Spring Leaf Drop

It’s normal for evergreen plants to shed their oldest leaves, which are closer to the trunk than the newly emerged younger leaves at the ends of branches. While the term “evergreen” may make one think that the leaves last forever, evergreen leaves may last between one to several years before being replaced by new leaves.

Don’t panic if you see your evergreen plants dropping large amounts of yellow leaves in the spring! These are actually older leaves giving up their nutrients to the flush of young leaves that seem to sprout up overnight as the temperature warms.  You might worry that leaves turning color and falling to the ground indicates that your evergreen plant isn’t happy or is stressing, but this spring leaf-fall is very much a vital, living process that prepares the plant for a giant growth spurt each spring. As I discussed in another post, leaf drop is only possible if the leaves are attached to a plant or branch that is alive. In fall, leaf drop is a preparation for winter dormancy; in spring, it is a sacrifice of nutrients from old leaves into young leaves that need an injection of critical nutrients to expand and mature quickly for the growing season.

Spring leaf drop from our evergreen Sugarbush (Rhus ovata). This is a normal response to the flush of new growth in spring and early summer. “Out with the old and in with the new” is the agenda that moves valuable nutrients from older leaves into new, fast-growing spring leaves. After transferring their nutrients to the new growth, the old leaves fall to decompose and release their remaining nutrients into the soil as a continuing fertilizer…so their death keeps recycling vital gifts.

People are used to seeing deciduous plants shed their foliage at the end of the growing season in autumn, over-wintering with bare limbs and branches. “Leaf-peepers” travel around the country every year to watch the spectacle of leaf colors as groves of deciduous trees prepare for their autumn leaf drop and winter sleep. But we are not as accustomed to the often significant leaf drop of evergreen trees just as spring entices new growth.

As spring triggers new growth, older leaves are busy sending their accumulated stores of essential minerals, sugars, and other hard-won products out of the leaf, and sending them to the fast-growing cells of new leaves. As minerals like iron, phosphorus, and other valuable nutrients are transported off-leaf, color changes paint the older leaves yellow and orange, a visible sign of the chemical transformations happening within each old leaf during its sacrifice before it falls.

So don’t stress over the overnight yellow apron of leaves underneath your evergreen trees and shrubs that appeared just as spring kicks into high gear. It’s just the elders making way for the next generation of youngsters…as we will all end up doing.


Notice that only the older leaves are turning yellow on this evergreen shrub, Sugarbush (Rhus ovata), not the newer leaves near the branch tips. The older leaves are transferring nutrients to the flush of new spring growth.



The Desert Underground

Who would guess that anyone could say, with any credible authority, “Mushrooms can help save the world?” And even more preposterous, that in the desert, fungal threads that connect to plant roots combat global warming? Even though these statements sound like science fiction, we are finding that both are proving to be absolutely true. 

These concepts were at the forefront of the vision to create an artistic, visually compelling book that would transport readers along a graphic “tour” of the unseen desert beneath the soil surface. Robin’s new book, “The Desert Underground,” will lead you on a virtual tour of the hidden but magnificent world of microorganisms intertwined with roots that silently works under our feet every day.

“The Desert Underground” book reveals the amazing partnerships that connect every native plant underground across the landscape. Journey through the interlocking biological and geological systems that work together to create a surprising carbon sponge, helping to combat climate change wherever desert soils remain intact.

Come on a virtual tour of the desert’s surprising living soils under your feet. Join in this tour within the pages of “The Desert Underground” book by Robin Kobaly.
Sample page from “The Desert Underground”

Robin’s newest book was released at a special exhibit featuring “The Desert Underground” at the Joshua Tree National Park Art Expo at the 29 Palms Inn in September 2019. A series of large graphic displays, created with the original artwork presented in Robin’s book, led viewers along an illustrated tour through our desert soils, delving deeper and deeper into the underground. The exhibit was created by Robin Kobaly and was presented by The SummerTree Institute, an environmental education nonprofit, of which Robin is Executive Director.

Visitors take a virtual “tour” of the desert underground at the
Joshua Tree National Park Art Expo in fall 2019.

For more information about
The Desert Underground” book,
or to order your own copy,
visit www.summertree.org/the-desert-underground/.

For an overview of the hidden but magnificent realm that works silently underground every day, read the feature article, “The Desert Under Our Feet” written by Robin Kobaly in the March 2019 issue of The Desert Report.


Check out our other events and workshops!  


Where can I see wildflowers?

The exceptional rains that have fallen over the fall and winter months are enticing colorful fields of wildflowers out for a spectacular spring showing. We were blessed with much-welcomed rain in southern California not just this past winter, but  in the fall, and those earlier rains produce our best wildflower shows. Please be mindful of trampling these delicate treasures; you can cut their life short and destroy the production of seeds for future years’ displays by crushing them under your feet while you enjoy them. You may track wildflower conditions throughout the Southwest desert on Desert USA’s website.

Five parks in Southern California regularly update wildflower reports on their websites during the viewing season:

A private retreat and wildlife sanctuary to view spectacular Mojave desert blooms is High Desert Eden in historic Pioneertown, California. You can stay in a luxurious southwest-style casita on 22 acres that backs up to protected lands within The Wildlands Conservancy and The Sand to Snow National Monument.



Mushrooms to the Rescue!

Mushrooms peeking up above ground tell of the presence of root partner relationships below ground.

Mushrooms peeking up above ground often tell of the presence of root partner relationships below ground.

Native desert plants are faced with a big dilemma: how to gather nutrients and moisture from soils that are practically devoid of either. To the rescue are fascinating, symbiotic root partners: fungal relatives of truffles, attached to plants’ roots by thread-like hyphae called mycelium. These living, microscopic fungal partners, or mycorrhizal fungi (literally “fungus root”), attach to plant roots and spread outward like a three-dimensional net, expanding the surface area and absorption capability of the plant’s root system by magnitudes, and boosting the plant’s immunity to disease.

Mycorrhizae: A Hidden Partnership
Between Plants & Fungi

You may be more familiar with the part of a fungus you can see above ground — the mushroom. But most of the fungus lives below ground as a spreading mass of tiny threads called mycelium. The mushroom you see is the  fruiting body of the root-like mycelium mass underground. Those fruiting bodies that peek above the soil, we call toadstools or mushrooms, and those that remain below ground we commonly call truffles. Spores released by the mushroom disperse into the air or across the soil and germinate when the soil is moist. For those mushrooms that form root partnerships with plants, the new mycelial threads from a germinating spore grow out in search of the roots of a new host plant to encase or penetrate, beginning a life-long partnership with that plant host. As the mycelium encounter dead plants or animals, they break down the debris to create rich new soil, and recycle the carbon, nitrogen, and other essential elements to absorb themselves or to pass on to their plant host.

A powder of brown spores is released by a muchroom across the soil.

A powder of brown spores is released by a mushroom across the soil.

Since these root partners are fungi, they have a tremendous capacity to break down, absorb and transport soil nutrients, along with soil moisture, directly into the roots of their plant host. These mushroom relatives cannot, however, make their own food, so they barter their gathered nutrient supply in exchange for food. In fact, potential plant hosts exude sweet sugars and other products from their youngest roots to entice mycorrhiza to connect with them. When that connection is established between the fungal mycelium and the plant’s roots, the host plant rewards the mycorrhiza for their efficient delivery of nutrients and water; the plant supplies their underground root partner with sugars manufactured above-ground in leaves that have been well-supplied with nutrients gathered by its below-ground root partner.

It is not the plant’s roots that absorb most of the water and nutrients needed for survival, but the plant’s microscopic underground fungal partners which live on or in those roots that do most of the work. Plant roots can only absorb nutrients in the immediate area around the roots, the rhizosphere, or a zone about 1/10 of an inch surrounding the roots. To extend its “nutrient reach”, plants’ roots secrete products to attract fungi (and beneficial bacteria), which hook into the roots and spread out in massive webs to bring in more nutrients to the plant. The thin fungal mycelium, only one cell layer thick, can contact much more surface area of soil than the plant roots, and can expand the effective root surface by up to 700% or more. In the end, a plant may invest about 10 to 20 % of the food it creates or collects (carbohydrates, vitamins, and amino acids) in exchange for a partnership with symbiotic fungi, but the plant may realize over 100 times the value of its investment in extending its nutrient reach by its roots fed by mycelium.

Plants Communicate Through the
“Fungus Internet”

Scientists have discovered that the mycelia connected to a plant host may also connect to one or more nearby plants, or to the mycelia radiating out from another plant’s roots. These interconnections allow for resource sharing between plants to help stabilize the whole plant community; if one plant has more moisture but lacks some vital nutrients, while its neighbor has access to those nutrients but needs more moisture, the networks of mycelium connecting those plants can transport the needed resources in both directions between plants. Wow—not the fierce competition we always thought existed between each plant, but an intentional sharing of resources to benefit the entire plant community…all achieved by what we call “lower life forms”.

Fungal threads of mycelia connecting neighboring plants can alert the neighbors of an attack by pests--so bad-tasting chemicals can be produced by the plants to repel the pests.

Root partner alert! Aphids attacking! Fungal threads of mycelia connecting neighboring plants can alert the neighbors of an attack by pests–so that bad-tasting chemicals can be produced by the plants to repel the pests.

One of the newer discoveries about mycorrhizal fungi is that they pass not only water and nutrients to their plant hosts, but also transfer information between plants, alerting neighboring plants about threats in their environment – such as invasions by insect pests – to stimulate their plant hosts to produce pest-repelling compounds. Anything that protects the plant host ultimately protects the root partner, so arming its host against invasion helps to ensure its own survival. Because of the rapidness of sharing both information and resources between underground fungal threads connecting plant to plant and to adjacent mycelia networks, mycelium is being called “Nature’s Internet”. As mycologist Paul Stamets writes, he believes that mycelium is the “neurological network of nature.”

Let’s think about this relationship in practical terms. When you are part of a community of root partners, you belong to an elite club. Being part of that club gives you access to special goods and services. Those goods allow you to withstand much more stress than you could ever have endured on your own. You are much more likely to survive, in fact, to thrive, because of the help of all your friends.

Mycelium, the thread-like hyphae of fungus, growing on dead wood.

Mycelium, the thread-like hyphae of fungus, growing on dead wood.

This is why living root partners are so critical to restoring disturbed habitats. If a long-lived native plant or a small island of native plants is left within a parcel that is bladed for development, you will be retaining a living, working segment of the “ROOT PARTNERS’ CLUB”, which acts like the “sourdough starter kit” for the soil to assist all other plants reintroduced into that parcel. The parcel still retains established native landscaping that requires no care, while new plants will benefit from the living underground resources ready to support them without added fertilizers, and most weeds will be held at bay. Everyone benefits when the Root Partners’ Club is left alone to flourish.

In our deserts, native plants with the longest roots are those that are capable of surviving the worst droughts. Deeper roots can reach moisture deep in the soil that may have fallen as rain many years earlier, while surface soils may have dried out years before. Long-lived desert plants have well-developed networks of both roots and mycorrhizal partners. Besides the recognized value that long-lived desert plants have in providing long-term food, shelter, and cover for many generations of wildlife, as well as contributing to overall ecosystem stability, new studies are unveiling surprising contributions of deep-rooted desert plants to global environmental issues.

An Unexpected Gift from the “Root Partners’ Club”

Recent studies by Dr. Michael Allen, et al*, are recording how the tiny threads of mycelium connected to deep-rooted desert plants stimulate the formation of calcium carbonate (caliche) crystals underground where the fungal hyphae and soil particles interface. Before you allow the gardener in you to start screaming, “Caliche—I hate that cement-like soil!”, let me introduce you to a little known value of caliche: it serves as a huge carbon “storage tank” in our desert soils, and is therefore helping to reduce excess carbon in our atmosphere. When soil calcium (which is not leached out of our desert soils because of so little rain) bonds with the carbon brought underground by plant roots (in the form of sugars and organic acids), the resulting calcium carbonate formed along the mycorrhizal fungal hyphae starts creating a layer of caliche crystals. This pocket of caliche can continue to develop and enlarge until a significant rain event leaches down through the soil, dissolving the crystals, and sometimes moving the caliche layer deeper.

The desert’s long-lived, deep-rooted plants work with their fungal root partners to store vast pools of carbon underground as caliche in desert soils.

The desert’s long-lived, deep-rooted plants work with their fungal root partners to store vast pools of carbon underground as caliche in desert soils.

The cycle of caliche formation will resume as soils dry out after a rain. The host plant absorbs carbon from the atmosphere, transforms it into sugars in its leaves, then transfers those sugars and other organic compounds down to its roots; there, the microbial root partners “eat” the carbon-rich sugars, and respire or “breathe out” carbon dioxide, some of which combines with calcium to form calcium carbonate, or caliche. By forming caliche, the “root partners club” effectively removes carbon from the air, moves it down into the soil, and “sequesters” or stores that fixed carbon. This microscopic transformation starts a process that may prove to have global implications for helping to keep greenhouse gases at safe living levels.

This unseen but critical partnership between long-lived desert plants and mycorrhizal fungi doesn’t just help desert vegetation to survive; it may be an extremely important component in guarding buried inorganic soil carbon stocks and maximizing the capacity for carbon sequestration in the most unlikely of places…our arid deserts. This should cause us all to have second thoughts about removing large tracts of vegetation across our deserts to build renewable energy arrays, whose proposed intent is to help combat rising carbon dioxide levels in our atmosphere. The removal of desert vegetation to build a renewable energy plant may greatly reduce or eliminate the carbon sequestration capability of those desert soils—and end up adding to higher atmospheric CO2 levels!

The next time you take a walk in the desert, think about the fascinating underground, 3-dimensional network of fungal threads you are walking over, connecting the roots of the native plants you are passing. It’s just another one of the  incredible strategies our desert plants have figured out to survive in one of the most extreme climates on our planet.


An information superhighway connects these desert plants underground -- it's nature's internet made of fungi.

An information superhighway connects these desert plants underground — it’s nature’s internet made of fungi.


*Allen, Michael F., G. Darrel Jenerette, Louis S. Santiago. (University of California, Riverside). 2013. Carbon Balance in California Deserts: Impacts of Widespread Solar Power Generation. California Energy Commission. Publication number: CEC-500-2013-063

Painted Lady Butterfly sips nectar from Rabbitbrush

Phenology is happening all around us. Here in the West, spring is coming earlier, summer is lasting longer, and winter is shorter. Flowers are blooming earlier, some birds that normally migrate are staying put, and it seems we’ve lost our normal summer rains. This affects you, your allergies, the birds, insects, and other wildlife in your yard, your gardening practices, and the plants that surround you. Desert plants are responding to these changes as well. Some are actually slowly moving on their own, including some of our desert’s most iconic symbols such as the Joshua Tree. Due to these changes, plants are moving—slowly trying to adapt to new areas of suitable conditions, while dying off in areas that are no longer suitable for their survival.


Hummingbird sips nectar from Eaton’s Firecracker


Nature’s timing is intricately connected to all other organisms. Plants’ reactions to our climate affect birds, bees and other pollinators, wildlife—and us. This reaction to nature’s calendar is called “phenology”, and it affects every plant, animal, and human every day. The relative timing of how each plant and animal reacts to nature’s calendar is critical, as it affects when birds migrate, when insects hatch to provide food for nesting birds and other creatures, when farmers plant crops, it triggers bees to pollinate, and determines when your allergies kick up. However, not all species are changing at the same rate. These changes will paint a new picture across our landscape.


For example, when our local birds begin nesting and other species begin to migrate through, it is incredibly important that their food source is available when they need it—not too early and not too late. If spring temperatures heat up too early, it stimulates the early hatching of insects and the early flowering of plants. If the birds are still on a nesting schedule that is timed with the season and day length, but their food source has already peaked, then there may not be enough food for them or their babies, whether it be insects, nectar, fruit or seeds. As the cliché goes, timing is everything.

Watching autumn leaves turn color and then fall to the ground may seem like a dying process, but it is very much a vital, living process. In fact, leaf drop is only possible if the leaves are attached to a plant or branch that is alive.

Have you ever seen a tree with a section of dead leaves clinging to one of the branches, or observed an entire tree full of dead leaves holding tight to the branches, never to fall? That branch or whole tree is surely dead itself if it holds persistently onto its dead leaves after they die. Eventually, wind, rain and storms will batter the dead leaves until they are ripped off the branches by weather and time, but they will never slip elegantly off the branches of a dead tree or plant like they do from living plants and trees each fall.

Plants that shed their foliage at the end of the growing season are termed deciduous, and people travel every year to watch the parade of leaf colors as groves of deciduous trees prepare for their autumn leaf drop and winter sleep.

The intricate mechanism that powers this annual shedding of leaves from deciduous trees and plants is ingenious. And the chemical gears that snip leaves off their stems can only continue to turn when powered by a live plant.

The first trigger that starts leaves in preparing for their skydive to earth is when the day length begins to shorten as winter approaches. During the growing season, the light-capturing marvel called chlorophyll (which makes leaves look green) is continually being manufactured and broken down during the longer summer days. As autumn approaches and night length increases, chlorophyll production slows down to a standstill until eventually all the chlorophyll disappears. Other pigments that are normally masked by the abundance of chlorophylls begin to show through, and we see a magnificent palette of yellows, oranges, and reds.

At the same time as this color transformation, each leaf is busy sending its accumulated stores of essential minerals, sugars, and other hard-won products out of the leaf, down the branches, and into the roots for safe storage underground all winter. As minerals like iron, phosphorus, and other valuable nutrients are transported off-leaf, other color changes are occurring, a visible testament to the chemical lab work being accomplished by each leaf before it falls.

In preparation for the undocking of leaf from stem, a layer of cork-like material has been forming to reduce and finally cut off the flow between leaf and stem. Any sugars, secondary pigments, or waste products still trapped in the leaves reveal themselves as the various autumn colors we wait for each year, now that the green chlorophyll is no longer king. When the leaf has surrendered all the nutrients it can transport, a magical thing happens: a chemical is sent to the very “seam”, or separation layer, where the leaf is attached to its stem. This veritable scalpel, a chemical called abscissic acid, snips the cell walls that attach leaf to stem, and breaks the bond holding each leaf to its living stem. A seal has already formed at the tear-line, so the branch is protected from fluid loss and bacterial entry. The leaf is released from its stem to perform its next chapter of life under its parent plant: as a cover to insulate the roots from frost, and as a time-release fertilizer for the roots as the leaves gradually break down into the soil.

The following spring, as the sap begins to flow, the roots pump back upward all the stored nutrients into the waiting buds of the branches, and new leaves emerge fed with resources recycled from their predecessors that are now serving a second “life” on the soil below.

The entire process of leaf drop is a response to chilling winter temperatures that would otherwise freeze the water inside each cell of a deciduous plant’s thin leaves, burst the cells, and damage the leaves beyond function, ending up with the complete loss of each leaf’s accumulated storehouse of nutrients, minerals, and sugars. Evergreen plants (think pines, cedars, etc.) have leaves that contain a functional “anti-freeze” to prevent them from freezing. In contrast, the strategy of deciduous plants is to avoid the whole threat of frozen leaves altogether by snipping off each leaf after its valuables are sent off to the roots, accumulating the fallen leaves over the plant’s roots like a warm blanket during their winter sleep, and reabsorbing the disassembled leaf nutrients each year as the blanket of leaves decomposes.

The role that fallen leaves contribute to the health of our trees, shrubs, and landscapes, and whole ecosystems, for that matter, should make us think twice before we rake up and haul away fallen leaves on our own property. Not only do last season’s leaves fertilize next season’s growth, the mulch they create helps to hold in precious soil moisture over drier seasons, and helps to deter weed growth. If we remove leaf “litter”, we are short-circuiting the naturally self-sustaining sequence of nutrient recycling, and nature’s carefully balanced systems.

What we witness as autumn leaves falling may look like a dying process, but it is very much a living cycle of perfectly orchestrated phases of growth, storage, and slumber.

What an ingenious cycle.