When you send a gift, the trees gifted disappear from your EcoTree account as they are being transferred. Once the recipient has claimed their gift code, the trees are automatically transferred to their EcoTree account.
If you change your mind, you can always cancel or modify a gift from your EcoTree account in the "Gifts" section as long as the beneficiary has not yet claimed their gift code. If you cancel the gift, the trees will appear in your account again.