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?
Maybe this disease on 2nd and 3rd photo (if the branch is not damaged)?: Bronze Leaf Disease - Salisbury Greenhouse Black spots on 5-9 are not much of concern - they are widespread and pretty harmless Aspen (Populus tremuloides)-Marssonina Leaf Spot. 1, 4 and 10 are normal autumn coloration of populus. Coloration is often affected by exposure to the Sun and frosts.
Thank you for the good links! It does not seem the condition discussed in the first one fits the condition of these trees. The leaves appeared normal color, healthy-looking, not drying and curling through the summer months. I'd have to go back and check to be positive, but I do not belive there was damage to any branches in the photos # 2,3. And, you mention the aspect of frost in your comment. Yesterday, the day photos # 8,9,10 were taken, was the first frost there has been since around early to mid-May. There is information in the second link that really seems to fit, as it is mentioned that the effect on the leaves is at its worst in wet conditions. From around mid- to late May, there had not been wet conditions. Only a few brief thundershowers scattered throughout the months of June - mid-September. The last part of September, there were several rainy periods, not just intermittent, brief showers. That is when the changes in the leaves appeared. And, this morning, I will amend the remark I made in the original post yesterday about not seeming to be premature droppage of leaves. The area where I took photos # 8,9,10 yesterday, now has many trees which had crowns with a lot of leaves yesterday which are just about completely bare today. Yesterday was not windy nor rainy. The killing frost yesterday morning must have had some affect on clearing those trees in that 24 hour period, probably?
2 and 3 may be mechanically damaged branches, maybe an elk or a deer broke those in the middle of growing season, so those leaves dried prematurely. Some first light frosts (that may not be even measurable at meteorological measuring height) may affect leaf coloring, but stronger frost can cause sudden massive dropping of leaves.
I honestly thank you for your helpful replies. At this point, it is looking like probably you were correct with both your links in the first reply, and both conditions may be present. These photos were all taken this afternoon in the area of photos # 2,3,4 in the original post, where there was a question of damage to the branch. Photo #1 here, is the branch inward end of the outward end seen in photo #2. To my untrained eye, that looks like the Bronze Leaf Disease addressed in your first link. In looking farther into the grove today, I found some of the smaller to medium, younger trees looking like photo #3, with only brown / black leaves, which seem very firmly attached, not about to fall any time soon. Again, seems to me consistent with description of Bronze Leaf Disease. At least 75-80% of the leaves in that general vicinity for at least .75 mile, still look like the ones in photo #4. There is a bit more of the condition in that same area, seen in photo #5, than there was yesterday morning, but there is no apparent massive loss of leaves from the trees like there was farther downstream, though the frost was equally heavy.
I uploaded a video about how a frost can affect aspen leaves, causing massive fall: I measure frosts in my garden by walking and scanning the ground surface with my IR thermometer, and although my garden is small, extremely plain and almost equally open to the elements, I can often measure up to 5ºC differences, sometimes even more. It does matter, if there is -2ºC or -7ºC. That's even without vertical differentiation. I suspect, that tree crowns could be quite radically differently exposed to the frost, depending on terrain, distance to the waterbody, cloud cover, tree height, absense of neighbours or position towards them, etc.
Very interesting to see what is occurring in that video. Thank you. There are very obvious differences in surrounding terrain, and nearness of neighboring trees in the different sections of that drainage. It is easy to understand how that could influence air flows and convective patterns which could easily result in differences in either mixing or layering of air at different heights. The fairly minor temperature range you reference could easily be arranged in layers in one area and not another, depending on the differences in the surrounding environment, it seems. Thank you.
With the return of growing season, there is apparent difference in trees which I now believe to be affected by bronze leaf disease and those which seem to be healthy. These are photos from one of the areas where I took some of the previous ones. There is about sixty yards between the two trees used in these photos. Though the other vegetation continues the same through that sixty yards, there are no live aspens in that space. It will be interesting to see if there is 100% die off of the aspens in the section where there are so many that do not appear healthy.
