DiMichele W A, Montañez I P, Poulsen C J, Tabor N J
National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA.
Geobiology. 2009 Mar;7(2):200-26. doi: 10.1111/j.1472-4669.2009.00192.x.
The late Paleozoic earth experienced alternation between glacial and non-glacial climates at multiple temporal scales, accompanied by atmospheric CO2 fluctuations and global warming intervals, often attended by significant vegetational changes in equatorial latitudes of Pangaea. We assess the nature of climate-vegetation interaction during two time intervals: middle-late Pennsylvanian transition and Pennsylvanian-Permian transition, each marked by tropical warming and drying. In case study 1, there is a catastrophic intra-biomic reorganization of dominance and diversity in wetland, evergreen vegetation growing under humid climates. This represents a threshold-type change, possibly a regime shift to an alternative stable state. Case study 2 is an inter-biome dominance change in western and central Pangaea from humid wetland and seasonally dry to semi-arid vegetation. Shifts between these vegetation types had been occurring in Euramerican portions of the equatorial region throughout the late middle and late Pennsylvanian, the drier vegetation reaching persistent dominance by Early Permian. The oscillatory transition between humid and seasonally dry vegetation appears to demonstrate a threshold-like behavior but probably not repeated transitions between alternative stable states. Rather, changes in dominance in lowland equatorial regions were driven by long-term, repetitive climatic oscillations, occurring with increasing intensity, within overall shift to seasonal dryness through time. In neither case study are there clear biotic or abiotic warning signs of looming changes in vegetational composition or geographic distribution, nor is it clear that there are specific, absolute values or rates of environmental change in temperature, rainfall distribution and amount, or atmospheric composition, approach to which might indicate proximity to a terrestrial biotic-change threshold.
晚古生代地球在多个时间尺度上经历了冰川气候与非冰川气候的交替，伴随着大气二氧化碳波动和全球变暖期，泛大陆赤道纬度地区往往还出现了显著的植被变化。我们评估了两个时间间隔内气候与植被相互作用的性质：宾夕法尼亚纪中晚期过渡阶段和宾夕法尼亚纪 - 二叠纪过渡阶段，每个阶段都以热带变暖和变干为特征。在案例研究1中，潮湿气候下生长的湿地常绿植被在生物群落内部出现了优势度和多样性的灾难性重组。这代表了一种阈值型变化，可能是向另一种稳定状态的 regime 转变。案例研究2是泛大陆西部和中部生物群落优势度的变化，从潮湿湿地和季节性干燥植被转变为半干旱植被。在整个宾夕法尼亚纪中晚期和晚期，赤道地区欧美部分这些植被类型之间一直在发生转变，到二叠纪早期较干燥的植被持续占据主导地位。潮湿植被和季节性干燥植被之间的振荡过渡似乎表现出类似阈值的行为，但可能不是在两种替代稳定状态之间反复转变。相反，赤道低地地区优势度的变化是由长期的、反复的气候振荡驱动的，随着时间的推移，这种振荡强度不断增加，总体上向季节性干燥转变。在这两个案例研究中，都没有明显的生物或非生物预警信号表明植被组成或地理分布即将发生变化，也不清楚在温度、降雨分布和数量或大气成分方面是否存在特定的、绝对的值或环境变化速率，接近这些值可能表明接近陆地生物变化阈值。
