Early monsoon drought and mid-summer vapor pressure deficit induce growth cessation of lower margin Picea crassifolia

TL;DR

This study investigates how early monsoon drought and mid-summer vapor pressure deficit contribute to growth cessation in Picea crassifolia at the lower distributional margin, highlighting climate change impacts on tree growth and survival.

Contribution

It reveals the climatic drivers of growth cessation at the lower margin of Picea crassifolia, emphasizing the roles of early monsoon drought and vapor pressure deficit under warming conditions.

Findings

Lower margin trees experienced increased growth cessation events since 2001.

Drought in June and high vapor pressure deficit in July drive growth cessation.

Warming and increased VPD may elevate mortality risk at the lower distributional margin.

Abstract

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.