  Extreme climatic events have been shown to be strong drivers of tree growth,
forest dynamics, and range contraction. Here we study the climatic drivers of
Picea crassifolia Kom., an endemic to northwest China where climate has
significantly warmed. Picea crassifolia was sampled from its lower
distributional margin to its upper distributional margin on the Helan Mountains
to test the hypothesis that 1) growth at the upper limit is limited by cool
temperatures and 2) is limited by drought at its lower limit. We found that
trees at the lower distributional margin have experienced a higher rate of
stem-growth cessation events since 2001 compared to trees at other elevations.
While all populations have a similar climatic sensitivity, stem-growth
cessation events in trees at lower distributional margin appear to be driven by
low precipitation in June as the monsoon begins to deliver moisture to the
region. Evidence indicates that mid-summer (July) vapor pressure deficit (VPD)
exacerbates the frequency of these events. These data and our analysis makes it
evident that an increase in severity and frequency of drought early in the
monsoon season could increase the frequency and severity of stem-growth
cessation in Picea crassifolia trees at lower elevations. Increases in VPD and
warming would likely exacerbate the growth stress of this species on Helan
Mountain. Hypothetically, if the combinations of low moisture and increased VPD
stress becomes more common, the mortality rate of lower distributional margin
trees could increase, especially of those that are already experiencing events
of temporary growth cessation.
