What factors cause the autumn change in aspen leaves to turn them from green to black instead of to yellow/golden? Is there a good source written in layman's terms in which to read about the factors involved in the changes? I have found some articles mentioning Marssonina and Septoria fungi affecting aspen leaves. It seems that they are just addressing the blackening and premature dropping of leaves from planted/transplanted trees in landscaping situations, and means of addressing that. The trees that have me curious are naturally-occurring, not in managed landscape environments. The descriptions of the appearance of the fungal effects on the leaves seems to match, but I'm not sure if there is a lot of premature droppage. Specifically, along a section of stream drainage, about 6.5 - 7 miles in length, with no more than 150 - 200 feet elevation difference from the upstream portion to the downstream portion. The elevation in the area is around 3100 - 3300 feet. At the upstream portion, the changes in leaf color are not as advanced as they are farther downstream, and appear to not be abnormal in colors. Photo #1 is from that area, but there are not very many which have changed to that stage. Most are still in green stages. Photos # 2,3,4 are from the area about 2 miles downstream from the area of #1. They don't seem particularly out of the ordinary. Photos # 5,6,7 are typical of the leaves in the portion beginning about .75 miles downstream from the area of # 2,3,4 and extending about 3 miles downstream from there. In the downstream area of that section, leaves in photos # 8,9 are pretty typical. Photo #10 shows one tree in the area of # 8,9 which does not seem to be as much affected in its upper section by the seemingly abnormal colorations. Downstream from the area of photos # 8,9,10, the stream and trees are on private property, so I do not have access to them. About 1 - 1.25 miles downstream of the area of photos # 8,9,10, there are some aspens on the private property which are quite visible at approx. 1/4+mile distance. From that far away, their leaf coloration looks quite normal and much more advanced, fully in the yellow/golden stage more so than still having a lot of green mixed in. Is there only a fungal cause, or also possible other factors which affect the leaves in that way?