After I cleared the bottom:

Sorry to hear your Michelia Alba withering story, however considering your temperature is close to 0 Celsius recently this might be just the tree is responding to the cold weather. In my city (Sydney Australia) in winter when temperature is close to 0 at night the Michelia Alba also wither a bit, some leaves might turn yellow or even drop and some thinner twigs/branches might wither and fall off, but these symptoms do not necessarily mean the tree is dying simply mean it's withstanding the coldness. The tree can withstand -2 Celsius night temperature for sure because we occasionally have this in winter and in Auckland New Zealand they have more frequent winter nights like -2/-3 temperatures but the Michelia Alba can survive there too. So all I meant is maybe what you see now is just a normal way your tree is responding to the weather. From the pictures you posted I saw it still has dark green trunk and several fresh green branches so these further indicate the tree is OK. Also I do not believe you should dig the trunk base to expose the roots since the roots are sensitive to cold temperature too, I would rather bury the trunk base with more mulches and/or garden soil to help the tree live through the winter days.

I would clean up the weeds and small plants around the trunk and give the tree more space. And then, I would add some fresh and clean top soil or potting soil on top of the roots to cover the roots up, about 1/2 to 1 inch, and sprinkle some water to work the soil into the roots. Air bubbles and empty spaces between roots can kill the root hairs on the roots. The soil will fill up those empty spaces. Root hairs are the main tissue to absorb water and nutrients to the tree. Do not add too much of water as we want to avoid water log and root rot. I avoid using organic top soil or organic potting soil as most of them contain manure or other unknown organic substances, especially when the tree is weak right now.

If I have some potting plants, I would move them near the tree to create micro-climate to keep it warm in winter, but not too close. Too close could create too much moisture, which promote fungal growth. I also would not let their branches touching each other to avoid scratching each other to create wounds. Move those potting plants away in other part of the year for fresh air and sun light.

I would feed it with half strength of Miracle Gro solution for the tree to reduce the transplanting shock, just for one time at this time. Winter is not its time for fertilizing. Too much of fertilizer will promote new growth, which will be killed by cold weather.

I agree with Grant Yang's assessment. Michelia alba is a tropical tree. The tree drops some leaves is responding to the cold weather.

The roots of Michelia alba are quite brittle. Once it was planted in about the right level with the ground, do not dig it up again. The roots should not be exposed to the air too. Cover the roots up with clean top soil. Bay Area is in rainy season right now. We do not need to water them unless the top soil is dry by a few sunny days. Good luck.

I transplanted my 5 years old Michelia alba from pot to ground 3 years ago. It is under a tall Russian Olive tree. I live in East Bay. It is hot in summer and cold in winter. From time to time, it is quite windy too. I have to stick it to keep it straight. I plan to prune it next year to keep it about 12 feet tall.

Thank you so much for the intricate and informative replies! I’ve collected a Few buckets of fine wood chips (we got several truckloads years ago and collect the fine, partially broken down mulch) and spread it to cover the roots and the surrounding area. I will take some potted trees to put nearby. I’m really hoping this is just a natural response as you said, It only worries me because it went through last winter looking much more alive.

It is inbetween two other fruit trees, one of which is covering a bit of its sunlight. I’m thinking it is better to keep it more protected for the while (they are deciduous fruit trees) from the cold rather than to get more light?

thanks again for the replies, it means a lot!