Árbol, Spanish for “tree” comes from the Latin arbor, for the same. We can see the Latin to Spanish evolution easily recognizing the common r‑to‑l swap, where the “r” and “l” sounds in many languages are often interchanged.
From the same Latin root, we get a variety of related English words, such as herb and arbor, as in Ann Arbor, home of the great University of Michigan. We also get some other Spanish words, such as hierba, meaning “grass”.
The pattern is easy to spot in the vowel-r‑b root: a‑r-b for árbol and e‑r-b for herb.