Miracle under the oaks

California is home to nine species of oak: valley, blue oak, coast live, Engelmann, canyon live, interior live, black, island, and Oregon white. Together, these stooped giants drop a thick carpet of sturdy, carbon-rich leaf matter beneath their canopies, providing the soil with a slow decaying feast of critical nutrients. Look at the difference between the top and bottom of this handful (above): the rich, dark earth, the gradient of crumble representing in-process decomposition, the fresh leaves on top. I also just noticed that there’s a worm, visible against my thumbnail.

Oak leaves can make for excellent compost. They’re nutrient-dense, carbon-heavy powerhouses that many people will rake up and add to their backyard piles, something that feels akin to “cleaning up.” However, the orderly backyard is often created at the expense of ecological balance—and when you take oak leaves for your pile, you are definitively taking them from the oak tree. I often advise people to source inputs for their compost from their surroundings, but I also take pains to remind them that they’re sharing those resources with their plant and animal neighbors.

The oak, specifically, really needs its leaves.

Oak trees count on their leaf litter for a number of reasons, like (one) nutrients, (two) enrichment of the soil surrounding their roots, and (three) the supression of unfavorable plants ("weeds") that would disrupt their feeding and reproduction. Oaks are very sensitive to what plants they share space with. They thrive best alongside coastal sage scrub and chaparral, but these types of plant communities are vanishing and you're now more likely to find oaks surrounded by European grasses, especially in residential neighborhoods. As a result, oaks are dying across the state. Trees weakened by poor landscaping practices are less likely to flower and less likely to produce acorns. If acorns are produced, they're less likely to germinate and even less likely to successfully grow.

To understand the principle at play, it helps to understand a little more about how an oak “eats.” Oaks are ectomycorrhizal, which means they utilize ectomycorrhizae (EcM) to help them find and intake nutrients. EcM forms a sheath around the root hairs of the oak, surrounding the cortical root cells, and then it goes searching for food. It reaches up into the oak’s layer of leaf litter, where it is able to source small amounts of nutrients made available by the slowly decaying leaves and transfer them back to the oak. This is a clever and symbiotic survival strategy, but it also makes the oak highly vulnerable to soil disturbance around its roots. For example: from the removal of its fallen leaves, and the subsequent enchroachment of invasively-behaving grasses that do not provide nutrients to the tree or (critically) its acorns.

If you have an oak in your yard, you may feel suddenly moved to rush out the store and purchase a commercially available mycorrhizal inoculant. I would caution you against this. Most inoculants are made of endomycorrhizae, which is a different type of differently-functioning mycorrhizae. You might also worry that you need to fertilize. Don’t do this either. Introducing external, synethic fertilizers will cause the oak to disconnect its (slowly-built) mycorrhizal relationships as it takes in a sudden excess of quickly available nutrition. You do not want to destroy that carefully built network. All you want to do is get out of its way, and let it do its thing.

So, as much as I love to compost everything in sight, I always recomend that you leave your oak leaves alone.

Love,

Cass

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