Selective leaf colour change in trees
The changes of colour in sugar maples at autumn time give us a vivid picture of this species’ internal process of readying itself for the long winter ahead. We see that each tree performs the withdrawal of nutrients for its canopy in various ways – some leaves turn shades of red, others turn shades of yellow – and each tree has an individual path it follows.
A sugar maple variety such as ‘Flashfire’ however, which has been selected by breeders for a particularly strong autumn colour, will visually be more uniform, consistent and predictable at this time – primarily because they are all selected from the same plant (an intentional selection to highlight specific features; in this case, colour).
In the following photos, we see that there is other information a tree reveals to us by the variations in colour...
Pic 1, above left: A sugar maple shows dramatic colour change in the majority of its leaves, yet one area of the tree has chosen to remain green. We don't know the reason, but it demonstrates that each part of the plant is compartmentalized - the leaf, branch, and limb of a tree - each has an abscission layer at its point of connection to the rest of the plant which enables it to be independent.
Pic 2, above right: In the centre of this street-side grouping, a young tree has kept most of its foliage green far longer than the older trees to each side of it. To the right is a 100-year old mature tree and, to the left, a middle-aged tree of perhaps 50 years.
The young tree does this because it is still maximizing the chance to prepare itself for the coming season’s growth spurt. Interestingly though, we can see that the leaves on several individual branches have gone as red as the older trees – they stand out compared to the remainder of the tree’s foliage, which has not even begun – visually – to ready itself for leaf fall.
So why, and how, would one limb of the tree begin its nutrient withdrawal in such a separate way from the tree as a whole?
Firstly, this individuality shows us that each limb of the tree is its own entity, which is a surprise to think of! It was set up that way when it first began to emerge from the main stem many years ago. A tree creates the ability to sever that limb without it jeopardizing the whole tree – and yet it is still one living entity. A layer of cells called the ‘abscission’ ring is laid down where the limb joins the trunk, in preparation for the time when the limb dies, so that the rest of the tree can seal itself off to prevent infection.
The early leaf change could indicate a problem in the limb, but generally it is not that. Rather it is simply the limb interpreting its need to prepare for winter earlier than the rest of the tree. In this case, the one tree has three limbs that are ‘early changers’. It has probably laid enough ‘stores’ in its cells for next spring and has let go of this year’s leaves quickly so it may ready its structure for the stresses of winter exposure.
Like humans, who all come from a similar genetic base but are each individuals, so, too, are trees.
When we look with wonder at this amazing canopy transition, we see that the individual limbs of this particular tree are also making their own decisions about how to respond to their circumstance, and in particular, the stresses of city life. 🏡