Concise Bible Dictionary:

Populus Alba (White Poplar) Leaves
It was probably the white poplar (Populus alba) which Jacob employed: it was “green” in the sense of being fresh, moist. The poplar affords a grateful shade from the heat of the sun and was therefore one of the trees chosen under which the Israelites burnt incense (Gen. 30:3737And Jacob took him rods of green poplar, and of the hazel and chesnut tree; and pilled white strakes in them, and made the white appear which was in the rods. (Genesis 30:37); Hos. 4:1313They sacrifice upon the tops of the mountains, and burn incense upon the hills, under oaks and poplars and elms, because the shadow thereof is good: therefore your daughters shall commit whoredom, and your spouses shall commit adultery. (Hosea 4:13)). Some judge the Hebrew word libneh to refer to the “storax tree” (the styrax officinale) which also grows in Palestine.
Populus Alba (White Poplar)
The Storax Tree

Strong’s Dictionary of Hebrew Words:

from 3835; some sort of whitish tree, perhaps the storax
KJV Usage: