I am not positive it is spiders. I think it could be a tent caterpillar.
The tree is here: https://www.inaturalist.org/observations/215576404
Witches' broom Gall (Cystotheca lanestris)
Edgewood Park and Natural Preserve,
Redwood City, CA
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Reference: Field Guide to PLANT GALLS of California and Other Western States by Ron Russo- ISBN 978-0-520-24886-1
Gall induced by a fungus (Cystotheca lanestris)
Edgewood Park and Natural Preserve,
Redwood City, CA
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Reference: Field Guide to PLANT GALLS of California and Other Western States by Ron Russo- ISBN 978-0-520-24886-1