One possible version:

     I used to send a lot of cards to my friends when New Year came. I didn't stop doing this until Professor

Wang came to give a lecture. In his speech, Professor Wang said that too many trees had been cut down

every year, because the cards we sent were made from trees. The professor also said that without trees, the

soil would be washed away by rainwater more easily, which would lead to serious floods in some areas. I

was shocked at the professor's speech and realized that it was not good to send cards. Nowadays I am used

to sending my wishes by e-mails.