Royer Dana L, Osborne Colin P, Beerling David J
Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK.
Nature. 2003 Jul 3;424(6944):60-2. doi: 10.1038/nature01737.
Fossils demonstrate that deciduous forests covered the polar regions for much of the past 250 million years when the climate was warm and atmospheric CO2 high. But the evolutionary significance of their deciduous character has remained a matter of conjecture for almost a century. The leading hypothesis argues that it was an adaptation to photoperiod, allowing the avoidance of carbon losses by respiration from a canopy of leaves unable to photosynthesize in the darkness of warm polar winters. Here we test this proposal with experiments using 'living fossil' tree species grown in a simulated polar climate with and without CO2 enrichment. We show that the quantity of carbon lost annually by shedding a deciduous canopy is significantly greater than that lost by evergreen trees through wintertime respiration and leaf litter production, irrespective of growth CO2 concentration. Scaling up our experimental observations indicates that the greater expense of being deciduous persists in mature forests, even up to latitudes of 83 degrees N, where the duration of the polar winter exceeds five months. We therefore reject the carbon-loss hypothesis as an explanation for the deciduous nature of polar forests.
化石表明，在过去2.5亿年的大部分时间里，当气候温暖且大气二氧化碳含量高时，落叶林覆盖着极地地区。但其落叶特性的进化意义近一个世纪以来一直是个猜测的问题。主要假说认为，这是对光周期的一种适应，能避免在温暖极地冬季的黑暗中无法进行光合作用的叶冠通过呼吸作用造成碳损失。在此，我们通过实验来检验这一观点，实验使用了在模拟极地气候下生长的“活化石”树种，且部分实验设置了二氧化碳浓度升高的情况。我们发现，无论生长环境中的二氧化碳浓度如何，每年通过落叶冠层落叶所损失的碳量都显著大于常绿树通过冬季呼吸作用和落叶产生所损失的碳量。扩大我们的实验观察范围表明，即使在北纬83度的地区，极地冬季持续时间超过五个月，成熟森林中落叶的成本更高这一情况依然存在。因此，我们拒绝接受用碳损失假说作为极地森林落叶特性的解释。
