The lack of any truly remarkable dryness in the precipitation data led me to consider the other side of the water balance equation, i.e. the rate of evaporation. Estimates of evaporation or evapotranspiration rates can be calculated from meteorological variables, or evaporation can be measured with an evaporation pan. I believe the latter measurements are taken at the UAF experiment station in Fairbanks, but I haven't yet found the data online. In lieu of these observations, and as a first look at potential evaporation rates, I calculated the vapor pressure deficit from hourly observations at Fairbanks airport. The vapor pressure deficit (VPD) is the difference between the saturation vapor pressure and the actual vapor pressure, and in theory is proportional to evaporation rates, excluding the influence of solar radiation and wind; the VPD is a key factor in the theoretical evapotranspiration - see here for an example.
The chart below shows the mean VPD, together with the mean temperature and dewpoint, for June through August of each year since 1950. The result is remarkable: the summer months of last year brought much higher VPD, and presumably therefore much higher evaporation, than normal, and the three-month mean VPD was the highest on record. Based on the hourly observations, the June-August mean temperature was more than 2 °C above the 1981-2010 normal, but the mean dewpoint was 0.2 °C below normal.
The series of charts below shows the same information, but for each month from May through September. Perhaps the most striking feature is the magnitude of the VPD anomaly in June 2013; the June mean VPD was the highest on record. Based on the hourly observations, the mean June temperature was higher than in 2004, but the mean dewpoint was 2.8 °C lower than in 2004 and only a little above normal.
An interesting aspect of the long-term history is that for each month except September, the 1950-2013 linear trend in dewpoint has a smaller slope than the trend for temperature, so this has contributed to increasing VPD over time. Of course, the VPD has also increased simply because the temperature has increased. In May and August, the dewpoint trend is actually negative, i.e. the air has been getting slightly drier in an absolute sense in these months. The difference between the temperature and dewpoint trends is most notable for May, and consequently the May VPD has increased by about 3.6 percent per decade.
In conclusion, it seems likely that the observed dryness of the white spruce in interior Alaska this year is mostly a result of the very high evapotranspiration during the 2013 growing season; and conditions this year so far are not helping to improve the situation. As a next step, it would be interesting to try to factor in the contribution of excess solar radiation, which probably also boosted evaporation rates last year, and also the effect of mean wind speeds.