Interesting fire-related phenomenon in box elder

Dead box elder (Acer negundo)

A straggling box elder tree in our white oak savanna showed an interesting behavior after a prescribed burn. It leafed out normally in the spring and produced a healthy crop of samaras (seed carriers). Then in early June all the leaves suddenly died! (see photo)

How to explain this?

It is well known that dormant buds of trees are relatively heat resistant. It appears that the cambium, which supplies water to the leaves, may be more fire sensitive and was heat girdled by the burn. So the buds opened and formed normal leaves, but as they expanded their demand for water could not be met. With the advent of summer heat, the leaves (and the tree) died.

This isn't the first time I've seen trees being affected by heat girdling. About ten years ago we had a few smallish red maples on the north side of Toby's Prairie. They appeared to survive a spring burn fine but in midsummer the leaves turned red and dropped and the trees died.

Here is a more complicated phenomenon involving heat girdling: smallish white oaks in our savanna appeared to survive a hot fire reasonably well. The leaves came out in early May normally but a few weeks later the lower leaves turned brown and died. The upper leaves remained fine. Interpretation: the fire was hot enough to heat girdle the lower leaves but the upper leaves were far enough away from the heat to be unaffected.

I've seen this latter phenomenon twice, in different years, in two separate savannas.

How much heat can a vital part of a tree stand? According to U.S. Forest Service literature, the lethal temperature is about 60 C (140 F). But it looks like dormant buds may be able to withstand higher temperatures than the small twigs that supply them with water and minerals.