Creature - Elemental
As an additional cost, you may sacrifice a Forest when you play Walking Glade. If you do, reduce its mana cost by (2) and put a +1/+1 counter on it.
When Walking Glade leaves the battlefield, if it had a +1/+1 counter on it, you may search your library for a Forest, then put it into the battlefield tapped. Then, shuffle your library.
Strategy and Tactics
Obviously, you will want to sacrifice the Forest when you play Walking Glade. In such cases, you will get a 4/5 with Trample for only 3 mana, which ramps you when it dies. Playing it on curve in this way can be a bit dangerous however as it leaves you going into turn 4 with only 3 mana, assuming you hit all your land drops. There are some fancy shenanigans you can pull off by bouncing the Walking Glade, and using it to fill your graveyard with lands while thinning your deck, but this is usually very impractical.