INCREASED SUSCEPTIBILITY TO DROUGHT-INDUCED MORTALITY IN SEQUOIA SEMPERVIRENS (CUPRESSACEAE) TREES UNDER CENOZOIC ATMOSPHERIC CARBON DIOXIDE STARVATION
INCREASED SUSCEPTIBILITY TO DROUGHT-INDUCED MORTALITY IN SEQUOIA SEMPERVIRENS (CUPRESSACEAE) TREES UNDER CENOZOIC ATMOSPHERIC CARBON DIOXIDE STARVATION
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DOI：
10.3732/ajb.1200435
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发表时间：
2013-03-01
影响因子：
3
通讯作者：
Beerling, David J.
中科院分区：
文献类型：
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作者：
Quirk, Joe;McDowell, Nate G.;Beerling, David J.
研究前提：气候引起的森林退缩具有深刻的生态和生物地球化学影响，但对过去树木死亡的生理机制知之甚少，限制了对气候变化下植被变化的预测。气候、干旱、火灾和放牧是晚新生代树木死亡的因素，但干旱与大气二氧化碳（[CO2](a)）从3400万年前的高水平下降到接近饥饿水平之间的相互作用被忽视了。在这里，通过水力功能、碳限制和防御代谢的相互依赖，这种相互作用框架了我们对树苗死亡率的研究。方法：我们通过在气候控制的生长室内以1500、500或200 ppm [CO2](a)的浓度种植红杉树，重现了新生代[CO2](a)的变化情况，捕捉了从34 Ma开始向最低浓度下降的过程。7个月后，我们施加了干旱条件，并测量了与碳利用、水力学和防御代谢相关的关键生理成分，作为树木死亡的假设相互依赖机制。关键结果：灾难性的水力传导失败，碳水化合物饥饿和树木死亡发生在200 ppm，而不是500或1500 ppm [CO2](a)。此外，[CO2](a)的下降减少了对富含碳的叶面防御化合物的投资，而这些化合物会降低对生物攻击的抵抗力，可能会加剧死亡率。结论：干旱下低[CO2](a)驱动的树木死亡率与更新世花粉记录一致，这些记录显示了冰期（180-200 ppm [CO2](a)）加利福尼亚红杉林的重复收缩，甚至可能导致了过去10 Ma在低[CO2](a)下多个大陆草原的扩张。这样，低[CO2](a)的地质间隔加上干旱可能造成树木补充的人口瓶颈，通过森林死亡推动植被转移。
Premise of the study: Climate-induced forest retreat has profound ecological and biogeochemical impacts, but the physiological mechanisms underlying past tree mortality are poorly understood, limiting prediction of vegetation shifts with climate variation. Climate, drought, fire, and grazing represent agents of tree mortality during the late Cenozoic, but the interaction between drought and declining atmospheric carbon dioxide ([CO2](a)) from high to near-starvation levels similar to 34 million years (Ma) ago has been overlooked. Here, this interaction frames our investigation of sapling mortality through the interdependence of hydraulic function, carbon limitation, and defense metabolism.Methods: We recreated a changing Cenozoic [CO2](a) regime by growing Sequoia sempervirens trees within climate-controlled growth chambers at 1500, 500, or 200 ppm [CO2](a), capturing the decline toward minimum concentrations from 34 Ma. After 7 months, we imposed drought conditions and measured key physiological components linking carbon utilization, hydraulics, and defense metabolism as hypothesized interdependent mechanisms of tree mortality.Key results: Catastrophic failure of hydraulic conductivity, carbohydrate starvation, and tree death occurred at 200 ppm, but not 500 or 1500 ppm [CO2](a). Furthermore, declining [CO2](a) reduced investment in carbon-rich foliar defense compounds that would diminish resistance to biotic attack, likely exacerbating mortality.Conclusions: Low-[CO2](a)-driven tree mortality under drought is consistent with Pleistocene pollen records charting repeated Californian Sequoia forest contraction during glacial periods (180-200 ppm [CO2](a)) and may even have contributed to forest retreat as grasslands expanded on multiple continents under low [CO2](a) over the past 10 Ma. In this way, geologic intervals of low [CO2](a) coupled with drought could impose a demographic bottleneck in tree recruitment, driving vegetation shifts through forest mortality.