Pecan. Carya illinoinensis.
The pecan is native to the south-central part of the United States and usually found naturally growing in lowlands. When the native tree is planted as a horticultural specimen in this area we find that the fruits are poorly formed or are smaller than what we see in the supermarkets where the nuts come from trees that are improved cultivated varieties.
The pecan tree pictured above near Peace Monument is likely our tallest tree on the Capitol Grounds at about 115' in height and is well suited to its growing conditions. If you look up into the branches on the west side you may see scars from a lightning strike that happened in 2011. Fortunately, damage was minimal and the tree has gradually recovered.
Full details on the trees of Capitol Grounds can be found at aoc.gov/trees.