Poplar, Cottonwood (Populus species). Maybe the tree in the background is the same species.

possibly the northwest native black cottonwood/western balsam poplar populus trichocarpa. however, whichever species it is, poplars and cottonwoods are not good for small spaces---they grow very big and very fast and have aggressive roots that can ruin septic lines and break up concrete pavement/house foundations. also with age the trees can become brittle and drop branches on you or your neighbors property or people. if you do have LOTS of space a potentially great tree for shelter, shade, or wildlife habitat but probably not in a suburban back yard, sad to say.,

Thanks for the answers. It seems like I have to replace it with something else before it gets bigger... Any suggestions for a small backyard? NW area, Portland.

How about Catalina Ironwood, Lyonothamnus floribundus subsp. aspleniifolius. Smaller tree, narrowish grower. Also this might be a good blog for you to look through, Portland Tree Tour--all sorts of trees actually growing the the Portland area (not my blog, I just read it).

Here is a link that might be useful: portland tree tour blog

Regarding the comment in the blog linked to above that the one Catalina ironwood is "huge" because they only grow 20 ft. tall in cultivation, Seattle had at least one 33 ft. tall and another 23 ft. tall ca. 2005 - and the species has been recorded 55 ft. tall in the wild.

Anyone planting the tree this far north should be aware of - and not bothered by - the risk of having it damaged or destroyed by a killer winter.

the options for small trees in your area are fairly large and which you choose may depend on what you want it for---shade, screen, ornament, all of the above? some trees to consider---kousa dogwood (cornus kousa) and the evergreen dogwood c. angustata (aka c. elliptica)---like the cultivar "empress of china", some of the larger Japanese maple cultivars (acer palmatum "bloodgood" for example), some of the larger crepe myrtles (lagerstromea "natchez" for example but several others), silk tree (albizia julibrissim "rosea" or "e.h. Wilson" for example), red flowering horsechestnut (aesculus Pavia), evergreen forms of the sweetbay magnolia (magnolia virginiana ssp. australis like "henry hicks" and similar types) and smaller forms of the southern magnolia (like "little gem" and "bracken's brown beauty"), strawberry tree (arbutus unedo) or arbutus "marina" both evergreen, the evergreen portugese laurel cherry (prunus lusitanica---NOT English laurel p. lauracerasus), and for the tropical look the Chinese windmill palm trachycarpus fortune (plant it in a spot protected from cold winter winds which can shred the foliage). hope this helps.