What was done
The authors reconstructed a winter-spring (November-March) precipitation history for the period 1386-1993 for Durango, Mexico, based on earlywood width chronologies of Douglas-fir tree rings collected at two sites in the Sierra Madre Occidental.

What was learned
In the words of the authors, their reconstruction "shows droughts of greater magnitude and longer duration than the worst historical drought that occurred in the 1950s and 1960s."  These earlier dramatic droughts included the long dry spell of the 1850s-1860s and what they call the megadrought of the mid- to late-16th century.

What it means
First of all, the data of Cleaveland et al. demonstrate that the worst droughts of the past 600 years did not occur during a period of unusual warmth, as climate alarmists are fond of saying they should, but during the Little Ice Age.  Second, they note that "Florescano et al. (1995) make a connection between drought, food scarcity, social upheaval and political instability, especially in the revolutions of 1810 and 1910," and they state that the great megadrought that lasted from 1540 to 1579 "may be related to the Chicimeca war (Stahle et al., 2000), the most protracted and bitterly fought of the many conflicts of natives with the Spanish settlers."  Hence, if these concurrent events were more than casually related, they suggest that warmer is far better than cooler in more ways than one.

References
Florescano, E., Swan, S., Menegus, M. and Galindo, I.  1995.  Breve Historia de la Sequia en Mexico.  Universidad Veracruzana, Veracruz, Mexico.

Stahle, D.W., Cook, E.R., Cleaveland, M.K., Therrell, M.D., Meko, D.M., Grissino-Mayer, H.D., Watson, E. and Luckman, B.H.  2000.  Tree-ring data document 16th century megadrought over North America.  EOS, American Geophysical Union, Transactions 81: 121, 125.