an ecologist with the Environmental Protection Agency. The broad scientific question that framed my research was how coastal wetlands - salt marshes and woody wetlands such as tidal swamps - would be affected by climate change. I really enjoyed my research during this period of my career and thought I would give an overview in a series of blog posts of our findings and mention a few of the many unanswered questions we still have about these fascinating coastal ecosystems.

After I started working with our EPA/USGS team, we quickly determined that we needed field data on how wetland plants were distributed along gradients of elevation and salinity in the Pacific Northwest. It is relatively well known that these factors play some role in how species are distributed spatially in salt marshes in general, but what are the patterns in our region? If future sea-level rise (SLR) affects the environmental gradients in estuaries to which wetland organisms respond, what will future wetland communities look like?

To quantify patterns of distribution, we designed a field sampling plan that included estuaries along the Oregon coast with a range of different hydrologies. For instance, one of our field sites was a bay in northern Oregon (Netarts) that has a small coastal watershed and is generally very marine-influenced because it has no major rivers flowing into it. Near the opposite end of the spectrum, we also sampled the Coquille estuary in southern Oregon which is a very river-dominated site. Our other sites (Alsea and Yaquina) were more intermediate.

During the course of a summer, we visited over 160 locations in four estuaries and collected data on vegetation (relative abundance of different species and total number of species) and many environmental variables including elevation, soil organic content, and soil salinity. Acquiring good data on elevation was the technically-challenging part of the research. The vertical range of the tides along the Oregon coast is several meters, but at the upper end of that tidal range (where marshes and tidal swamps occur) change of only several decimeters can make a big difference in how often a particular wetland is flooded. Flooding, in turn, affects which species grow in a given spot and how productive those species are. We needed a method for determining elevation to less than 10 cm accuracy at our sampling locations spread in wetlands of all sizes and shapes over four estuaries along the coast.

The answer for us was GPS, though not the off-the-counter recreational GPS. Rather, we used a survey-grade GPS that could measure horizontal and vertical positions to within centimeters. For the first year, I used an older model GPS rover that was available at EPA. Data collection required at least 10 minutes per site, limiting the number of measurements we could conduct during a day. It was slow going, but after several months we completed all of the measurements needed for our survey. (Eventually our lab purchased a new GPS capable of linking via cellphone into a statewide network that would fine-tune our data and give us cm-level accuracy after just a few seconds! This new instrument became my favorite tool/toy/child and lived in my office for my last two years at EPA.)

Our sampling lasted a full summer and continued into winter months as we continued to make GPS measurements and assess winter-time soil salinities at our marked plots. Finally with a large data set of information on tidal wetland plants, algae, sediment chlorophyll a, soil carbon and nitrogen content, soil salinity, elevation, and soil grain size, we were ready to address some questions about how vegetation composition related to these environmental factors.

The first research paper we assembled was on the algae of our tidal wetlands. This turned out to be a logical initial step for me because I had worked on wetland algae as a PhD student and it was a smaller data set than the plants. Additionally, there seemed to be so little known at all about algae in vegetated tidal wetlands in the Pacific Northwest.

With the algal work, however, a few preliminary sets of lab analyses were necessary before writing the paper. For one analysis, we took surface mud samples and extracted chlorophyll a to obtain estimates of how many microalgae live on the sediments of these marshes and swamps. These microscopic “plants” are easily overlooked, but they can be very important parts of coastal environments. For example, research with stable isotopes shows that they turn up in the diets of animals, indicating that they make an important contribution to coastal food webs.

Our data from Oregon wetlands showed a very prominent role for elevation in structuring the abundance and diversity of macroalgae and sediment microalgae in the estuaries. Unsurprisingly (because algae are mostly aquatic organisms), they were more abundant and diverse in tidal marshes found at lower elevations, but essentially absent from high tidal marshes that are rarely flooded. The figure below illustrates how total macroalgal cover on the wetland surface changed with elevation in the dataset.

Macroalgal cover (open circles) along the tidal wetland elevation gradient. Above mean higher high water (MHHW), the wetlands are seldom inundated (blue line) and have essentially no macroalgae. Pictures to the right show some common genera of seaweeds found in estuarine wetlands in Oregon: Fucus, Gracilaria, and Ulva.

Salinity seemed to play a secondary role in structuring algal communities (as far as could be determined from an observational, not experimental study). Sediment chlorophyll a and macroalgal diversity was higher in areas with more saline soils, but the relationships were not strong.

Our analysis of sediment chlorophyll a took a fair amount of effort in the lab, but unfortunately it is not an adequate technique for assessing which kinds of microalgae live in different wetland environments. Most tidal wetland sediments in Oregon are probably dominated by diatoms, but many species may be involved. Do sediments at different tidal elevations or under different kinds of plant canopies have different microalgal communities? One of the observations I made repeatedly in the field, but was never able to carefully investigate, was the occurrence of dark globular cyanobacterial colonies in some wetlands. By light microscopy I determined that these colonies were comprised of Rivularia, a cyanobacterium capable of nitrogen fixation. What are the environmental and biological factors that affect where this fascinating alga grows?

Rivularia colonies on sediment (left) and squashed on a microscope slide (right). At the end of the individual green filaments of cells there are brownish spherical cells. These are heterocytes, cells that specialize in nitrogen fixation.

What does the algal perspective suggest about changes to coastal wetland ecosystems in light of sea-level rise? First, if rising water levels outpace the vertical growth of the wetland surface, the abundance of low salt marsh in coastal estuaries is likely to increase. Macroalgae and microalgae are then expected to become a more prevalent component of the coastal wetland landscape. This may potentially have effects on coastal food webs. For example, will groups of consumers that more readily consume algae over vascular plant matter be favored?

Second, sea-level rise could have consequences for wetland accretion if it stimulates algal production but decreases plant productivity (more on this latter question in a future post). This is because the organic material produced by vascular plants in a key ingredient of the new sediment added to growing marshes. Algal production may be less likely to serve as a substitute because it much more easily decomposes. Could all of this constitute a negative feedback between sea-level rise and accretion potential?

Reference

JanousekCN and Folger CL. 2012. Patterns of distribution and environmental correlates of macroalgal assemblages and sediment chlorophyll a in Oregon tidal wetlands. Journal of Phycology 48:1448-1457. 

*The posts in this series represent the views of the author only and not necessarily those of the US EPA or US government.