https://en.wikipedia.org/wiki/South_Lawn#Specimen_trees

and this looks like a detailed explanation when most trees were planted and what trees they are. http://www.prop1.org/park/99plan/126-147.htm

26 looks like one of the flanking trees and that is mentioned as a Southern Magnolia planted by Democratic Party founder Andrew Jackson.

Edit. That was old info apparently. https://www.chicagotribune.com/lifestyles/home-and-garden/ct-life-sun-0121-garden-morton-20180117-story.html

Can't find what tree replaced it but I would assume the same as it looks similar.

https://en.wikipedia.org/wiki/South_Lawn#Specimen_trees

and this looks like a detailed explanation when most trees were planted and what trees they are. http://www.prop1.org/park/99plan/126-147.htm

26 looks like one of the flanking trees and that is mentioned as a Southern Magnolia planted by Democratic Party founder Andrew Jackson.