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

作   者：Shoujia Sun, Lanfen Qiu, Chunxia He, Chunyou Li, Jinsong Zhang, Ping Meng
期刊名称：Forests
影响因子：1.956
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关键词：Populus simonii Carr. (poplar); intrinsic water-use efficiency; tree rings; basal area increment; long-term drought
论文摘要：
Abstract: 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 d13Ccor
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.