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I observed a series of trees along the 1100 block of Meares St. in Victoria, BC. From my observations, I have determined that there are three different species of lichen growing on the bark of the plum trees in this area. Their distributions on the bark 
of the trees seems consistent with patterns of direct sunlight and shade. Regions of lichen growth inhibition near the bottoms of the trunks are not consistent between trees and follow no discernible pattern of gradient along the street.
All of the plum trees seemed to have lost some of their leaves and I wonder if there is a relationship between the number or size of leaves that are being dropped and the position of the tree relative to a nearby construction site and busy roadway. Most of the trees along the street are of similar size and trunk diameter, suggesting they are of similar age. The trees at the west end of the street, near the construction and major roadway are also some of the most heavily-shaded, which may be a confounding variable to any comparisons with this location. The number of leaves dropped by trees in different areas appears fairly equal, but it is hard to say without making measurements if there is a relationship between dropped leaf size and tree.
If there is a difference in the size of dropped leaves, and assuming all trees are of a similar age, this might suggest that trees that are dropping smaller leaves are doing so prematurely, or their leaves are not achieving as large a size at maturity. Either way, the difference might be caused by the growing conditions of the tree (light, water, soil type) or perhaps something is affecting the trees directly. The area of the street near the construction site is noticeably dustier than other areas. We know that trees rely on transpiration through the stomata on their leaves to draw water up from their roots. Perhaps the dust is interfering with this process and leading the trees to drop leaves earlier than normal.
I hypothesize that the size of dropped leaves from trees near the construction site and busy roadway is smaller than the size of dropped leaves from trees at the eastern end of the street. I predict that trees near the construction site will drop leaves of a smaller mean size than trees away from the construction site.
A response variable for this experiment would be the mean length, in millimetres, of leaves (continuous). An explanatory variable would be the distance of the tree from which to leaves are dropped, in meters, to the construction site (continuous).
Hello!
I will say first that if there was a competition for best blog post title, you would most certainly win!
As far as the content, I believe with a few tweaks you could have a clear and falsifiable prediction. You seem to be toying with both leaf size and leaf number – perhaps both could be studied? I am fairly unfamiliar with the intricacies of why trees drop their leaves, other than in response to changing temperatures and day length. What have you come across in your review of primary literature of the subject? Are light, water, and soil type major factors that contribute to leaf drop? If so, studying these predictor variables may be a simpler method. Could dust from the construction site be influencing the trees’ ability to sense light? If this were true, perhaps you might notice a difference in pigment between trees closer to the construction site vs. those that are farther away. A confounding variable to leaf size and number of leaves dropped may be anthropogenic interference as leaves are often purposefully blown away from sidewalks and urban areas. It is unclear at this point how you will directly link your predictor variables to your response variable. One positive aspect is that all trees were likely planted at the same time and are similar in age, which is one less variable to deal with.
Hope this helps!