| 英文摘要: | Plants in most biomes are thought to be living at their hydraulic limits, and alterations to precipitation patterns consistent with climate change trends are causing die-back in forests across the globe1, 2, 3, 4. However, within- and among-species variation in plant traits that promote persistence and adaptation under these new rainfall regimes may reduce mortality in these changing climates5, 6. Storage of non-structural carbohydrates (NSCs) is posited as an important trait for resistance and resilience of forests to climate-change-induced drought, but the underlying mechanisms remain unclear7, 8, 9, 10. Here we demonstrate a positive relationship between NSCs and drought survival by manipulating NSC concentrations within seedlings of ten tropical tree species. Seedlings experimentally enriched in NSCs showed higher stem water potentials and sustained NSCs during drought. NSC use for maintenance of osmoregulation and hydraulic function therefore seems to underlie improved drought resistance. That drought mortality is delayed by higher NSC concentrations has implications for predicting the impacts of climate change on forest die-back2, 4 and may help focus restoration efforts on species that increase the resistance and resilience of forests to climate change. Precipitation patterns are changing across the globe causing drought-induced forest die-back and altering ecosystem function1, 2, 3, 11, and recent evidence shows that plants in nearly every forest biome are living at the edge of their functional hydraulic limits4. However, within- and among-species variation of traits contributing to drought resistance may improve survival of species and adaptation to a changing climate5. In this way, biodiversity can be seen to have an insurance value6 by maintaining the presence of traits that support ecosystem resilience—in this case of forest ecosystems against drought. Identifying traits that promote plant resistance to drought is therefore important for predicting the effect of climate change on the persistence of species and communities11, 12. For poorly understood tropical forest communities, which sustain extremely high biodiversity and provide essential carbon sinks, the task of defining the functional traits influencing plant response to global change is a particularly important and pressing goal. Inter-specific differences in non-structural carbohydrate (NSC) stores are assumed to be an important trait for plant survival under stress because they reflect, in part, the balance between photosynthesis and respiration and as such could influence carbon availability for growth depending on species-specific life-history strategies10, 13. NSC concentrations correlate with resistance to herbivory and with survival under low-light conditions10, 14. Although NSCs are also suspected to play a role in drought resistance and have been manipulated during drought15, a direct relationship between drought resistance and NSC stores has not been demonstrated unambiguously3, 7, 8, 9, 10, 16 in part owing to the difficulty of experimentally manipulating NSC concentrations without altering potential confounding factors such as plant size, hydraulics and morphology. Although plant mortality from drought is a complex process dependent on multiple interrelated mechanisms16, 17, recent work has proposed three pathways to drought mortality: hydraulic failure, carbon starvation due to depletion of stored NSCs and the interaction between the two inhibiting transport and use of stored NSCs (refs 7, 15, 18). Hydraulic failure occurs when insufficient control of water loss during severe drought leads to the formation of embolisms, xylem damage and desiccation. Alternatively, when plants maintain water potentials through stomatal closure, photosynthesis is inhibited, which may lead to mortality from carbon starvation7, 19. Although depletions in plant NSC concentrations under drought have been observed in some systems19, 20, accumulation or maintenance of NSC stores is commonly documented3, 7, 21. This accumulation is probably due to a decoupling of growth and photosynthesis as cell expansion and division are more sensitive to water deficit than photosynthesis21, 22. NSC concentrations also play a functional role in non-growth mechanisms such as plant metabolism, maintaining cell turgor, osmoregulation and embolism repair7, 21, 23, 24, 25, and it has been proposed that active storage of NSCs by plants, which is in direct competition with growth, may also maintain basic metabolic functions to optimize long-term growth and survival7, 10, 13. However, a direct link between stored NSC and drought resistance and its importance relative to other variables remains unclear7, 26. Regardless of the process, we hypothesize that increased NSC concentrations support and prolong basic plant functions, thereby improving tolerance of water deficit. We set out to test the extent to which higher NSC concentrations improve survival during drought in tropical forest species and the mechanism by which this resistance is achieved. We used a new approach to experimentally manipulate NSC concentrations in which seedlings experienced either high-then-low or low-then-high light conditions under the relatively constant aseasonal climate of our study system (Fig. 1). This manipulation produced seedlings either relatively enriched (low-to-high light) or depleted (high-to-low light) in NSC concentrations while maintaining similar seedling size and morphology, thus controlling for potentially confounding factors (Supplementary Figs 1–4). We used ten species of Bornean shade-tolerant seedlings (Supplementary Table 1) in this study because seedlings have limited NSC stores and a relatively small stature that allows a direct test of the role of NSC. Furthermore, Borneo provides a diverse, ecologically sensitive forest system, which is facing increased drought under climate change27. We monitored seedling mortality, NSC levels, pre-dawn stem water potential and stomatal conductance under drought of NSC-enriched and NSC-depleted seedlings of all ten species to examine the extent to which NSC concentrations affect survival. We deliberately maintained drought conditions until all seedlings were dead with a hypothesis that greater NSC concentration would extend the time to death both within- and among-species either through prolonged hydraulic integrity or reduced carbon starvation.
| http://www.nature.com/nclimate/journal/v4/n8/full/nclimate2281.html
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