Trample
Domain — Whenever Briar Hydra deals combat damage to a player, put X +1/+1 counters on target creature you control, where X is the number of basic land types among lands you control.
Domain abilities count the number of basic land types among lands you control, not how many lands you control or how many of any type.
The basic land types are Plains, Island, Swamp, Mountain, and Forest. Land types other than basic land types (such as Desert) don’t contribute to domain abilities.
The value of X is determined as Briar Hydra’s last ability resolves.