The Vulnerability of Qilian Juniper to Extreme Drought Events.

Xiaofeng Wang,Fredrik Charpentier Ljungqvist,Bao Yang

doi:10.3389/fpls.2019.01191

Abstract

Identifying which trees are more vulnerable to extreme climatic events is a challenging problem in our understanding of forest and even ecosystem dynamics under climate change scenarios. As one of the most widely distributed tree species across the arid and semi-arid northeastern Tibetan Plateau, Qilian juniper (Juniperus przewalskii Kom.), is the main component of the local forest ecosystem, providing critical insurance for the ecological security of the surrounding areas. However, this species’s ability to cope with climate extremes (especially drought) has not been adequately assessed. Here, we apply a dendroecological approach that considers indices of resistance and resilience to quantify the vulnerability of Qilian junipers to the extreme drought events of 1957, 1966, 1979, and 1995. A total of 532 Qilian juniper trees from different age stages (100–1,100 years) and altitudes [3,500–4,000 m above sea level (a.s.l.)] were studied to assess their response characteristics during these four drought extremes. We conclude that drought extremes have a significant negative impact on the growth of Qilian juniper. The oldest Qilian junipers at the lower altitudes constituted the most vulnerable populations across the northeastern Tibetan Plateau and were characterized by the lowest resistance values, the narrowest annual rings, and the highest proportion of missing rings during the four drought years. Tree resilience after droughts was strongly related to the intensity of the drought event and did not change with tree age or elevation. A threshold of tree tolerance to drought may exist, with the more vulnerable tree individuals (e.g., the oldest Qilian junipers from lower altitudes) being exposed to the highest mortality risk when drought intensity exceeds the threshold value. Such a threshold needs further consideration, through the study of trees that have died (or are about to die) due to extreme droughts.

Highlights

Droughts can reduce tree growth and forest productivity through changes in photosynthesis rate (Grassi and Magnani, 2005; Hinckley et al, 1979), carbon assimilation (Chaves, 1991; Lawlor and Cornic, 2002), phenology (Misson et al, 2011), tree morphology (Abrams et al, 1992; Aspelmeier and Leuschner, 2006), and others, with adverse impacts on ecosystem stability
We studied Qilian junipers that are widely distributed across the northeastern Tibetan Plateau (NETP) to explore the vulnerability of different tree individuals to severe drought events
The effects of age and elevation both strongly affect the ability of Qilian junipers to cope with drought extremes: older trees from lower elevations are more vulnerable to drought and may even be exposed to a higher risk of mortality should water shortages become aggravated in the future

Summary

Introduction

Droughts can reduce tree growth and forest productivity through changes in photosynthesis rate (Grassi and Magnani, 2005; Hinckley et al, 1979), carbon assimilation (Chaves, 1991; Lawlor and Cornic, 2002), phenology (Misson et al, 2011), tree morphology (Abrams et al, 1992; Aspelmeier and Leuschner, 2006), and others, with adverse impacts on ecosystem stability. Qilian juniper (Juniperus przewalskii Kom.) is the main component of natural forests on the NETP and is widely distributed up to elevations of 3,500–4,100 m above sea level (a.s.l.) on the sunny and partly sunny slopes (Shao et al, 2005) of the Gobi Desert margins. Their distribution is relatively scattered, it is one of the most critical components of the local fragile terrestrial ecosystem due to its wide distribution and the high number of trees, which play a crucial role in maintaining ecological stability and preventing desert expansion or erosion. We need to compare how the responses to hydraulic deficits of trees under different site conditions (e.g., altitude) and within different age bands vary, and identify which trees are more resistant and resilient to water shortage, making them more likely than other trees to survive after drought extremes

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Discussion

Conclusion