What actually kills an outdoor tree when it is kept indoors?

Bonsai beginners will commonly purchase a juniper (often Juniperus procumbens ‘Nana’), which is an outdoor tree, and keep it inside all year. The tree invariably dies. It is commonly explained to the beginners that this is because junipers are outdoor trees and require a winter dormancy.

My question is what exactly goes wrong that leads to the death of the plant? Let’s assume that the person gives the tree appropriate care, e.g. assume that desiccation or root rot or other more direct causes of death are not the reason. (Of course, a valid answer is that it is impossible to give “appropriate care” for some specific reason.)

I have two hypotheses, in more detail but not in a rigorously scientifically biological detail.

  1. The first is that the warmth and low humidity of the indoor environment cause the plant to continue to transpire water and attempt to grow or maintain foliar mass, while the reduced daylight hours signal the plant to “do something” (what, though?) that results in lower metabolic action. And so the tree expends its stored energy (starches and sugars?). Then in spring, the when the tree tries to put on a strong flush of growth, it has nothing left to build on. So it struggles and eventually dies.

  2. The second is that there is a daily metabolic cycle that is disrupted when the tree is kept in the more or less constant indoor environment. That is, there is a cycle that depends on the daily differences in temperature and humidity the tree experiences during daytime and nighttime. Through my own measurements, I have found, for example, that the vapor pressure deficit (VPD) reduces to near zero outdoors during the night in springtime as temperatures drop and relative humidity rises. This, I think, means the respiration demands on the tree are near zero. That is not the case indoors, where the VPD is more or less constant.

Is either of those at all consistent with what really happens? And what does really happen from a biological perspective? That is, what part of the plant’s metabolism fails? Is there something that happens during dormancy that doesn’t happen when it doesn’t get a cold dormant period? Is there something detrimental that happens when it should be dormant but isn’t?

If anyone has any pointers to resources where I could do further research on this question, that would be much appreciated, too, and I will update here with any further information I can find.

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I can’t answer this at the level that you wish to discuss it, but I think the main point is the very low light levels indoors, which reduces the production of sugars and starches by photosynthesis to almost zero thus starving the tree to death over a prolonged period of time. The much lower humidity, higher temperatures and lack of breeze are also factors which stress the tree, more rapidly depleting it’s reserves. In terms of dormancy, trees have foliar growth seasons and vascular growth seasons in cycles which support and provide the tree with the necessary processes and storage of food to continue the cycle. If the indoor environment fails to trigger dormancy it would mean the tree is always trying to grow depleting it’s resources and not spending the time building it’s roots and vascular tissue to store resources and support that growth. Thus in the lopsided world it finds itself, it will struggle to to have enough in place for the second growing season and by the third will typically give up.

Sorry if that’s not scientific enough but it’s how I understand it. I hope it helps. Obviously species by species and even tree by tree all these factors vary but most trees seems to struggle inside and many die, especially conifers.

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That’s great, actually. I know nothing of biology. But I consider myself an armchair scientist, so this is a great subject to investigate in my spare time! Input like yours is very helpful. It adds detail to my hypothesis #1, and gives me some leads to follow up on. Sounds like a good thing to learn more about is the foliar growth seasons and vascular growth seasons, and what biological mechanisms are related to them.

This sort of information is one of Ryan’s strengths so you will pick up this and much more as you watch the streams. He likes to explain this over and over and over.

In summer when leaves are out the tree uses the accumulated sugars to build new phloem and xylem (the vessels in the woody tissue which transport sap). Also roots are build and all these structure are loaded up with the sugars produced by the leaves (or needles). As winter reaches it’s peak the stored sugars then start their journey to the buds and start producing leaves in spring building new solar panels for generating more sugar for the following season.

Yes, I have been noticing Ryan’s discussion of this cycle, and it is super helpful for me to begin understanding why things are done at the times when they are (as opposed to just memorizing someone else’s “rule”).

I’m currently (as we type) doing some google research about this and have found that there are “endodormancy” and “ecodormancy” periods, and that there are genetic factors that influence how long those need to last, and compounds you can apply that affect when they end. That seems to be a fruitful direction to pursue.

I found a couple of papers (including one from 2017) that say that there’s a compound that commercial fruit tree growers use to induce their trees to leave dormancy at the same time, but that the biological mechanism that underlies its dormancy-breaking effect is not yet understood.

In addition to the other responses here, carbon starvation is another contributing mechanism. Photosynthesis slows to a crawl in the winter, but respiration stays high as it is strongly temperature regulated. Burning energy while producing none is a recipe for death.

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