In bonsai practice, 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 strong 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.

  • $\begingroup$ Here's a link with a discussion that is a step in the right direction. omafra.gov.on.ca/english/crops/hort/news/orchnews/2016/… Specifically, it says that there is an endodormancy period where the tree will be dormant of its own volition and will stay dormant until it has been cold long enough. This is followed by an ecodormancy period where the tree remains dormant so long as external conditions are not favorable to growth. It references a paper called "(Westwood, 1993)" but with no corresponding bibliographic entry. $\endgroup$ – cape1232 May 9 at 19:16

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