I find the leaf shapes to be quite different.

Liquidambar is noticeably star shaped:

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While Sycamore (Platanus) is not:

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... and note the spiky seedball on liquidambar.

Sycamore seed balls are smooth, by comparison.

Also, liquidambar generally have wrinkled, ribbed/warted bark, whereas sycamore bark is white/grey/tan and fairly smooth, with large flaking patches.

Josh

Sycamores have real big, brittle leaves that rattle all the time, and the wood is brittle, always come down in hurricanes. Sweetgum leaves are soft, and the trees are strong and hold up well in hurricanes. Personally, I much prefer sweetgum.
Sherry

sweetgum.. immediately on being crushed.. have a very heady aroma.. that lasts only a few seconds ... something like turpentine ...

sycamore.. on mature trees.. have very fuzzy back side ... at least the ones in the hood when i grew up ...

ken

Also Sycamores can be identified easy from a distance even, the bark is mostly whitish/creamish in color.

also .. on mature trees. ...

sycamores will be the ones with leaves falling all year long .... and sticks.. and twigs .... they are just down right messy trees ....

not to mention the hordes of fuzzy balls ...

ken

Do you have quite a lot of sycamores and sweetgums in southern california?

I assume most are talking about the eastern species here, but there are western species that I think that the original poster could have been referring to.

Sweet gums are nice but they do drip sap. Don't plant too close to where you park cars. The spike balls can be a pain.

Thanks to everyone for their answers. Yes, we do have both Sweet Gums (Liquidambars) and Sycamores here in the West. We have some Sycamores that are native to CA. I think I can now easily tell the difference between the two of them.

Actually, we recently had a 60' Liquidambar come crashing down in a storm (though it was nothing like a hurricane), despite what Misshenry said about Sycamores being more likely to fall. Interesting.

Thanks again!!

Sacramento and Davis, CA have tons of sycamores! Avenues of them! Massive, messy trees.

Sycamores, in my experience, tend to be prone to "summer limb drop." Cars are crushed every year in Sacramento.

Whereas, liquidambar limbs tend to snap when the copious leaves are over-laden with rainwater.

Neither wood seems especially sturdy.

Josh

My father had a sweetgum next to his driveway that was topped by two hurricanes about ten years apart, at the same spot about 15 feet up. Each time the tree rebounded and soon looked attractive again.

It was removed a few months ago to allow more room for it's neighbors, a live oak and a bald cypress.

It was a hard call to make. Miss that tree.

Well, I don't know what's the matter with my sweetgums, but they withstood the hurricane very well, although they did lose a lot of branches.
Mag grands didn't even lose many branches.
Sherry

Question-

I've heard that Sweetgum (Liquidambar) has naturalized in parts of California. How does an Eastern tree naturalize in a place with such dry summers, when it's drought-sensitive here in the East?

Ours did fine in that horrible drought we had last summer. Also I think they are better pioneer species and enjoy taking over open tracts of land (they used to be quite uncommon here until people started clearcutting forests). I'm sure they have lots of room in California to grow. More open spaces than even here.

Western sycamore leaves resemble sweetgum but the lobes nearest the stem form a straight line while sweetgum leaf lobes closest to the stem form the same angle as other pairs of sweetgum leaf lobes.