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TREES, PLEASE! How Do Trees Become “Native”?

Photo by Mara Koenig/USFWS  (CC - Free to use image - US Fish and Wildlife Service)
Monarch nectaring a Eucalyptus tree.

By Robin Y. Rivet.

A global pandemic is forcing humans to consider their collective vulnerability, while humbled individuals are expected to hover in place - regardless of origin. Meanwhile, trees are oblivious. It’s vaguely possible they’ve noticed we’ve decelerated greenhouse gas emissions, even created a slight blip in temperature or pollution levels, but local plants are mostly trudging along - business as usual.

In San Diego, it’s summer in the city. Gardeners pinch their peaches, pluck cucumbers, and admire pollinator-friendly zinnias - with little thought to their native habitat. When local beaches, parks and pools abruptly closed, people sought refuge with gardening. Vegetable seeds vaporized online, while shade trees cooled the thermometer, reducing epidemic angst.

Our stunning, floriferous Chilopsis linearis is considered a CA native tree, but during glorious June bloom, our jacaranda (hailing from South America) dropped hourly accumulations of lovely, lavender litter, and nesting hooded orioles flocked to eat any emerging aphids from both. We observed giant swallowtail butterfly chrysalides attach to our large white sapote, while it provided welcome heat-island respite. It’s difficult to distinguish what’s ecologically beneficial – native or not.

Planting diverse species in SoCal urban zones, yields multi-purpose outcomes for food, shade and habitat. Could San Diego’s native tree populations sufficiently supply ecological diversity to cool and calm, but also feed our increasingly hot, crowded, and people-populated concrete jungle? Let’s consider options, like what it means to be a tree, and how Calscape determines “native-ness”.

Generally, a “tree” is simply a woody, perennial plant with a trunk. The Urban Forest Ecosystem Institute adds a ten foot minimum as a characteristic. San Diego was once dominated by chaparral plants – but most under 10 feet. The “tree” noun is vague, as genetic fruit dwarfs, potted palms, and even Bonsai – often claim to be trees. As we increasingly measure urban canopy cover and carbon storage to mitigate climate change, tree definitions may matter.

GIF of Tectonic Plates: (creative commons illustration)
Tectonic Plates.

What trees are considered San Diego natives? Not too many. In California, people say they’re a “native” - if they were born here, but not so for plants. Tropical trees have been found preserved in Antarctic ice, although we don’t treat them like they’re “native” to Antarctica. We consider pre-colonial plants as “native”, but given the variation of European colonization within the US – is that very scientific? Plate tectonics is now an established fact. Genera moved around the globe, as continents shifted. Plus, much of our region was under the sea for long periods. Species vanished as they drifted, some thrived or evolved, and others resettled. But continents – well, they’re still moving. Angiosperms surfaced roughly during the Cretaceous period when the continents were clearly conjoined much more than they are today. Should we look further into our past, or consider future ecosystems for urban planning – as the only certainty might be change?

Humans have relocated plants since we crawled out of caves, and so have animals, insects and wind. Amazingly, roughly 1/3 of California’s butterflies lay eggs on, or consume non-native food sources. Plants also endure plagues. Are humans native here? Will we ever be? When did the clock start? When does naturalization and adaptation become ecologically stable? Is being “native” mostly an anthropomorphic idea? I have no answers, but many questions.

Just plant more trees - please …


Member Robin Rivet is an ISA Certified Arborist – contact her:


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