Today, I went by the Arnold Arboretum in Boston to take in the warm weather (55F) before the cold finally works its way over to New England.
I noticed that the Poncirus there are currently semi-evergreen. They've already experienced several nights in the teens and at least one night at around 5-6F.

The trees are planted in what is normally a shady area. Could this have anything to do w/ their semi-evergreen appearance. I've noticed a couple of branches on my Flying Dragon are also almost evergreen. These branches were shielded from the sun by an adjacent fern this past fall.

That tree must have some magic antifreeze, or is a hybrid. My trifoliata have all been leafless of months now.

The fruit certainly looks like Poncirus trifoliata, though.

From the Arboretum's website they received it as seed from the Boyce Thompson Institute for Plant Research (http://bti.cornell.edu/) in 1980, which is apparently affiliated with Cornell University.

Since it was from seed, I guess it could be a hybrid. However, if it is a hybrid, it must be extremely hardy. Boston has seen several nights below 0F and at least one night at -7F since the mid 1980s.

There is an 8' tree in NW SC which is a seedling of this Arboretum tree. It is mostly deciduous, it currently has just a few leaves near the tips of the branches.

Perhaps this selection needs a longer fall period to form an abscission layer.

A large 10+ year old tree near where I live still has most of its leaves also. The lowest temps we've had so far is in the low 20's F

I have a theory about this, based on the fact that one of my Flying Dragon Poncirus is currently partially leaved. As you can see in the photo most of the top of the plant has leaves, much of the bottom is leafless.

Now this plant was about to come into leaf and flower in April last year, when it suffered a hard frost. The buds on the top half of the plant were killed, but leaves on the bottom half continued to develop. The top half grew new leaves a couple of months later and it is these that remain, while the first flush leaves have fallen.
So I suggest that, in addition to the normal temperature requirement for leaf abscission in autumn, the leaves have to be a certain age - perhaps at least 6 months old. Otherwise they do not fall, and they hang on while the plant goes dormant over winter.
So, this could explain why leaves remain on plants in much colder regions where leaf burst in spring is much later.
I think I will name this 'Michael's hypothesis of partial deciduousness'.

Very interesting observations! Right now my FD seems to have the opposite pattern, although not nearly as pronounced. A few branches right above the ground still have about one third of their leaves even after a low of ~3F. The the rest of the tree is nearly leafless. The few leaves that remain higher up on the tree show significant bronzing, but the lower leaves are nice and green.







The lower branches were covered by some adjacent ferns and dead leaves from nearby trees this fall. So I was thinking that the amount of sunlight on the leaves while the tree is transitioning to dormancy may play a role.
That is clearly not the case with your tree, as with most things there are most likely many factors at play.

It is also possible that there are some differences between trees due to genetic variation. At the Arnold Arboretum, there are two trees planted ~4 feet apart. One of them was mostly evergreen, whereas the other (which I didn't take clear pictures of) was mostly leafless. If you look closely in the first picture of my original post, you can see two trunks one almost directly in front of the other. In this pic, the branches to the lower left, which are nearly leafless, are from the second tree.

I wonder if this PT still has green leaves? My PT probably looses it's leaves in November?

MY FD are now blooming.

My FD has a blanket of 6-8 inches of snow, with more on the way!

Mine is still in dormancy. Temps are about 40F at night and 55F during the day in this time.

Totally decidious this year, but a small seedling on the ground is totally evergreen. It's often the case.

About the first pic, I guess this PT has taken advantage of the canopy just above in early fall, and the leaves finally and quickly turned to yellow and felt after the shooting.

Franck

I have 3 PT's that I planted 2 years ago when they were about one year old seedlings: two are planted in full sun and exposed to high winds in the open, and the third one is planted on the north edge of a forest tree line and doesn't get as much sun. The two getting more sun and less protection from winds are twice as tall as the other one and leafless but that third one on the treeline is semi evergreen with fully green leaves at the top of the plant. I assume this more shaded one is more juvenile and is therefore not fully dormant. My lowest this winter was 19 degrees for over 24 hours with major freezing rain event. It's been lows in upper 20's here for the past two weeks.

Our outside temperatures have been in the 3 - 7 F temperature range with lots of snow. However inside the greenhouse its 53-nights and 70's day. My Flying Dragon is still dormant (leafless).- Millet

Now, temps are here around 50F during nights, and 68F during days. So that I finally noticed the buds were swelling back in early April ! It's the first time dormancy is broken so late. Both leaves and flowers.

Franck

Buds are now starting to break on my FD too.

I went back to the PT trees at the Arnold arboretum a few days ago to see if they still had any leaves. They probably have lost about 50% of the leaves which they had in the January pictures. Overall probably only 15-25% of the leaves remain.



In late January the trees went through a period of 200+ hours below freezing and saw several low temps below 10F. The absolute low was probably somewhere between 0 and 4F where the tree is located.