While the East Coast is getting blasted with rain and wind, here on the other side of the Appalachians, we’re dry, dry, dry.

The tuliptrees have given up on the year and are shedding their crisped leaves.

Whereas the more drought-tolerant hickories and oaks retain their summer freshness.

You can pick out the tuliptrees from a distance. Their yellowing leaves stand out against the rest of the forest.

Why do tree species differ in their ability to withstand drought? McDowell et al.’s review sums things up nicely: “Numerous hypotheses to explain mechanisms of survival and mortality have been generated via theoretical, modeling, and experimental analyses. However, a broader framework that encompasses these different hypotheses is lacking, and most hypotheses remain untested.” Despite this state of ignorance, it appears that differences in the plumbing systems of trees provide a partial answer. Oaks and hickories have water-conducting vessels that let them squeeze a living out of  dry soil whereas the tuliptrees’ system stops working in drought, cutting off the water supply to the leaves. The particularities of the width and shape of the vessels account for these differences.

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