What was done
The authors reconstructed a history of winter-spring (November-April) precipitation for the Mexican state of Chihuahua, which time period accounts for one-third of the region's annual precipitation, for the period 1647-1992, based on earlywood width chronologies of over 300 Douglas fir trees growing at four locations along the western and southern borders of Chihuahua and at two locations in the United States just above Chihuahua's northeast border.  This 346-year precipitation history was then transformed by operating on its annual data with a 10-year smoothing cubic spline.

What was learned
The authors correctly note that "three of the 5 worst winter-spring drought years in the past three-and-a-half centuries are estimated to have occurred during the 20th century."  Although this observation tends to make the 20th century look highly anomalous in this regard, it is not; for two of those three worst drought years occurred during a period of average to slightly-above-average precipitation, as defined by the 10-year smoothing cubic spline representation of the data, so they were not representative of long-term droughty conditions.

The authors also correctly note that "the longest drought indicated by the smoothed reconstruction lasted 17 years (1948-1964)," which is again correct and seemingly indicative of abnormally dry conditions during the 20th century.  However, for several of the 17 years of that below-normal-precipitation interval, precipitation values were only slightly below normal.  For all practical purposes, therefore, there were four very similar dry periods interspersed throughout the preceding two and a half centuries: one in the late 1850s and early 1860s, one in the late 1790s and early 1800s, one in the late 1720s and early 1730s, and one in the late 1660s and early 1670s.

With respect to the 20th century alone, there was also a long period of high winter-spring precipitation that stretched from 1905 to 1932; and following the major drought of the 1950s, precipitation remained at or just slightly above normal for the remainder of the record.  Finally, with respect to the entire 346 years, there is no long-term trend in the data, nor is there any evidence of any long-term departure from that trend over the course of the 20th century.

What it means
Chihuahua's 20th-century precipitation history differs in no substantial way from its precipitation history of the prior 250 years, suggesting that neither 20th-century anthropogenic CO2 emissions nor natural 20th-century warming have significantly impacted rainfall in that region of the world.