All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Wearing short trousers above the knee does not nullify ablution because there is nothing in the Book of Allaah or the Sunnah of the Prophet according to our knowledge which indicates that if one wears trousers above the knee his ablution is nullified.
For more benefit, please refer to Fatwa: 84093 about the nullifiers of ablution.
You should also know that a man may not expose any part of this 'Awrah in front of anyone except his wife. The man's 'Awrah is from the belly button down to his knees as we stated in Fatwa: 81982.
Allaah Knows best.