Since lithoids get fancy species traits, it would only be fair if our leafy bois got some too. Keeping with the vanilla bonus-per-pop thing the rocks have going on I had some ideas. I wanted to evoke a feeling of your pops taking root on their planet, slowly grow more powerful as their forest-cities literally grow along with them.

Symbiotic Undergrowth
-0.5% Pop and building upkeep per pop on the planet, capping at -50%
Your shrubbery does your laundry and waste disposal. Doesn't a helpful undergrowth slowly growing across the planet as you tame alien frontiers sound cool?

Deep Roots
+2% Pop resource production per pop on the planet, capping at +20%
IRL roots can go deep as hell (up to 400 feet for some trees) and can be tough as balls. Imagine all that space, integrated with your species' advanced technology and perfectly adapted for them. Nice.

I don't think these traits are particularly OP, since it takes time and resource investment in a planet to fully exploit them, and the final bonuses are't too far away from what void dwellers, robots and other combos can achieve.