The image shows a Pyrenean oak (Quercus pyrenaica) heavily adorned with a leafy lichen like Evernia prunastri. The reddish-brown, spherical growth on the branch is an oak gall (commonly called an "agallón" in Spain), which is a tumor-like plant tissue created in response to a gall wasp's larvae.
Oak gall wasps (Tribe Cynipini) are highly specialized insects that induce abnormal plant growths, or "galls," on oak trees. Comprising roughly 1,000 species worldwide, these tiny wasps manipulate plant hormones to create distinct structures—from tiny spheres to large "oak apples"—that provide their larvae with both food and shelter.
