You'll get a different answer from just about every different book!

Some treat it as a cultivar of Populus nigra, others treat it as a botanical variety or subspecies Populus nigra var. afghanica or Populus nigra subsp. thevestina, and yet others treat it as a distinct species Populus afghanica.

It is usually clonally propagated, so I tend to go with those who regard it as a cultivar, but which species it should be considered a cultivar of is trickier. Take your pick!

Resin

Resin is right that there is no consensus in the literature. The best treatment of the subject was done by S. Bugala - a Polish dendrologist and poplar expert. He argues that 'Thevestina' is really a cultivar of P. usbekistanica (an eastern P. nigra relative) and should really be called 'Afghanica'. This refers to a clone that is extensively grown throuhout southeastern Europe, North Africa, the Middle east and Mediterranean. It is either female - or hermaphroditic, fastigiate in form, has white bark and leaves that are longer than wide. Also, it can be distinguished from 'Italica' by its cylindrical stem (buttressed in 'Italica'), lack of burs and epicormic shoots that are typical of 'Italica'. See the pics below....

Although there is one clone that is widespread and was thought to have been distributed by the Turks and Arabs via the Silk Road, in reality there are several clones - some male and female. In Turkey, it is treated as a variety (P. usbekistanica var. afghanica). Specific cultivars have been selected in Iran, Romania and Iraq as well.

Here in the U.S. is a different story althogether. Although the true 'Afghanica' is found in Seattle and possibly elsewhere, the clone that is sold as 'Thevestina'/'Afghanica' is mislabeled. It is in fact, 'Plantierensis'. This clone is a cross between the western European P. nigra var. betulifolia and 'Italica' (several clones were selected by the Simon-Louis Nursery at Plantiers, France in the 1800s, so 'Plantierensis' really applies to several clones - a male is predominant). It is characterized by a very leafy and slightly broader crown (than 'Italica'). It also has slightly pubescent new leaf petioles (inherited from var. betulifolia). I have never seen the true 'Afghanica' sold in the U.S. and according to a friend in Holland, this same situation occurs in Europe.

I have propagated from the Seattle trees and also have several other clones from Turkey. They are very promising for warmer areas (I'm in north Florida). This spring I also raised about 300 seedlings from seed from the Seattle trees. I don't know if they are hermaphroditic and the seedlings could be 'Afghanica' x 'Italica', 'Afghanica' x 'trichocarpa or something else. I will select out the best, disease-free seedlings for further testing.

here is a photo showing the difference in leaves between 'Afghanica' and 'Plantierensis'
http://i234.photobucket.com/albums/ee306/smit1278/P1080833.jpg

here is a photo of the trunk of an old 'Afghanica'
http://i234.photobucket.com/albums/ee306/smit1278/P1040771.jpg

here is another one
http://i234.photobucket.com/albums/ee306/smit1278/oldAfghanicabark2.jpg

here is a photo of the trunk of an old 'Italica' (or 'Plantierensis) for comparsion
http://i234.photobucket.com/albums/ee306/smit1278/P1040703.jpg

here is a clone selected in Turkey
http://i234.photobucket.com/albums/ee306/smit1278/Gazi.jpg

here is another old 'Afghanica' in Seattle
http://i234.photobucket.com/albums/ee306/smit1278/P1050027.jpg

Thanks pineresin and thanks salicaceae very much for all that great interesting information. Those old beautiful 'Afghanica' in Seattle are very nice.

Does it look like to you salicaceae that mine are in the 'Plantierensis' group in the pictures below? I bought this "Populus nigra 'Thevestina'" from Empire National Nursery, LLC. this spring. See this link for their information on this tree: Empire National Nursery Theves Poplar I did not see any pubescent new leaf petioles on my poplar tree. It does have a resin smell around the buds like the Lombardy poplar, I guess 'Afghanica' would have a resin smell also maybe.

Here is my tree today:
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The main reason I bought it was I have always loved the look and sound of the Lombardy Poplar ('Italica') and they only live around 15 to 20 years here in AL because of the humidity causes canker problems and kills them early. So I read these "Theves Poplars" live longer so I bought one in hopes they would live a normal life of 80 or more years and not likely die like the Lombardy Poplar aways does in 15 to 20 years.

If mine is a 'Plantierensis' do they live normal and longer than the Lombardy Poplar 'Italica' here in the humid southeast?

Thanks.

Empire National Nursery seems to have shoddy business practices, so who knows what you truly have.