"Knotty" is a homophone of "naughty". Knotty means extremely difficult or intricate. Naughty means disobedient, or badly behaved. A knot is a mass formed in a tree trunk. A sapling is a young tree. Admonished means to reprimand someone. The sapling was admonished by his father for being knotty, or naughty. The joke is playing on the word "knotty" and its homophone "naughty". 
The young pine sapling was admonished by his father . Apparently he ' d been knotty .