Quickbeam, Upstart Ent

Whenever Quickbeam or another Treefolk you control enters, up to two target creatures each get +2/+2 and gain trample until end of turn.

Old-Growth Educator

Vigilance, reach
Infusion — When this creature enters, put two +1/+1 counters on it if you gained life this turn.

Rootless Yew

When this creature dies, search your library for a creature card with power or toughness 6 or greater, reveal it, put it into your hand, then shuffle.

Timber Protector

Other Treefolk creatures you control get +1/+1.
Other Treefolk and Forests you control have indestructible.

Sun-Dappled Celebrant

Convoke (Your creatures can help cast this spell. Each creature you tap while casting this spell pays for {1} or one mana of that creature's color.)
Vigilance

Enraged Huorn

Trample
When this creature enters, the Ring tempts you.

Doran, Besieged by Time

Each creature spell you cast with toughness greater than its power costs {1} less to cast.
Whenever a creature you control attacks or blocks, it gets +X/+X until end of turn, where X is the difference between its power and toughness.

Moonlit Lamenter

This creature enters with a -1/-1 counter on it.
{1}{W}, Remove a counter from this creature: Draw a card. Activate only as a sorcery.

Reaping Willow

Lifelink
This creature enters with two -1/-1 counters on it.
{1}{W/B}, Remove two counters from this creature: Return target creature card with mana value 3 or less from your graveyard to the battlefield. Activate only as a sorcery.

Blighted Blackthorn

Whenever this creature enters or attacks, you may blight 2. If you do, you draw a card and lose 1 life. (To blight 2, put two -1/-1 counters on a creature you control.)

Magnigoth Treefolk

Domain — For each basic land type among lands you control, this creature has landwalk of that type. (It can't be blocked as long as defending player controls a land of that type.)

Fangorn, Tree Shepherd

Treefolk you control have vigilance.
Whenever one or more Treefolk you control attack, add twice that much {G}.
You don't lose unspent green mana as steps and phases end.

Tend the Sprigs

Search your library for a basic land card, put it onto the battlefield tapped, then shuffle. Then if you control seven or more lands and/or Treefolk, create a 3/4 green Treefolk creature token with reach. (It can block creatures with flying.)

Old Man Willow

Old Man Willow's power and toughness are each equal to the number of lands you control.
Whenever Old Man Willow attacks, you may sacrifice another creature or a token. When you do, target creature an opponent controls gets -2/-2 until end of turn.

Gnarlbark Elm

This creature enters with two -1/-1 counters on it.
{2}{B}, Remove two counters from this creature: Target creature gets -2/-2 until end of turn. Activate only as a sorcery.