Wet Decades; Dry Decades

I'm not much of a dendrochronologist.  In fact, I know very little about it.  But, I do know that, in semi-arid southern Utah, where I live, moisture availability is often the limiting factor in tree growth.  If water is available, trees can grow quickly; if it is not, they cannot.  In fact, on very limited data, I have found that precipitation rates can explain up to 70 or 80 percent of the variation in tree growth rates.

Anyway, a year or two ago, I cut and measured a relatively large 150 year old ponderosa pine that I found dying (attacked by a beetle) on a ridge at about 9200 feet.  The red curve in the graph above marks its growth rate.  Then, the other day, I was working on an Engelmann spruce tree of about the same age that was dying (attacked by a different beetle; same genus, though) on a flat at about 9900 feet.  When I saw the spruce curve (in blue, above), it reminded me of the pondo curve, so I plotted them together.

For the thirty years from 1900 to 1930, there is no surprise.  It was a well known wet period in Utah and other parts of the western United States.  My two sample trees grew quite quickly as a result.  After 1930, too, everybody knows the story:  Dustbowl.  The 1930s were extremely dry across North America.  That dry period extended into the 1940s, and, perhaps, the 1950s, too.  None of this is surprising, and is also well supported by my data.

What I don't understand, however, is the rapid growth of my two sample trees in the decade prior to 1900.  The period from 1890 to 1900 was, by other data sets I've seen, a dry one.  In fact, part of that decade was considered to be a drought in northern Utah.  So why did my trees grow so well during that decade?  One explanation might be a localized wet period in southern Utah.  We've all experienced a storm cycle where one city gets pounded, while another, 50 miles away, enjoys nice weather.  Another explanation might be that I don't know very much about dendrochronology.