My wife and I just spent some time in upstate New York and Vermont enjoying the Fall colors. A curious question popped into our heads. We discussed it at length but we did not come up with a satisfactory answer:
Q: When would a leaf of a deciduous tree be considered “dead”?
I’ll chime in later with some of the details we mulled over.
I would request that any responses to be as technical and scientifically logical as possible. Analogies to the life histories of other plants might be appropriate but I ask that the discussion not get side-tracked on “What is Life?” from a more philosophical framework.
So I would think whether the leaf is alive or not is irrelevant as the tree is technically what’s being observed. Maybe that’s getting too philosophical about what constitutes an organism. Let’s not get into a Tree of Theseus…
It’s my understanding that once the leaves release all of their chlorophyll, photosynthesis stops. So, sometime around peak color, when all of the [insert color of leaf during growing season here] is gone. I would consider it “dead” at that point. Admittedly, I see an unavoidable philosophical rabbit hole looming here though…
I agree, the leaf is not the organism, which is the tree. The leaf is not capable of reproducing on its own (like a fruit or seed which could be considered its own organism). To make an analogy, it’s like a skin cell falling off of a human. The cell will be alive for a little bit, but then dead. Regardless of the cell’s individual state, though, we would ID it as Human (assuming we could), because it is evidence of the presence of a human, but not the human entire. It would be like fur or antlers or bark or any other shed piece of an organism.
With the leaf, I would not annotate as either Alive or Dead, but, if I did, I would annotate as “Alive” since dead trees don’t make leaves. Well, unless I had specific knowledge that the tree that produced that specific leaf was dead.
" Researchers said that a leaf’s ability to perform photosynthesis — and produce oxygen as a byproduct — is permanently lost above 116 degrees Fahrenheit and results in its death. New research discovered that some tropical leaves are already surpassing that critical temperature."
This might answer some questions. Although leaves also die for other reasons than heat.
The correct answer is: When leaves stop the process of photosynthesis. But then once again, when exactly is that?
edit: some leaves have green stems in the winter (such as Aspen trees → Populus) Which are capable of doing photosynthesis on their own. Although it doesn’t take CO2 from the air, the tree makes some on its own.
edit2: When a leaf is plucked, it doesn’t necessarily mean it stops with photosynthesis. Infact, most research is done with plucked leaves. As long as the conditions are right, the leaf will continue with the process.
(plucked leaves are not the same as fallen of leaves. Those fall off because they don’t help the tree anymore/the tree doesn’t need them anymore./The tree is investing in other parts of the tree. Or because of outside factors.)
I would guess that once an organ / part of a complex organism is separated from the whole, the status of the original organism’s function and systems are no longer relevant. At this point then, we are looking at tissues and cells. Once the cells are no longer able to perform their function they can be argued as dead. Once they can no longer sustain themselves at all most people would agree they’re dead. Once they’ve been ripped apart by predators (fungi, bacteria) and mechanical forces I don’t think anyone would call them alive.
What if a leaf gets plucked? On its own it’s most likely gonna die, because conditions need to be optimal/perfect. In that case, you’re right. What if a human keeps it alive though? They create the optimal conditions, in order to keep it alive. In which case it’s abled to do its functions as it would have been doing on a tree.
They’re not sustaining themselves now. Because without the help of the human, it would have been dead. Sure, it’s creating sugars, in which case it would be sustaining life. But that’s all because of the intervention of the human.
Hah, botany lecture alert… In the context of deciduous trees, dropping the leaves is an active process. The tree shuts down normal leaf function (photosynthesis) with a combination of environmental cues and hormone signals and starts breaking down subcellular components and extracting nutrients from the leaves to be dropped. Hence the color change in the fall as chlorophylls are broken down and cartonoids and other pigments are starting to show through. Programmed cell death is involved but I think metabolism still continues for a while after photosynthesis stops, breaking down the chloroplasts and loading nutrients into the phloem to transport valuable resources such as sugars, nitrogen, and phosphorous back into the tree and down to the roots.
The leaves aren’t just dying, they are actively shed. The tree cuts the leaf off by forming a cork layer at its base that eventually becomes a leaf scar and part of the bark. By doing this, the tree protects itself from injury. On the leaf side of this, a separation layer forms that consists of cells undergoing changes such as digestion of their cells walls to direct exactly where the leaf will break off. The cork and abscission layers seem to be built from the outside in with the last part to be cut off being the vascular bundle. So I would say the point of death for the leaf would be once those layers are completed and the leaf has been cut off entirely from the tree’s vascular system. It’s then a matter of time (and wind and rain) before the leaf separates completely and drops off.
Here’s a picture of one of the microscope slides of a maple leaf abscission zone that we use in our botany labs with a couple of labels to explain what everything is. It’s a longitudinal section (top to bottom) through the connection between the stem and leaf. Note how the separation layer forms exactly at the point between the bud for next year, which will stay on the tree and grow out again in spring, and the leaf, which will get dropped.
Yes, I think of tree leaves as similar to feathers in that they come off of an organism, as @annkatrinrose put it,
In my opinion, I would think of leaves as feathers:
A feather from a live bluejay, one that I saw to be alive when the feather came off of the bird’s body, I would annotate as alive. If I had an obs of a dead bluejay, or just one picture close up of a feather that undoubtedly is attatched to the bird’s carcass, I would annotate that as dead.
A tree from a live maple, even if it has fallen, I would annotate as alive, since the organism it came from is still alive. If I took a picture of a leaf on a tree that was cut down, I would annotate it as dead.
I think everyone who has been kind enough to read this Forum entry can see the subtle nuances that had my wife and I scratching our heads as we walked through a beautiful deciduous forest in Vermont enjoying the Fall colors of the live/dead foliage!
I very much appreciate @annkatrinrose’s botany lecture. It is precisely the detail I was hoping for. And it’s easy to see from the variety of examples offered up–such as leaves which can generate a whole new plant, etc.–that I sought to confine my question to a deciduous tree (e.g. in a North Temperate woodland). My question becomes a gordian knot if it were expanded to address just any foliage of any higher plant.
“Nature is not only more complicated than we think, it is more complicated than we can think!”
Well, death of an organism vs. cellular death are quite a bit different and they do not necessarily coincide. I’ve read somewhere the question when death occurs, depends on what we think death is. For animals, there is the concept of brain death but that of course does not apply to plants. Plants also have amazing regenerative capabilities. You can rip up a plant and put it into a blender and then coax the still living cells in the resulting sludge through tissue culture into regenerating into hundreds of clones of the plant you destroyed.