Wood = Air.
Use the Air Element for trees and plants ... and oxygen. Also, even in European traditions, the sky can be the Tree, like Yggdrasil, and so on. In Jewish mysticism, the element of Air corresponds to the Tree of Life. In the I Ching, the hexagram of wood (double wood trigrams), talks about wind.
I know of and like the I
Ching equation of Wood and Air, but feel that if I'm going to make a solid/liquid/gas triune I'd either make Wind the gas form of wood to reference that concept or make it the gas form of water to reference how the sea currents drive wind patterns of the planet.
Though that gets into complicated issues of Land/Water/Sky where Water and Sky are more intricately linked there than Land is. I do understand, however. that the sea currents also depend on the magnetism, rotation, and mantle convection currents of the earth and thus all are related. Perhaps Wind shouldn't be closely related to Water then, but then I'm left with a question of what the "Sky" form of Water should be (clouds? those are actually ice crystals. I like Fire/Ice/Lightning as a trinity of forces, but Ice is really a solid, unlike the gas-plasmatic forms of fire and lightning. Steam? Hmm.).
I'm not certain how I'd do it, but there are interesting thoughts to ponder. If Wind is the gas-plasma form of Wood, then is Wood the solid or the liquid? Would Coal be the solid and wood the liquid (due to a plant's growth and the "flow" of both the form of wood's xylem and phloem and the sap forth from the wood)? I'd kind of want to consider Coal/Oil/Fire a trinity, though, just in terms of combustibles…
It's a lot to think about.
I'm also a big fan of how Golden Sun did it:
4 Elementals:
Earth - Venus = Land, Rocks, Metal, Sand, Plants
Fire - Mars = Flames, Magma, Dryness, Heat
Wind - Jupiter = Gusts, Smoke, Haze, Lightning, Thunder
Water - Mercury = Waves, Bubbles, Rain, Ice, Snow, Clouds
2 Fundamentals:
Light - Sol = All four elements combined, sun, radiance, brightness, life
Dark - Luna = All four elements voided, moon, darkness, shadow, death