Resolving the Dust Bowl paradox of grassland responses to extreme drought
Resolving the Dust Bowl paradox of grassland responses to extreme drought
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DOI：
10.1073/pnas.1922030117
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发表时间：
2020-09-08
影响因子：
11.1
通讯作者：
Smith, Melinda D.
中科院分区：
文献类型：
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作者：
Knapp, Alan K.;Chen, Anping;Smith, Melinda D.
在 20 世纪 30 年代美国中部沙尘暴干旱期间，具有 C-3 光合途径的物种在整个以 C-4 为主的草原上扩张。在十年的低降雨量和高温期间，C-3 草的广泛增加与 C-3 与 C-4 途径的众所周知的特征不一致。事实上，C-3 物种的水分利用效率普遍较低，光合作用对高温更敏感，这与 C-3 草原在全球较冷环境和高纬度地区的主要分布一致。我们在堪萨斯州和怀俄明州的混合 C-3/C-4 草原上进行了为期 4 年的极端干旱试验，与沙尘暴观测类似，还记录了 C-3/C-4 生物量比率增加了三到五倍。为了解释这些矛盾的反应，我们首先分析了长期气候记录，表明在美国中部的名义条件下，C-4 草在降水量和气温密切相关的地方（最温暖的月份是最潮湿的月份）占主导地位。相比之下，C-3 草在降水输入与温暖气温关联性较小的地方繁衍生息。然后我们发现，在极端干旱年份，降水与温度的关系减弱，而凉爽月份降水量下降的比例增加。降水季节性的这种变化为 C-3 草提供了一种机制，可以对多年干旱做出积极反应，从而解决沙尘暴悖论。草原是全球重要的生物群落，越来越容易受到极端气候的直接影响。我们的研究结果强调了极端干旱如何间接改变降水季节性并改变生态系统物候，以无法从 C-3 和 C-4 物种的关键性状预测的方式影响功能。
During the 1930s Dust Bowl drought in the central United States, species with the C-3 photosynthetic pathway expanded throughout C-4-dominated grasslands. This widespread increase in C-3 grasses during a decade of low rainfall and high temperatures is inconsistent with well-known traits of C-3 vs. C-4 pathways. Indeed, water use efficiency is generally lower, and photosynthesis is more sensitive to high temperatures in C-3 than C-4 species, consistent with the predominant distribution of C-3 grasslands in cooler environments and at higher latitudes globally. We experimentally imposed extreme drought for 4 y in mixed C-3/C-4 grasslands in Kansas and Wyoming and, similar to Dust Bowl observations, also documented three- to fivefold increases in C-3/C-4 biomass ratios. To explain these paradoxical responses, we first analyzed long-term climate records to show that under nominal conditions in the central United States, C-4 grasses dominate where precipitation and air temperature are strongly related (warmest months are wettest months). In contrast, C-3 grasses flourish where precipitation inputs are less strongly coupled to warm temperatures. We then show that during extreme drought years, precipitation-temperature relationships weaken, and the proportion of precipitation falling during cooler months increases. This shift in precipitation seasonality provides a mechanism for C-3 grasses to respond positively to multiyear drought, resolving the Dust Bowl paradox. Grasslands are globally important biomes and increasingly vulnerable to direct effects of climate extremes. Our findings highlight how extreme drought can indirectly alter precipitation seasonality and shift ecosystem phenology, affecting function in ways not predictable from key traits of C-3 and C-4 species.