Certainly a Populus, but not P. tremula, that has orbicular leaves, except at the top of strong stems. Pic #2 has some lower crown leaves visible with an acuminate apex, which P. tremula wouldn't (see example below; public domain photo from wiki commons). Several possible species, more information needed for identification. Can you get some more pics of lower crown foliage? Ideally a shoot with several leaves on, showing the upper sides and under sides of the leaves. Also, rub a developing bud and see if it has a strong resinous scent or not. Note that even with extra information, it still won't be easy, as poplars frequently hybridise, with intermediate trees virtually impossible to identify!
