Question
I live in central Texas (Austin area) and planted a lemon tree last spring. It was fine all spring and summer, had a few fruits that didn't grow very big (maybe an inch or two around), and had healthy green leaves. We get a couple of hard freezes per winter here, so I made sure to cover it well with a few blankets when the freezes were expected.

Around January I noticed the leaves turning solid yellow and some dropped off. I thought they were all going to drop off, but a few hung on for a while, until a week or two ago. Now the entire tree has a yellow hue to the bark (it's a few feet tall). I thought it was dead, but I cut off a small branch at the top and the wood is still green underneath. I haven't noticed any insects or anything on it.

The tree has pine bark mulch around the bottom of it - I don't know if that's important. Do you have any idea what's wrong with it? I can send a picture if you'd like to see it.

Thanks for your help.

Answer
The pine bark mulch really shouldn't be a problem as the citrus trees generally like an acid condition.  It is normal for some leaves to fall off, but it is quite unusual in your climate (unless you have had a several cold spell) that all of the leaves would fall.  Trees can survive temperatures down to the mid 20s without much permanent damage, so I don't think it would be a temperature related issue at this time of year.

I am away from home, and I don't have any of my reference materials with me.  Nothing comes to mind that would relate directly to the bark, so give me some time to research this for you.  My very first opportunity is on Tuesday.  

It is a good sign, though, that you have green tissue beneath the bark.  

Update:

Since I arrived home, I have done some research.  There are many published reports of defoliation of citrus trees in winter time that have been infested (and thus weakened) by Texas Citrus mites.  The defoliation occurs when the winter winds detach the already-weakened leaf petioles.  That could account for the defoliation at this time of year.

Texas citrus mites do not require specific control action in most seasons if trees and fruit are protected from other mite and insect pests. Numbers exceeding 10 adult mites per leaf over a period of 30 days have been associated with winter defoliation.  Examine your tree again very closely over the growing season for insects, and treat with general pesticides to control those pests.  Your local garden center can help with the selection of appropriate pesticides in your area.  (Better yet, find an old-fashioned garden center instead of one of the big-box hardware stores.)

Further, if there has been a great deal of rain(typical of winter in the southern US), the chlorosis (yellowing) could be a result of the a very wet root system.

If you see no new growth during the coming weeks, then the tree is dead.  Give it some time.