Trees and Wetlands- World Wetlands Day 2024

Wetlands and the Diablo Mountain Range (source)

Today, February 2nd, is World Wetlands Day! This celebration of wetlands was created in 2021 by the UN General Assembly in order to raise awareness about the importance of wetlands and the dangers this critical habitat faces. For 2024, the World Wetlands Day theme “Wetlands and human wellbeing” strives to educate the public on how human health and wellbeing is tied to the health of wetland ecosystems. While Our City Forest does not work directly with wetland habitats, growing the urban forest has positive impacts for many ecosystems, including wetlands. In honor of World Wetlands Day, this article will give a brief overview of wetlands and how they are connected to trees and the urban forest.

What is a Wetland?

The EPA defines wetlands as “areas where water covers the soil, or is present either at or near the surface of the soil all year or for varying periods of time during the year, including during the growing season”. This broad definition means that wetlands are incredibly biodiverse! The National Oceanic and Atmospheric Administration (NOAA) divides wetlands into 5 types: marine (ocean), estuarine (estuary), riverine (river), lacustrine (lake), and palustrine (marsh). These categories include both saltwater and freshwater environments, as well as brackish (a mix between freshwater and saltwater found in areas like estuaries where the salinity levels fluctuate). The prolonged presence of water has resulted in plants adapting to these wet conditions. These plants are called “hydrophytes”. For example, the freshwater marsh community in the San Francisco Bay includes hydrophytes such as cattails, rushes, sedges, willows, ferns, and tules (nps.gov).

Tule (Schoenoplectus acutus var. occidentalis) (source)

Why are wetlands important?

Wetlands are biodiversity hotspots that have many critical ecosystem roles and provide food for over 1 billion people. Wetlands filter water of excess nutrients and pollutants, provide flood and erosion control, and provide food and shelter for both migrating and permanent wildlife. According to the UN, wetlands, while only covering approximately 6 percent of the Earth’s land surface, “40 percent of all plant and animal species live or breed in wetlands”. Wetlands are also “carbon sinks”, meaning that they capture more carbon from the atmosphere than they release. In fact, they capture more atmospheric carbon dioxide than any other ecosystem on earth.

Threats to Wetlands

In the US, 60,000 acres of wetlands are lost per year. This loss is due to many different causes such as being drained for agricultural use and development for industry, habitat fragmentation, pollution from run-off, eutrophication from agricultural run-off and commercial fertilizer, competition from invasive species, and changes in water level and quality. Unfortunately, California has lost 95% of its historic wetlands. In the post-gold rush era, wetlands were seen as unproductive, wasted land. Many wetlands were intentionally drained for agricultural use. In San Jose during the early 20th century, Laguna Seca, a 1,000+ acre freshwater wetland located at the present intersection of Santa Teresa and Bailey Avenues, was drained and burned for agricultural purposes. The Peninsula Open Space Trust (POST) describes Laguna Seca as a place where “water slowed and fanned out across the landscape, creating a mosaic of wet meadows and ponds, providing habitat for a variety of wildlife and percolating into a groundwater basin that now supplies San Jose with drinking water”.

Laguna Seca (source)

How are trees and wetlands connected?

Trees and forests, including the urban forest,  play a critical role in protecting wetlands and other parts of the watershed by regulating water flow, providing water filtration, and preventing erosion. When the tree canopy is removed and replaced by impermeable surface such as roads, parking lots, concrete, etc., there is an immediate impact on the watershed (PSU). Water runs off of the many impermeable surfaces found in urban landscapes rather than being slowed by the tree canopy and absorbed into the soil. These large amounts of water are often carried directly to streams, lakes, or rivers without first being naturally filtered by trees and other plant species. This large amount of water increases erosion and deposits large amounts of sediment, both of which can destroy important habitats and breeding grounds. The canopy is first to slow incoming rainfall which then drips down into the soil where it is absorbed and gradually released into the watershed, including important groundwater stores. According to Penn State University, 

“Average interception of rainfall by a forest canopy ranges from 10-40% depending on species, time of year, and precipitation rates per storm event. In urban and suburban settings a single deciduous tree can intercept from 500 to 760 gallons per year; and a mature evergreen can intercept more than 4,000 gallons per year”.

(source)

This water is then filtered by trees and other plants before entering the watershed. Excess nutrients, such as nitrates and phosphates, come from agricultural sources as well as urban fertilizer use (such as for lawns). While abundant nutrients may sound like a good thing, high levels cause a detrimental effect called eutrophication. Excessive nutrients result in unbridled growth of plants and microorganisms such as algae. When these plants and microorganisms decay, the amount of bacteria needed to break them down consumes the dissolved oxygen in the water, creating what is often referred to as “dead zones'. These dead zones, stripped of oxygen, suffocate animals such as fish which have a key role in the food web . Furthermore, other contaminants such as metals and pesticides are also removed from the water by plants, particularly woody plants like trees and shrubs. While wetlands can filter pollutants and nutrients, the accumulation of too many nutrients can outweigh the ecosystem’s filtering capacity.

Eutrophic water with algal bloom (source)

Where do wetlands fit into this picture? Without trees and other plants that make up wild and urban forests, these excessive flows of water, nutrients, and pollutants enter straight into the watershed and into down-stream wetland systems. Eutrophication of a wetland can change the water quality and chemical composition and completely destroy the plant and animal life.

How can I help?

While Laguna Seca was mostly drained, it is not completely gone! It remains as San Jose’s largest freshwater wetland and is located in the critical Coyote Valley habitat corridor. You can volunteer with Santa Clara Open Space Authority to help save and restore this important habitat. Grassroots Ecology, based out of Palo Alto, also does restoration work in San Jose’s Alviso wetlands where the Guadalupe river meets the San Francisco Bay. You can also plant trees or convert your lawn to help capture more stormwater and filter it before the water drains into the watershed.

More Than Trees: Creating Habitat for the Urban Environment

What do you see when you think of a forest? Do you see oaks or redwoods? Look below the canopy, between and below the trees. There are mosses and lichens hanging and climbing amongst branches, trunks, and rocks. The creeks and rivers are full of fish and aquatic insects. Below the soil, fungi connect the forest in a mycelial network that orchestrates communication and nutrient exchanges. Just like wild* forests, urban forests are composed of much more than just trees. Urban forests are ecosystems that integrate humans and our built environments with cultivated and wild plants, wild and domesticated animals, fungi, microorganisms, bodies of water, soil, minerals, and organic matter. Our City Forest is dedicated to planting not just trees, but also understory plants that create habitat and save water.

*For lack of a better term, I use the word “wild”. The reason this word can be problematic is that the word wild implies a place where nature lives outside of human society. It brings up images of pristine places untouched by humans. However, the myth of the pristine wilderness is highly inaccurate, especially in North America where we know that forests and other ecosystems  were tended to for thousands of years by Indigenous peoples and continue to be tended to this day.

The Urban Environment

Integrating the built environment with urban forests often requires innovation to solve problems. Ideally, the built environment (which consists of buildings, infrastructure, agricultural lands, etc.) would mesh with the rest of the environment with no conflict. However, many of the systems in urban and suburban environments make the built environment difficult to survive in and even uninhabitable in some situations. For example, the Urban Heat Island Effect describes how surfaces common in urban settings, such as glass, concrete, and asphalt, absorb and retain heat, causing higher temperatures. These higher temperatures put more physical stress on trees, animals, and other organisms in the urban forest while also evaporating more water.

Worldwide urban development has also caused habitat loss and habitat fragmentation, which are both tied to one another. Habitat loss is the reduction in total land while habitat fragmentation is the isolation of habitat islands from others. Generally, habitat loss leads to fragmentation, making it difficult and dangerous for animals to migrate, establish new ranges, escape wildlifes, and adapt to climate changes. However, it doesn’t need to be this way. Creating and maintaining habitat corridors, safe road crossing (under or over), green roofs, lawn conversions, and more can help solve the conflict between the environment and urban development and lead to integration. Our City Forest has a role in this integration not by just planting trees but also by converting lawns into drought-tolerant landscapes.

Lawns

One of the biggest issues regarding the urban environment across the United States is the lawn. In the United States, there are over 40 million acres of lawn, roughly the same size as the state of Florida! Lawns, including artificial grass, pose many negative consequences. Grass lawns consume up to 31 gallons per square foot, per week. The average residential lawn in the U.S. is 10,871 square feet (in California the average is 5,575 square feet). That’s up to 337,001 gallons of water per week to water the average U.S. lawn and up to 172,825 gallons per week in California! To put these numbers into context, the average shower is 17 gallons of water. The upper limit of water used to water the average U.S. lawn is equal to 639 showers! Alternatively, many drought-tolerant native plants usually do not need any watering once they have become established. 

Besides the issue of water use and waste, lawns and artificial grass have no ecological value. They are desolate places for most plants and wildlife. They do not provide food, shade, or shelter for animals. Many grasses used for lawns are invasive, spread easily, and can be difficult to remove or kill, such as Bermuda Grass which is common in warmer climates and is used  in San Jose. Grass like this pushes out native plants that animals and other organisms have evolved to depend on.

Non-native grass lawns are also not able to capture as much water as native grasses and other native plants. The image below demonstrates the difference in root systems between lawns and prairie plants. To the far left, you can see that the root system of typical lawn grass is only 3-4 inches, whereas the native plants to the right have root systems up to 12 feet long! Longer root systems help to stabilize the soil and prevent erosion, capture more stormwater, filter pollutants in the water before it ends up in the ocean, and help to restore groundwater.

Lawn Busters

The Lawn Busters Team at Our City Forest is dedicated to converting lawns into drought-tolerant landscapes that save water and create habitat in a county that has suffered much habitat loss and fragmentation. We plant a wide variety of native and drought-tolerant plants from California Lilacs, Penstemon, succulents, native grasses like Creeping Red Fescue and California Fescue, tree-like shrubs such as Toyon and Elderberry, various Mediterranean species, and more! In some cases we also include trees in a landscape design, but overall we focus on grasses and shrubs. While trees are important to the urban forest, shrubs are just as important and provide food, shade, and habitat for non-arboreal species.

Recently, the Lawn Busters Team sent out a wildlife reporting survey to homeowners who had their lawns converted by Our City Forest. The majority of respondents reported an increased presence of wildlife in their new landscape, especially pollinators such as bees, butterflies, and hummingbirds as well as lizards, hawks and other bird species, and even opossums!

In addition to transforming lawns, we also teach the community how they can transform their lawns by themselves through workshops held at our Urban Forestry Education Center. Many of the plants we use in lawn conversions can be seen in our various gardens at our Education Center and more gardens are being added, such as a bioswale, a native wildflower meadow, and a Lawn Buster’s demonstration garden.

If you would like to continue learning about the urban forests and the projects Our City Forest works on to increase San Jose’s canopy and understory, consider volunteering with us! As a volunteer, you can learn how to plant trees and shrubs, learn gardening skills, learn about native and drought-tolerant plants, climate change, community science, and more!

Our City Forest - California Native Plants

The native plants and trees in our ecosystems are indispensable. Native plant species are superior in supporting pollinators and local wildlife, such as bees, butterflies, amphibians, reptiles, and mammals, as demonstrated by ecologists, wildlife biologists, and entomologists. On the other hand, invasive plant species have the power to disrupt environments, diminish biodiversity, drive native species to extinction, and clash with them for scarce resources. This invasive species takeover can and is occurring everywhere, including California. For Californians looking to add some greenery to their yards, it  can be especially challenging to tell native plants from invasive ones. Fortunately, the general public can plant and obtain a wide variety of native California trees from Our City Forest in San Jose, CA. They take tremendous pride in the quality of their plants and go to considerable lengths to ensure that they all mature into robust, healthy trees. These are a few from their wide variety available at their nursery as of November 2023:

1. The Acer Macrophyllum (Big Leaf Maple) is a huge deciduous tree. Although it’s typically 50 to 65 feet tall, it can grow to a height of even 100. The diameter of the trunk on the other hand, can grow to over 3 feet. Its native range extends from southernmost Alaska to southern California in western North America, primarily along the Pacific coast. 

2. The Aesculus Californica (California Buckeye) is either seen as a tiny tree or a big shrub. It usually spreads out and has several trunks, with a broad and tall crown. This species, belonging to the Sapindaceae family, is the sole native buckeye in California. Early spring usually sees it leaf out and its creamy-white to pale pink flower spikes begin appearing- butterflies adore the blooms!

3. The Cercis Occidentalis (Western Redbud) is a little deciduous tree that grows in California's highlands and foothills. Early in the season, the glossy heart-shaped leaves on the slender brown branches are light green, but as the season progresses, they turn a darker shade. Bright pink or magenta, beautiful flowers appear in clusters throughout the shrub in the spring and give the plant a vibrant, eye-catching appearance in the landscape.

4. The Myrica Californica (Pacific Wax Myrtle) is a naturally occurring shrub of to the Myrtle family that is mainly found in northern and central California along the coast. It can also be found as far north as British Columbia and as far south as Los Angeles County. It has a lengthy lifespan and grows rather quickly. It develops actively in the spring and summer, reaching a tall shape up to 33 feet in height. Early summer is where you can bear witness to the blooming of its yellow flowers.

5. The Quercus Agrifolia (Coast Live Oak) is a recognizable, magnificent tree that supports the local flora and animals. Its massive canopy and twisted branches make it easy to identify. A diverse range of birds and butterflies are drawn to the acorns of the Coast Live Oak, which blooms in the spring and provides food and habitat for over 270 different kinds of birds and insects. These hardy trees can reach mature heights of 30 to 80 feet. They can also live for decades, frequently exceeding 250 years.

6. The Quercus Douglasii (Blue Oak) is a deciduous tree that can withstand droughts and gives local wildlife food and shelter. It provides food for insects, squirrels, and birds and serves as a host plant for numerous moth and butterfly species. Additionally, the blue-green foliage of this tree is what gave rise to its name. Blue oaks can reach heights of more than 80 feet, and their canopy can enlarge to at least 30 feet in width. 

7. The Quercus Lobata (Valley Oak) is the biggest oak species in North America. They can grow up to 60 feet in 20 years, 20 feet in 5 years, 40 feet in 10 years, and so on; fully grown specimens can live up to 600 years. Its wavy bark, which has a pewter tint, contributes to this species' appealing appearance. October is when acorns fall throughout most of the range. They are consumed by a wide range of mammals and birds, such as the California Ground Squirrel, Acorn Woodpecker, Western Scrub Jay, and Yellow-billed Magpie. Like many oaks, this tree is resistant to wildfires.

These are just a few of the many, many plants available at Our City Forest. To learn more on the different species/plants available at the nursery, you can visit their website ourcityforest.org or visit them in-person at 1000 Spring St, San Jose, CA 95110. Do your part in promoting biodiversity today by considering planting some native trees. Thank you :)

Sources: ourcityforest.org & calscape.org