http://www.danubeparks.org/files/1903_BlackPoplarMonography.pdf?fbclid=IwAR0tYSylgoFSudPO0EVTCmurQyXj8SO4uI2vLa14_NFRo0hcCXmjW2vKNIQ
http://www.hrpub.org/download/20160130/UJPS2-10305549.pdf