beewildered, bees usually swarm in the spring, even in Southern Cal, so I think you're pretty safe in saying that they're interested in the nectar. The increase in bee activity could be due to a nearby beehive which swarmed this spring and moved into a location nearby, making the tree an ideal target for honey.

I'd say thisbud4u's response is spot on, including his comment about a "new" hive in your vicinity.
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I have several Bradford Pears on my property and it's the first source of nectar for my bees in the Spring since it blooms so early (small white blossoms). I absolutely love to walk up to the tree(s) when the bees are working it. If you listen closely, the whole tree seems alive with a soft, gentle humm. The bees are not aggressive when they're out foraging as long as you don't bug them (pardon the pun)...they are far too busy taking care to collect as much nectar as their home needs to begin raising the young'uns. That sound reminds me that mother nature has just turned the page and Winter is over; it's a most glorious time!

Chinese elms are pretty good sources of pollen, although I don't know how much nectar there is. Hmmm....I've got a couple, but didn't know they were in flower. I'll have to look tomorrow.

(BTW....I have seen two swarms in the past two weeks, so even though it is off season, there are some out there)

The Chinese Elm in my backyard is at least 50 years old. I've lived here all of that and more. This is the first time I've noticed bees in the tree. It's September! I heard their hum and was curious. What a blessing to discover! This makes my day.😊

The same thing happened to me! I am walking with my morning coffee and heard the Chinese elm tree humming. Amazing :-)

I’ve lived in my new house for 5 years now and every year around this time the bees come back and all all over my Chinese Elm tree. We look forward to it every year. it’s beautiful to see and hear!