Garfield County Utah
Almost certainly your soil is alkaline (has a high pH), and high pH soils bind iron and make it unavailable to some plants, including aspens. The resulting yellow color is called iron chlorosis. When they get very yellow the lack of chlorophyll in the leaves causes the tissue to get bleached and damaged by the sun, causing there to be dead brown and black areas. Fertilizer and manure won't help. You can put on iron chelate like Miller's Ferriplus, but it will have to be done every year and with ten trees you have a lot of soil to treat and it will be expensive. I recommend that you consider not trying to grow aspens or other species prone to iron chlorosis and instead grow proven species that do well in your immediate area. Junipers, for example, don't usually get iron chlorosis. You also might want to look at our Tree Browser website at www.treebrowser.org.