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Open AccessArticle

Drought-Affected Populus simonii Carr. Show Lower Growth and Long-Term Increases in Intrinsic Water-Use Efficiency Prior to Tree Mortality

1
Key Laboratory of Tree Breeding and Cultivation of State Forestry Administration, Research Institute of Forestry, Chinese Academy of Forestry, Beijing 100091, China
2
Collaborative Innovation Center of Sustainable Forestry in Southern China, Nanjing Forestry University, Nanjing 210037, China
3
Beijing Key Laboratory of Ecologic Function Assessment and Regulation Technology of Green Space, Beijing Institute of Landscape Architecture, Beijing 100102, China
4
College of Landscape and Travel, Agricultural University of Hebei Baoding, Baoding 071000, China
*
Author to whom correspondence should be addressed.
Forests 2018, 9(9), 564; https://doi.org/10.3390/f9090564
Received: 20 August 2018 / Revised: 11 September 2018 / Accepted: 12 September 2018 / Published: 13 September 2018
(This article belongs to the Special Issue Physiological Responses to Abiotic and Biotic Stress in Forest Trees)
The Three-North Shelter Forest (TNSF) is a critical ecological barrier against sandstorms in northern China, but has shown extensive decline and death in Populus simonii Carr. in the last decade. We investigated the characteristics—tree-ring width, basal area increment (BAI), carbon isotope signature (13Ccor), and intrinsic water-use efficiency (iWUE)—of now-dead, dieback, and non-dieback trees in TNSF shelterbelts of Zhangbei County. Results from the three groups were compared to understand the long-term process of preceding drought-induced death and to identify potential early-warning proxies of drought-triggered damage. The diameter at breast height (DBH) was found to decrease with the severity of dieback, showing an inverse relationship. In all three groups, both tree-ring width and BAI showed quadratic relationships with age, and peaks earlier in the now-dead and dieback groups than in the non-dieback group. The tree-ring width and BAI became significantly lower in the now-dead and dieback groups than in the non-dieback group from 17 to 26 years before death, thus, these parameters can serve as early-warning signals for future drought-induced death. The now-dead and dieback groups had significantly higher δ13Ccor and iWUEs than the non-dieback group at 7–16 years prior to the mortality, indicating a more conservative water-use strategy under drought stress compared with non-dieback trees, possibly at the cost of canopy defoliation and long-term shoot dieback. The iWUE became significantly higher in the now-dead group than in the dieback group at 0–7 years before death, about 10 years later than the divergence of BAI. After the iWUE became significantly different among the groups, the now-dead trees showed lower growth and died over the next few years. This indicates that, for the TNSF shelterbelts studied, an abrupt iWUE increase can be used as a warning signal for acceleration of impending drought-induced tree death. In general, we found that long-term drought decreased growth and increased iWUE of poplar tree. Successive droughts could drive dieback and now-dead trees to their physiological limits of drought tolerance, potentially leading to decline and mortality episodes. View Full-Text
Keywords: Populus simonii Carr. (poplar); intrinsic water-use efficiency; tree rings; basal area increment; long-term drought Populus simonii Carr. (poplar); intrinsic water-use efficiency; tree rings; basal area increment; long-term drought
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Sun, S.; Qiu, L.; He, C.; Li, C.; Zhang, J.; Meng, P. Drought-Affected Populus simonii Carr. Show Lower Growth and Long-Term Increases in Intrinsic Water-Use Efficiency Prior to Tree Mortality. Forests 2018, 9, 564.

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