Early monsoon failure and mid-summer dryness induces growth cessation of lower range margin Picea crassifolia

Extreme climatic events are increasingly recognized as important drivers of tree growth, forest dynamics, and range contractions. Understanding tree growth responses to extreme events is important for forest conservation and management, especially under climate change. Here, we studied the patterns of growth cessation of Picea crassifolia Kom., an endemic species to Central Asia, across its distributional range in the Helan Mountains of Northwestern China, to test the hypotheses that (1) tree growth and growth cessations are limited by moisture availability, and (2) that this relationship is constant over the diverse set of conditions covered in our elevational gradient. While tree growth across our gradient was significantly limited by low precipitation in June, we found growth cessation events at the lower distributional margin to increase in severity and frequency in recent decades. We found that the combination of low precipitation early in the monsoon season (June) and high mid-summer (July) vapor pressure deficit likely caused the increased frequency of growth cessation events. Because these populations are already experiencing a high frequency of growth cessation events, the mortality rate of the lower distributional margin trees could further increase if the current trends of decreasing moisture and increasing heat stress continue into the future. Our results strongly suggest that growth cessation events are more than the dendrochronological curiosity they are usually considered to be, and can be important indicators of increased tree growth stress and be potentially useful in identifying tipping points prior to forest change.