Last fall I got asked an interesting question. Why would two Ash (Fraxinus) trees drop their leaves at different times? They shared adjoining backyards. One was already bare, the other fully green. Interesting! As the company’s certified arborist people turned to me to explain this. Except I didn’t have any 100% sure explanations.
Two Ash trees
Fall 2016, Langley, BC. The Ash tree on the right has lost its leaves, the one on the right is still green. Why? Can we expect two trees of the same species living in adjoining yards to drop their leaves at the same time?
Water
The yard on the right has a kid’s splash pool and it’s built slightly higher than the yard on the left. Is it possible that the kids compacted the root zone and the bare tree struggled to get water? Soil compaction is a silent killer. People often don’t think about it. As the root zone gets compacted, the tree struggles to get water because water just runs off the hard surface. Is it just running down to the green tree down below? The still green tree has plenty of water and can afford to hold on to its leaves.
Senescence
Trees respond to light levels and temperatures. That’s how they decide to call it a season and drop their leaves. If the tree on the right called it a season, the still green one might have reacted to the extra available light and just kept on making food through photosynthesis. Why not?
Incidentally, leaf drop is also a way for trees to get rid off unwanted substances. Like humans visiting the washroom in the morning.
Characters
According to Peter Wohlleben, writing in his excellent book “The Hidden Life of Trees”, trees are characters. They aren’t clones or genetically close plants. They are characters. We can’t expect two trees of the same species living close to each other to drop their leaves at the same time. They call it a season when they decide to.
Peter describes a case from Germany of three oaks (Quercus) living in very close proximity. So close, the crowns look like one big tree. One decides to call it a season, the other two are still green or just changing their colors. Characters!
Now we know.
