Trees and carbon intake: the bigger the better

By Lauren Lewis

One of the defining characteristics of trees as compared to other plants is their longevity. Their growth patterns at the cellular level evolved not just for reaching up and out but also for “secondary growth”- the layering, repetitive cell growth that thickens the plant, making it highly stable and durable. This and other longevity adaptations mean trees have much to teach us about the environment of their past, and an important protective role to play as we navigate a changing climate.

Take the bristlecone pine: the oldest non-cloning lifeform on earth. The oldest bristlecones are around 5000 years old, and 3000 years would not be unusual. A big part of this longevity is that bristlecones live in extreme mountain environments where few other plants can live. This means the pines have little competition for resources and also that plant mass that could fuel fires around them is minimal. But bristlecones also have some special adaptations that keep them going. Their wood is high in resin that is an effective deterrent to pests. And they can compartmentalize to minimize damage- when a large root dies, the corresponding section of the tree and it’s bark dies too, which keeps the damage from spreading inside the tree. Eucalyptus does a similar thing, dropping a limb when the tree is stressed, to reduce the resources needed by the tree rather than letting the whole tree struggle.

Tree rings have long been used to understand the age of trees, but they’re now also being used to better understand climate change. After California’s severe drought in 2012-14, some physical geographers used cores of living blue oaks and blue oak stumps to address the question of whether the drought was really very unusual or not. They correlated tree ring thickness with precipitation and temperatures records from the last 100 or so years, and then measured tree rings going back 1200 years to infer the climate in those years. They found that the precipitation wasn’t exceptionally low as compared to previous droughts, but that when you added in the high temperature (and therefore the greater evaporation and overall lower available moisture to the trees), the 2014 drought was indeed exceptionally extreme.

So trees can teach us about ancient climate, and schoolchildren know that Earth’s rainforests act like the planet’s lungs by taking in carbon dioxide and releasing oxygen. But it also turns out that bigger trees are doing relatively more of this crucial work than smaller ones. In a study of over 400 tree species from various latitudes and longitudes, researchers found that as tree size goes up, so does mass growth rate (or rate that the tree builds mass), and therefore carbon accumulation. As an example, in the study’s American old growth forest sites, trees of >100cm diameter “comprised 6% of trees, yet contributed 33% of the annual forest mass growth.” So while it’s great to plant new trees to offset air travel, it’s Earth’s huge, old trees that do more to slow the accumulation of atmospheric carbon. They merit passionate preservation for this and so many other services they provide.

 

Native bees offer low effort, high payoff species conservation

By Lauren Lewis

You’ve probably seen media about the alarming loss of bees in the last decade or so, and it has most likely been framed as a biodiversity conservation issue. That angle on the issue is misleading because the vast majority of this public attention has been directed, often unknowingly, toward honeybees only. Honeybees are not native to North America and are essentially a domesticated species: highly managed and transported for crop pollination and honey production. The threat to crop pollination from honeybee decline is hugely important — valuable foods like avocados, blueberries, almond all rely on pollination for the fruits to develop — but it’s not exactly a conservation issue with ecosystem preservation as the goal. What does fit into that category is the issue of native bee population decline.

There are around 4,000 species of native bees in the United States, and many of these have shown alarming population decline in recent years. This is a concern if we care purely about the inherent value of biodiversity, but also because native insects are typically linked through their roles as pollinators and prey to so many other species in an ecosystem. (You can’t lose one species in an ecosystem without other species being affected- they’re all connected!) Research has linked native bee population decline to diminished pollen resources following land development, and sadly also to native bees’ higher susceptibility to agricultural chemicals, as compared to honeybees. Some research in California’s Central Coast has shown that honeybees and native bees are directly competing for scarce food resources and native bees may be losing the battle.

The silver lining here is that more research is beginning to show just how important cities can be as refuges for native bees and other pollinators. Studies have shown greater diversity of native bees in cities than surrounding agricultural land, suggesting that the urban environment is more hospitable to the bees. The presence of gardens, which can provide both food (pollen in flowers) and shelter (for native bees that’s most often bare ground or rotting wood for burrowing), is key. Given these relatively minor requirements for survival, supporting native bees is potentially low effort and high impact. City governments could put out PSAs that encourage folks to grow some native flowers, in whatever space they have available, and to leave a bit of bare dirt for bee burrows. (Pointing out that native bees are mostly solitary, rather than swarming, and also far less likely to sting than honeybees, would probably be helpful, too.) Happily, a blooming garden that supports native bees also brings beauty and joy to us humans.

 

The ecosystem diversity of old SF

By Lauren Lewis and Elisa Baier

Before the SF peninsula was settled by Europeans and urbanized, the landscape of many diverse ecosystems — shrubby, grassy, shore, etc — allowed native coastal Californians to thrive. They lived here without developing any farming methods that we associate with traditional agriculture; instead they carefully tended and collected useful plants, foraged shellfish, and created conditions favorable for hunting. The vast majority of these ecosystems are gone now, but if you know the plants and patterns to look for, you can find each of them in small corners of the city today.

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Of all the native plant communities, coastal scrub is the one that modern SF retains the most of, in Glen Canyon, parts of Mount Davidson, and the Presidio. It is categorized by California sage, coyote bush, blackberry, poison oak; shrubby, hardy plants that sheltered small game animals and provided edible berries and a lot of medicinal plant material to native people. A walk through this environment is rich with scents of the leaves’ oils and songs from the chaparral birds.

The Yelamu gained some resources from coastal scrub, but they gained substantially more calories from the large game that grazed on grasslands, so they burned the land regularly to keep it as grassland. (Unburned land eventually became colonized by shrubs rather than grasses, and turned into scrubland.) Because people managed the land this way, native prairie was the dominant plant community when settlers arrived. You can find bits of native grassland on the tops of SF’s hills now, and in the spring when the wildflowers are abundant, you can imagine the beauty of the old expansive flowered grassland with views of the water.

Along the edges of the city were extensive sand dunes, which supported a unique set of super-hardy plants tolerant of salt spray, wind, low-nutrient soil and unstable ground – dune strawberry might be the most recognizable. But because unstable ground is antithetical to development, plants such as ice plant and European beachgrass have been extensively planted for over 100 years to stabilize the dunes, and the vast majority of the habitat has been lost.

IMG_4777A similar explanation applies to the riparian and wetland habitats of old SF. Unsurprisingly because of the presence of water, these habitats were biologically very rich, but bay fill and culverting has resulted in just small remnants. The free flowing portion of Islais Creek in Glen Canyon Park gives a sense of what creek habitats may have been like and Heron’s Head Park resembles the marshy habitat once surrounding the peninsula on the bay’s edge.

We’re lucky that some parts of the natural world will probably be apparent in San Francisco as it grows – the steep hills, ocean and bay will likely be obvious forever. To see the more subtle parts of our natural history we’ll have to conserve what still exists and maybe attempt to reconstruct other areas, like old marshes or creeks. At Small Spot Gardens, we suspect that having access to California landscapes in San Francisco helps us connect to a bigger picture – maybe living with and learning about the old ecosystems and the people who tended them will help us understand how to individually handle an increasingly fast paced world and how to collectively approach a quickly changing climate.

 

Fungi in your soil: a reason to celebrate

By Lauren Lewis

The weeks following our late winter rains, when the soil is as soaked as it’s ever going to be in SF, is when we’re most likely to find mushrooms in our gardens. If you do, it’s a reason to rejoice, because a mushroom is the above-ground evidence of fungi in the soil, and it’s hard to overstate just how important fungi are for healthy soil and healthy plants. We mostly don’t even notice its presence, but our plants depend on fungi for their growth.

Around 90% of all plants form a mycorrhiza with fungi, a symbiotic relationship whereby the hyphae of the fungus (a fungus’s massive underground network of microscopic filaments) connect to the plants’ roots and nutrients are passed between plant and fungus. Fungi receive carbon that was photosynthesized by the plant, and the plant receives…so much assistance. Nutrients: mycchorizae supply roots with necessary nutrients like phosphorus that are hard for the plant to absorb independently. The fungi exude acids that break down rock and solidified soil, which turns existing nutrients into a form that’s absorbable by plant roots. Water: the network of hyphae in effect expands a plant’s surface area in the soil, letting the plant reach more available water. Mycorrhizal soil also tends to have greater water retention capacity, so there’s more water for the roots and hyphae to reach. Communication: this underground network also creates a means of communication between plants– a plant enduring a pest attack sends out chemical signals to neighboring plants, and the mycchorizal network allows the warning signal to reach farther. The list of benefits received between fungus and plant goes on.

From a gardening perspective, we can celebrate fungi for their ability to support perennial plant growth over annual (i.e. weedy) growth. Research has shown that fungal soils can deter growth of weeds, while supporting other plants in the ways described above. Over time, myccorhizae create a soil environment that’s hostile to more short-lived plant species and welcoming to long-lived plants, mimicking mature “wild” plant communities.

Fungi support our gardening ambitions, and they can also help fix the messes we humans have made. In urban settings, where soils are often contaminated with oils and petrochemicals, fungi offer a grassroots solution via mycoremediation. Most fungi get their energy by breaking down large carbon-based molecules, like those in wood, which means they’re also good at breaking down petroleum and related carbon-based chemicals. Communities are starting to employ controlled use of fungi to clean up soils, and there is data to support the practice. It’s a method that can be done piecemeal, by leaving discovered garden mushrooms to do their thing and avoiding fungicides, or on a community level to solve more serious contamination. (To sate all your fungal curiosity, check out the indispensable book Mycelium Running: How Mushrooms Can Help Save the World, by Paul Stamets.)

 

Small but meaningful action through wildlife corridors

By Lauren Lewis

As the title of this blog implies, our primary focus is: how does an individual garden in the city fit into the bigger picture? And the answer is: there are so many different ways that it’s exhilarating to think about (if you’re nerdy like us!). Arguably the most approachable example of how a garden connects to the world around it is the movement of wildlife in and out of a garden. The vast majority of urban animal species, both invertebrate and vertebrates, are not particularly restricted by fences between gardens, but their viability is hindered by limited green space and limited appropriate vegetation. So that’s where our gardening choices become important. They help determine the presence or absence of wildlife corridors.

Urban wildlife corridors exist at the intersection of urban ecology and movement ecology, which is a very new branch of study in ecology. On a larger physical scale, movement ecology has enjoyed a recent uptick in attention because of executive branch efforts to open up previously-protected US land to natural resource extraction, which creates more barriers to animal movement. The New York Times just highlighted new research showing the extent to which human activity restricts animal movement globally, and also the surprisingly bipartisan efforts to protect migratory paths in the American West. In an urban context the animals are smaller and less attention-grabbing, but gardening in a way that promotes urban wildlife movement can be a small but meaningful action.

img_5431.jpgA perfect example is CalAcademy biologist Tim Wong (@timtast1c), who learned about the struggling Pipevine Swallowtail butterfly population in SF and took action that has substantially improved the species’ population. The Pipevine Swallowtail caterpillar feeds only on California pipevine, which has become rare in SF, so Tim found the plant and began growing it in his butterfly-friendly garden at home. As the butterflies have thrived there he has brought both the plant and the caterpillars to the California Native garden at the SF Botanical Garden, and the population is growing.

For the rest of us in SF, there’s a wonderful resource for learning and garden planning- the Green Connections Ecology Guides. The city’s Green Connections program will create a bunch of long “paths” through the city over the next decade that are specifically designed to be safe and pleasant for travel by foot or bike. The idea of the program’s Ecology Guides is that if humans can travel along those routes, then appropriate plantings can make them pleasant routes for animals we want to support too. Check if there’s a route near your garden that can help guide some of your planting choices, but even if not, some plant-focused routes, like the Coyote Bush route along Kirkham out to the beach, are very informative about the ecosystems our city was built upon.

Using weedy plants to fight weeds

By Lauren Lewis

A weed is a weed almost always because it grows fast. It sprouts quickly and grows quickly, and can therefore take up more water, sunlight, and nutrients than its neighbor plants. At Small Spot Gardens, our primary strategy for weed control is finding ways to help our desired plants outcompete the weeds that are always trying to get a foothold. In a brand new garden, this often means planting some larger plants that take up space and sunlight, which makes the environment a little more challenging for little weeds. It also means choosing plants with the same speedy growth as weeds. If our chosen plants can grow as quickly or quicker than their weedy competitors, weeds will be less of an issue over time. These plants are some of our favorite fast growers.

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Photo via plantright.org

Centranthus ruber, often known as Jupiter’s Beard: Once you learn about this plant, you’ll start to notice it everywhere. Its bright red or pink flowers produce seeds with little fluffy wings like dandelions, so it really does grow everywhere, and therefore is sometimes considered an aggressive weed. In South Africa, where the climate is Mediterranean like ours and many native plants are endemic, Centranthus is treated as an invasive and banned from use. Here in California it’s not viewed as invasive by the California Invasive Plant Council, but it is counted among those plants that could become invasive. In our gardens it provides vibrant color and pollinator food, and its reseeding capabilities make it overall low maintenance. If you want to keep the reseeding to a minimum, trim off dying flowers and put trimmings in the green bin to isolate seeds.

 

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Photo via calscape.org

Fragaria chiloensis, beach strawberries: Beach strawberry is native to our region, and while Native Californians did collect and eat the fruit, it was a small treat rather than a staple of the diet because the fruit are tiny and relatively sparse. What’s great in our gardens is that this plant reproduces by sending out runners, and quickly forms a lovely thick mat that covers the soil surface. This kind of growth is ideal for crowding out weeds, and also for protecting the soil surface from erosion during rain and from drying out at all other times. The cute white flowers and occasional fruit are aesthetic bonuses.

 

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Photo via davesgarden.com

Oxalis, wood sorrels: You probably think of oxalis, with its highlighter-yellow flowers and clover-like leaves that take over, as your garden’s biggest springtime nuisance, but that is specifically bermuda buttercup (oxalis pes-caprae), which we would never intentionally plant. However, the oxalis family has hundreds of varieties, and many of the others are beautiful and hearty, but not nearly as invasive. Our choices, like oxalis vulcanicola, create gently-floating groundcover, and they can be dispersed around the garden purposefully by simply cutting stems and sticking them in the ground.

Follow us on Instagram for more wonderfully weedy species this month!

Seed Power

By Lauren Lewis

One of the defining characteristics of plants is unfortunately a limitation: their inability to move around. They have to reach out from where they are to find water and mates, or let those resources come to them, and they’re certainly disadvantaged when it comes to escaping danger (although many have evolved ingenious compensating strategies). Because of this limitation, the seed, the small but mobile plant part, holds great power. The seed has arguably the greatest power over a plant’s survival: it determines where the plant will forever live and grow by “choosing” when to germinate. Recent groundbreaking research has illustrated the almost decision-making power seeds possess for this purpose.

IMG_5067As a general rule, seeds move through space in a dormant state, waiting until conditions are right for germination. The timing of germination is crucial- the environment might be too cold if germination is too early, but too late and the plant will likely be outcompeted by surrounding plants who are already bigger. Seeds respond to environmental cues by producing the hormones abscisic acid (ABA) and gibberellin (GA), which promote dormancy and germination, respectively.

In a 2017 study of Arabidopsis, a fast-growing cress common in plant research, researchers mapped the 3-4000 cells in a seed and found that different cells produced these two antagonistic hormones, and that these cells were clustered in a dormancy group (ABA) and a germination group (GA) separated but near each other, and in the root tip of the seed. This layout is somewhat analogous to a “brain,” where differentated cells can send signals between each other, and the space between them is meaningful. The production and transport of more dormancy hormone (ABA) in this “brain” center of the seed maintains dormancy, and greater germination hormone (GA) stimulates germination. The researchers found that the physical separation of the cells is key because it allows the plant to more precisely respond to variations in temperature, which is an environmental signal of changing season.

Discoveries like this “brainy” cell layout in seeds are finally helping us appreciate agency that plants are capable of. In a managed setting like a garden, plants have us to determine their fates through propagation techniques. A plant off on its own appears to have a comparative disadvantage, but happily, recent science is giving us a better understanding of a plant’s own active role in its rebirth and survival.