Slough Restoration

LESSONS ON PLANT RESTORATION FROM WHISTLESTOP LAGOON

Salt marsh habitat at Elkhorn Slough is sandwiched between mudflats at low elevation, where no plants grow, and the upper reach of the king tides (the highest high tides). Plants in this zone have special adaptations to cope with salty soil, but sometimes conditions can become too harsh for even these specialists to endure. Unfortunately, the activities required to restore salt marsh also tend to create harsh growing conditions. Grading is often needed to create a gentle incline with the appropriate elevation range to support salt marsh, and moisture evaporates quickly from bare, exposed soil. Rapid evaporation also concentrates salts present, and both processes contribute to plant stress. We know that in shade, soil moisture evaporation is slower, so one way we might be able to reduce transplant stress is by clustering plants together – that way plants shade each other, and collectively shade a larger patch of soil. Research in other systems has shown that clustering neighbors together can reduce some sources of stress (Silliman et. al, 2015). However, the benefits of clustering might come at a cost, especially as plants grow – when plants are close together, they may be more likely to compete for soil moisture, nutrients, and space to spread out.

For the last year and a half, we have been monitoring the effects of planting pattern on transplant performance at Whistlestop Lagoon. We cleared plots in February 2017 and planted them with 5 individuals of either Frankenia salina or Jaumea carnosa, arranged in a Tight or Loose neighbor treatment.

Restoration plots at Whistlestop Lagoon, planted in either Tight or Loose neighbor treatments. The photo on the left was taken after planting in February 2017, and the photo on the right was taken in April 2018. All seedlings were initially caged to protect against herbivores. The king tide line is visible in both photos, at the abrupt transition from low-growing, salt tolerant species to taller, salt-intolerant upland species.

Karen Tanner and UCSC undergraduate Joanna Tran collect percent cover data at one of the Whistlestop plots.

We had an amazing rain year in 2017, with ~27” of rain falling at the Reserve – and the plants really took off! More than 95% of our transplants survived, in contrast to an earlier experiment where more than 50% of transplants died in the high marsh. Because there were no differences in mortality across the Tight and Loose treatment, clustering did not appear to affect survival.

We have also been monitoring how much bare space our transplants have spread to cover in the Tight versus Loose treatment. Our results show that cover is greater in the Loose neighbor plots – suggesting that the cost of competition outweighs any benefits of clustering for these two high marsh species. However, it is important to keep in mind that high rainfall prevented young seedlings from experiencing much (if any) drought or salinity stress in 2017. So, it’s possible that the balance between the cost and benefit of clustering could lead to a different outcome in a dry year (for example, we might see higher seedling survival in the Tight treatment if clustering reduces drought stress).

We conducted similar planting experiments at our 47-acre Hester Marsh restoration site in 2019. By repeating this experiment in a different year, we have the opportunity to observe how weather affects the outcome. We also tested 3 additional species (Distichlis spicata, Extriplex californica, and Spergularia macrotheca), so we can assess whether outcomes are consistent across the suite of perennials that contribute the most to native biodiversity in the high marsh. Taken together, these experiments give Reserve staff valuable information that can be used to develop more effective planting designs for future restoration efforts.

Reference: Brian R. Silliman, Elizabeth Schrack, Qiang He, Rebecca Cope, Amanda Santoni, Tjisse van der Heide, Ralph Jacobi, Mike Jacobi, and Johan van de Koppel. 2015. Facilitation shifts paradigms and can amplify coastal restoration efforts. PNAS vol. 112, no. 46, pgs. 14295–14300.

Photos at left show flowering Frankenia salina (top) and Jaumea carnosa (bottom). Photos of both species planted in the Tight pattern, after the 5 individuals have grown together, are shown at center; photos of both species in the Loose pattern, where individuals are still visible, are shown at right. Cleared plots are 2 meters square, and for each species more of the plot footprint is covered by plant tissue in the Loose planting pattern.

For more information about this research, contact Karen Tanner.