TREES, PLEASE! Life Is Alive And Well In Our Urban Trees
- k-england
- 2 days ago
- 4 min read
By Robin Y. Rivet, for Let’s Talk Plants! June 2025.
Life is Alive and Well in our Urban Trees
San Diego county recently competed in a friendly, global competition called the “City Nature Challenge”. The goal was to observe and document as many plant and animal species as possible where people work, live and recreate. Between April 25 – 28, ~1,214 individuals tallied their observations into a project dataset using the iNaturalist app for recording purposes. Collectively, residents submitted a total of 31,082 observations - which included 2,853 different species! World-wide, 102,945 people participated in this same challenge, making 3,310,131 observations for a record 73,765 species!
San Diego started with a distinct advantage. Our county is considered a “hotspot” as part of the California Floristic Province. However, to qualify as a biodiversity hotspot, a region must meet two strict criteria:
1. It must have at least 1,500 vascular plants as endemics — which is to say, it must have a high percentage of plant life found nowhere else on the planet. A hotspot, in other words, is irreplaceable.
2. It must have 30% or less of its original natural vegetation. In other words, it must be threatened.
So where do urban trees fit into this equation? Ironically, almost all trees in our local cities were planted by humans, and a high percentage of those - are also not endemic California species. So, they don’t count. In other words, we may have been gifted with a vast, pre-colonial palette of floral and faunal diversity, and Southern California is losing its “original natural vegetation” rapidly, but most of our city’s trees are still considered “interlopers”.
Where is diversity flourishing?
· Mission Trails Regional Park is an open-space park, ostensibly focused on our endemic species. In fairness, non-native and invasive plants are removed wherever feasible, but for better or worse, plenty of non-native flora and fauna are thriving there. After all, it was once a location for ranching, a dam, and plant cultivation. iNaturalist documents 120 introduced species out of the 905 “established” species, or about 13%.
· In our Cleveland National Forest, wildfires hit our native trees hard, and although the oaks are rebounding from fire, GSOB has been merciless, and many native pines have flatlined from drought, fire and borers. Is that impacting birds, insects and the overall ecosystem? Yes. Add climate changes to this mix, and strictly anecdotally, I’ve encountered lots of bird species in recent decades that have “migrated” into urban forests - where many now find refuge. On the other hand, “introduced” plant and animal species have made new homes in our native forests. iNaturalist documents 328 species as introduced into the Cleveland National Forest out of 3,010 species noted as already established – or almost 11%.
· The San Elijo Lagoon Marine Ecological Reserve is an example of a watershed region trying to stabilize our human presence and reduce everything from overfishing and pollution to maintaining breeding grounds and protection from sea-level rise. iNaturalist observers recorded 567 species with ~16% of those as introduced.
As a reminder, this is citizen science, so there are minimal “rules” to dictate what an observer chooses to document. As a local arborist, I help identify all regional trees entered into the database, but it upsets me that any tree that was planted by a human, (even if it’s 100 years old), is not deemed “research grade” - even if it’s full of endemic wildlife - and not considered invasive. Without humans, Balboa Park wouldn’t have any trees, nor would other city hotspots, reservoirs, backyards, streets, botanical gardens, or parking lots. Think about that. This is a personal peeve I have with iNaturalist. If you sight a western bluebird munching on a magnolia fruit, lucky you. But that magnolia won’t count as “research grade” science, just the bluebird. Same goes for a hummingbird nest spotted in your pepper tree. The nest is welcomed as an example of a regional life form, but not the tree. Birds and insects are generally considered “wild”, even if they are not endemic, while trees are almost always are considered “captive or cultivated” - unless someone happens to think they self-sowed. I think this notion shortchanges our authentic tree values. I believe our shaded parks; home orchards and ornamental trees provide essential habitat for local flora and fauna.
Our yard teems with wildlife AND urban trees.
What do my readers think?
