Demise of the Bristlecone?

Western five-needle pines, especially those that typically grow at high elevations, are currently facing the triple whammy of a warming climate (approximately .5 to 1 C), a major insect epidemic (mountain pine beetle, Dendroctonus ponderosae), and an exotic fungus (white pine blister rust, Cronartium ribicola).  In fact, the combination of pine beetle and blister rust has killed so many whitebark pine (Pinus albicaulis) trees in the past ten years that the species has become a candidate for listing under the Endangered Species Act.  Mercifully, however, our own Rocky Mountain bristlecone pine (Pinus aristata) has thus far been spared, though it seems only a matter of time.  (White pine blister rust has been found on a bristlecone in Colorado.)

All of this brings me to the question that has been bothering me all week:  If we don't have beetles or blister rust in our southern Utah bristlecone, what is killing them?  EDO and I were out walking in a locally famous bristlecone stand called the "Twisted Forest" earlier in the week (see top picture).  I'll grant that this is not a nice site, but these trees have been here for a thousand years.  And, when I was walking here a year or two ago, they all looked normal.  Now they don't.  They are dying at an unsustainable rate.  Look at the foliage on the specimens to the left of EDO's head:  going, going, gone.  Inside of two years, these trees have died from the tips of the branches in.  The pattern seems to be discoloration and dropping of needles from the ends of the branches.  Slowly the whole branch dies.  Eventually the entire tree is defoliated . . . and dead.

I thought about this for a couple of days, and then I ran out to my local (lower elevation) bristlecone pine, the one nearest to The Homestead.  It looks a little better, but is losing leaves just the same.  The big surprise, though, is that the die-back is happening in the local pinyon (Pinus edulis) forest (lower picture).  I really did not expect that.  The bristlecone and the pinyon are not generally susceptible to the same pathogens, so what is going on?  The only thing I can think of is a climatic effect:  drought?  winter desiccation?  frost?  But, if my reader knows the answer, I'm all ears.

Honestly, I don't really care about the pinyon pines.  They are prolific seeders, and can be found from sea to shining sea.  But, the bristlecone is fairly rare, and regenerates slowly, if at all.  On my walk through the Twisted Forest, I looked for cones and seedlings.  I found two cones, and about the same number of seedlings, but the seedlings were affected, too.  I hate to say it, but that particular stand is dying quickly and does not have the ability to replace itself.  The Twisted Forest is so named because of the gnarled, krumholtz forms of the older trees.  Those stems will not be going anywhere for a long time (dead or alive), so I suppose that the name of the area doesn't have to change.  But, this will soon be a skeleton forest.  If you want to see it while there are live trees, don't wait until next year.