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.