I am looking for information on how to identify a poplar (Populus) tree. It is located in Dresden, Germany, next to a small stream. The city tree database lists it as Populus nigra. However, it appears different from other individuals of this species, and I have found inaccuracies in this database before, thus I do not trust it. In particular, the leaves are very large (I estimated ~15 cm) and firm.
There is an old individual (in rather bad shape) with furrowed "woven-looking" bark at the base, but turning smooth not much higher. This one is shown in the last photo below. There are several much younger individuals directly next to it, with smooth grey bark (see photo). Many of the leaves have some brown discolouration on the underside. Some of this can be seen in the photo below, although this leaf is cleaner than the rest.