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

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 ( 13 C cor ), 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 δ 13 C cor 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.

Sand Dune Height Increases Water Use Efficiency at the Expense of Growth and Leaf Area in Mongolian Pine Growing in Hulunbeier Steppe, Inner Mongolia, China

“…In this study, we thought the Mongolian pine's death was not related to the dramatic increase of water use efficiency. According to our observation, the Mongolian pine in our research site has already exceeded the extreme drought, considered a more critical factor of the death than the sudden increase of its water use efficiency in response to drought, like in previous studies [23,[44][45][46].…”

“…Temporary increases in iWUE help protect trees from drought, but prolonged increases inhibit growth, eventually leading to death [23]. Previous studies have found that intense drought stress significantly increases the risk of death among drought-stressed plants [40,41].…”

“…This phenomenon was observed in the Taklamakan desert dunes, where the growth of trees has decreased as the dunes have become taller [20]. Plants are known to show improved water use efficiency under drought conditions [21][22][23]. One major change seen during drought stress is a reduction in plant height, which decreases the water transport distance from root to leaf [24][25][26].…”

“…In 2010, [27] suggested that the intrinsic water use efficiency (iWUE) and leaf mass per leaf unit area (LMA) of leaves differs among tree species, possibly according to growth patterns and susceptibility to drought stress in semiarid conditions. However, although physiological analysis of iWUE has recently been performed using tree rings and leaves [22,23], no studies have analyzed the effects of sand dune height on tree growth in arid and semiarid areas. In addition, species are being selected for transplantation in areas such as grasslands and sand dunes without full regard for the natural environment.…”

Radial Growth Patterns Associated with Tree Mortality in Nothofagus pumilio Forest

“…in the European Alps. Studies focusing on other species have also reported slower growth rates in declining or dead individuals vs. healthy individuals, serving as a potential early warning signal for tree death [19,21,[62][63][64][65][66]. Particularly for the evergreen Nothofagus dombeyi in Patagonia, Suarez et al [33] found that dead trees during the extreme drought event in 1998-1999 showed lower growth rates 30 years prior to the drought when compared with trees that survived.…”