The Effect of Wetland Restoration on Soil Health

ABSTRACT

The San Francisco Bay Wetlands have been heavily impacted by invasive species that alter soil chemistry and suppress native species. This study examines how restoration efforts led by the Save the Bay nonprofit organization affect soil health and native species growth across six Wetland sites—three restored and three not yet restored. Soil samples from each site had their pH and salinity measured using the saturated soil paste method, and Grindelia stricta was grown in each soil sample to assess the effects of restoration efforts on germination and growth. Results showed that soil from restored sites produced 47.2% more total growth and had twice the germination rate compared to control soils. Despite these results, there was no significant correlation between pH or salinity and plant growth. This suggests that improvements in restored sites may be due to other soil biotic and abiotic factors such as nutrient availability or microbial composition. Additionally, more recently restored sites showed greater growth than those restored over ten years ago, demonstrating that the effects of restoration might diminish over time. Overall, the results suggest that restoration improves soil quality and supports native plant growth, even though traditional soil metrics like pH and salinity show no direct correlation.

INTRODUCTION.

The San Francisco Bay has lost over 95% of its salt marsh and wetlands habitat since 1850 [1]. Wetlands serve vital roles to the greater ecosystem. By slowing the flow of runoff, wetlands prevent erosion and act like a natural filter before water reaches streams or the bay itself, trapping sediments and absorbing pollutants. Moreover, by storing water, wetlands aid in the recharge of local groundwater, which is vital during California drought seasons [2]. Finally, wetlands offer a home to hundreds of plant and animal species, promoting a diverse ecosystem [3].

Of the remaining 125 square kilometers of wetland habitat [1], invasive species have begun to replace native ecosystems. These invasive species are problematic because they can drastically alter existing habitats. For example, due to its rapid growth rate of 2.5 ft/week, the Giant reed masks sunlight, killing surrounding native species, and can cause erosion and wildfires due to its loose roots and high flammability, respectively [4]. Opposite leaf Russian thistle can harm native species like pickleweed by altering soil salinity and pH [5], threatening the food source of the already endangered salt marsh harvest mouse. Problems related to invasive species are compounded because these species typically lack any natural predators to control their populations [6]. Maintaining native ecosystems is crucial as the spread of invasive species can lead to catastrophic collapses – a phenomenon where massive native life is lost due to a chain reaction of codependent species.

One approach to restore wetlands and other ecosystems is to physically remove invasive species and subsequently recultivate the native populations [7]. Recultivating native species without the removal or suppression of invasive species is not sufficient as many alien species are more competitive for light, water, and nutrients [8]. Beyond resource competition, invasive species can alter the long-term health of soil, making it uninhabitable for native species. Soil microbial diversity and density can change because of invasive species [9] and natural nitrogen cycles can be disturbed due to excess litter and consumption from harmful invaders [10,11]. Additionally, invasive species can alter pH and salinity levels through the leaching of organic acids and uptake of salts [5]. Removal of these species results in long term restoration of soil health.

Recent grassroots restoration efforts have been made to fight the expansion of invasive wetland species. Save the Bay (STB), a nonprofit organization established in 1961 based in San Francisco, relies on volunteers to restore local wetlands. In addition to removing invasive species and reintroducing native species, STB also advocates for legislation that protects the wetland habitats. Over the past year they have removed 35,000 pounds of invasive species, managed 80 acres of habitat and planted 20,000 native species. Sites of restoration efforts include the Mundy lot (2022), Nursery Shoreline (2012), and Entry Vista (2022) regions of Palo Alto.

This work studies the long-term effects of restoration efforts by STB on the wetland area. Existing literature on wetland restoration is limited; in their review paper Karin et al. highlighted that most experimental studies on this topic used small plot sizes (< 1 m²), short time frames (one growing season), and observed predominantly grassland ecosystems [12]. Thus, there is a present need to study the ramification of invasive restoration efforts in the bay area wetlands across numerous, diverse sites over time. This study identifies the long-term impact of restoration on soil biomarkers like pH and salinity and characterizes soil health by growing native species in soil samples from control and restored wetland sites.

MATERIALS AND METHODS.

Save the Bay Site Treatment Overview.

The same method was applied to each of the sites that STB restored. The sites were first inspected to determine invasive species present (typically dominated by Russian Thistle) and the best native species to plant given the environmental conditions (e.g., sticky monkey flower would be planted in regions further from the water whereas a species like alkali heath would be planted closer to the bay). After inspection was completed, STB worked to remove invasive species from the sites by manual removal.

Three aforementioned sites, “Mundy “Lot” (2022), “Nursery Shoreline” (2012), and “Entry Vista” (2022), serve as “restored” sites for the purposes of this study. Additionally, three control group sites, “Mudflat”, “Outpost”, and “Bowl”, which have plans to be restored in the near future, serve as “control” sites for the basis of soil health comparison. Control sites were primarily overrun with Russian thistle, whereas all restored sites showed little to no invasive species.

pH and Salinity Methods.

6 soil samples of 300mL each were collected from 6 different San Francisco wetland sites (Fig. 1a). 3 sites were treated by STB while the other 3 sites were not and therefore suffered from invasive species (Fig 1b). Samples were then analyzed using the saturated soil paste technique.

Soil was first air-dried in a clean environment for 1 week. After the soil was dried, a mortar and pestle were used to grind the soil into a powder. The granularity of the powder was defined using a 2mm sift. 100ml of powdered soil was placed into a dish and mixed with 80ml of water. The paste was allowed to stand for 24 hours.

A Buchner funnel was attached to a vacuum tube and a pump. Filter paper was inserted into the funnel. 100ml of saturated soil was spread onto the filter paper. The vacuum pump was turned on for 1 hour, extracting water from the soil continuously. The pH and electrical conductivity of the filtrate were measured using a calibrated pH/EC meter. For each filtrate, 3 measurements were taken after calibrating the meter to ensure repeatability.

Grindelia Stricta Growth Methods.

500 mL pots were filled with soil from 6 different wetland sites. 3 sites were treated for invasive species through STB volunteering efforts while the 3 other sites weren’t treated and suffered from invasive species. The soil was hydrated before the seeds were planted into the pots with 65 mL of water. 25 Grindelia Stricta seeds were put into each potted soil sample and gently pushed down until they were approximately 5-10 mm beneath the soil and equally spread apart. The pots were watered with 30 mL of water every day. Any non-Grindelia Stricta sprouts were weeded.

For each pot, each day, the # of saplings that have germinated were taken. Every day, the measurements of the heights of every plant in each pot were recorded. Data was recorded for 65 days after the initial planting.

RESULTS.

The pH of the restored sites (figure 3A) ranged from 7.6 to 6.0. Of the restored sites, “N. Shoreline” shows the largest variation across samples, having a range of 0.8. The pH of the control sites ranged from 7.5 to 5.1. In contrast to the restored sites, with significant variability at all sites. The average pH of restored sites was 6.9, while the pH of control sites was 6.5 on average.

In contrast, the measured salinity (figure 3B) of each restored and control site showed little to no variation; however, Nursery Shoreline demonstrated an outlier of 1.2 mS/cm compared to the 13.1 and 15.5 mS/cm readings at the other two samples. This is in keeping with the high variability of its pH measurement across samples.

A statistically significant negative correlation between sample pH and salinity was observed, independent of restoration status, with the pH-salinity linear regression having an R² value of 0.7 (figure 3C). This strong negative correlation between site pH and salinity is a result of the wetland’s brackish water. First, increased salinity in the water results in an increased number of current-carrying ions, leading to larger conductivity. Common ions found in seawater include sulfates (SO₄^2-), which, when put into contact with microbial organisms, have been shown to form iron sulfides that oxidize and form sulfuric acid, lowering pH [13]. Moreover, seawater results in a high concentration of Na+ ions in the soil. Beyond contributing to conductivity, these Na⁺ ions displace existing pH buffering ions like Ca²⁺, Mg²⁺, and H⁺, resulting in a decrease in pH [14].

The total growth height of the Grindelia stricta grown in soil from restored sites was 47.2% more than the growth measured in soil from control sites (figure 3D). This indicates that the restored sites had a positive effect on the growth of Grindelia stricta. Figure 3E shows the breakdown of growth by individual site. The maximum growth for restored sites was at “Mundy Lot” with 65 cm of growth over 65 days since planting, nearly twice that of the maximum control site growth of 36.6 cm at “Mudflat”. Moreover, the total germination rate of 8.0% for restored sites and only 4.0.% for control heights indicates that restored sites have more suitable conditions for Grindelia Stricta germination.

Figure 3F shows the average growth height as a function of days since seed germination, characterized by the day of first plant growth sighting. Both control and restored sites showed similar growth rates when averaged and normalized to days after germination.

The pH of the soil also did not influence total growth height (figure 4). For example, “Mundy lot” had a slightly acidic mean pH of 6.1 and a total growth height 6.1cm more than Entry Vista (mean pH of 7.5). Initially, this indicates lower pH should result in more growth. However, “Bowl” had a mean pH of 5.5 and showed 22.3 cm less total growth than “Mudflat”, which had a higher pH of 7.1. Similarly, soil salinity showed no trend with total growth height (figure 5).

 

DISCUSSION.

The lack of correlation between pH or salinity and the total growth height indicates that there could be other biotic or abiotic factors contributing to the increased growth of grindelia stricta in restored sites. The microbial makeup of soil with a healthy native population will be better suited for growing the native grindelia stricta used in these experiments. The lack of correlation also serves as a tool to show that the effects of wetland restoration extend beyond extrinsic factors like pH and salinity, which can be impacted by the environment of the sample in addition to the invasive/native makeup. As seen in figures 3A and 3B, a given site can have a wide range of pH and salinity values; however, consistently these restored sites had more total growth and higher germination rates.

The variability and lack of growth from the “Nursery Shoreline” site is also of note. This site was restored in 2012, a decade before the other two restored sites. Because it showed high variability in pH and salinity, like the unrestored sites, and showed no germination or growth, it could suggest that there is a time effect related to site restoration and soil health. The two site that were more recently treated (2022) both grew > 35 cm of height in total and had less variability in their pH and salinity measurements.

CONCLUSION.

This paper examines the effects of restoration work in San Francisco Bay area wetland on the health of wetland soil. The growth of Grindelia stricta was monitored across restored and control sites, with findings suggesting a strong correlation between the status of restored sites and the total growth of Grindelia Stricta.

The pH and salinity were measured across the 3 restored sites and 3 control sites. These factors were not a significant determinant to total Grindelia Stricta growth, indicating other factors, such as microbial makeup, nitrogen and phosphorus levels in the soil, or other invasive species present, could be at play.

Unlike many previous invasive species / habitat restoration studies, this work was conducted across numerous sites with restoration efforts from varying time periods. This yields interesting findings like the impact of time seen at the “Nursey Shoreline” site, which had been treated a decade prior to the other restored sites, and exhibited no germination and high pH and salinity variation.

This study investigated the effect of wetland restoration on soil health via native flora growth, eliminating pH and salinity as growth factors. Future studies should examine other environmental and ecological factors, such as soil nitrogen, phosphorus, or microbial bacteria as an indicator of soil health. Identifying the exact factors affecting soil health and native plant growth in the future will help to strengthen future restoration efforts, support restoration missions, and fill a critical gap in ecological understanding.

ACKNOWLEDGMENTS.

I would like to sincerely thank my teacher, Margaret Deng, for her continuous support and guidance throughout this research project. I am also grateful to my mentor, Collin Finnan, for his valuable advice and encouragement during the entire process. Special thanks to park rangers Alison Hlady and Lisa Myers for their assistance and expertise. Finally, I am grateful to Jessie Olson, the Save the Bay restoration lead, for her collaboration and guidance in coordinating the restoration efforts I studied.

REFERENCES

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Posted by buchanle on Thursday, May 14, 2026 in May 2026.

Tags: Grindelia Stricta, Restoration, Soil Health