Why is there so little shade on the Mall near the museums?

When Washington was first established as a city, the Mall was an open area dotted with groupings of trees. Between 1800 and 1840, the land was cleared for timber, leaving an open plain. In the mid-1800s, landscape designers proposed planting new groves to form an outdoor museum of American trees. The Mall as we see it today is largely the result of the Senate Park Commission's 1901 plan that called for clearing trees to create an open central space bordered by ordered rows of elm trees. The National Park Service implemented the plan in the 1930s, despite the objections of some Washingtonians.

Why is there so little shade on the Mall near the museums?

In 1800, the area designated as the Mall was described by one of Washington city's earliest residents, Margaret Bayard Smith, as "a fine park" containing groves and clumps of trees. When Smith's memoir of life in the capital was published 37 years later, all of those trees had been removed. Builders of the new city needed wood for constructing homes, businesses, and federal offices and chopped down the Mall's trees. As the city grew, the Mall opened as grassy land with few old-growth trees. 

In 1851, Andrew Jackson Downing drafted a new landscape design for the Mall. He wanted the Mall to be like a museum of native American trees and shrubs. The plan shows the Smithsonian Castle ringed by trees, with additional small groves of dense tree plantings throughout the Mall. The Smithsonian adopted Downing's design and planted attractive groves and small garden areas that increased the amount of shade on the Mall.

In 1902, the Senate Park Commission published the McMillan Plan to improve the design and guide development of the park system in Washington DC. They argued that their idea to transform the Mall into a long open vista from the Capitol to the Washington Monument evoked the original Mall plan designed by Charles Pierre L'Enfant in 1791. The Commission specified for the old trees to be removed and for new rows of American Elms to be planted. Elms already dotted other DC parks.

The McMillan plan was implemented slowly. The National Park Service did not move forward with the removal of existing trees and planting of young American elms until the 1930s. Not everyone who worked on the Mall was happy with the plan. This letter to the editor was written by a woman whose office faced the Mall and who enjoyed the shade of an oak tree just outside the building. She objected to the removal of the perfectly healthy oak tree for what seemed to be no good reason. 

By the time the elms were planted on the Mall, a new blight began spreading across the United States, that wasn't present at the time of the Park Commission proposal: Dutch Elm Disease. The disease spread easily from tree to tree, and once it appeared on the Mall many of the original elms planted in the 1930s died. Some elms were resistant to the disease, which is why the Jefferson Elm, an original tree from the 1930s still stands near the Smithsonian Castle and the Freer Gallery. Scientists study this tree in the to save other American elms.