Abstract
The primary factor controlling C3/C4 relative abundance in terrestrial ecosystem since the Last Glacial has been widely debated. Now more and more researchers recognize that climate, rather than atmospheric CO2 concentration, is the dominant factor. However, for a specific area, conflicting viewpoints regarding the more influential one between temperature and precipitation still exist. As temperature and precipitation in a specific area usually not only vary within limited ranges, but also covary with each other, it is difficult to get a clear understanding of the mechanism driving C3/C4 relative abundance. Therefore, systematic analysis on greater spatial scales may promote our understanding of the driving force. In this paper, records of C3/C4 relative abundance since the Last Glacial on a global scale have been reviewed, and we conclude that: except the Mediterranean climate zone, C3 plants predominated the high latitudes during both the Last Glacial and the Holocene; from the Last Glacial to the Holocene, C4 relative abundances increased in the middle latitudes, but decreased in the low latitudes. Combining with studies of modern process, we propose a simplified model to explain the variations of C3/C4 relative abundance in global ecosystem since the Last Glacial. On the background of atmospheric CO2 concentration since the Last Glacial, temperature is the primary factor controlling C3/C4 relative abundance; when temperature is high enough, precipitation then exerts more influence. In detail, in low latitudes, temperature was high enough for the growth of C4 plants during both the Last Glacial and the Holocene; but increased precipitation in the Holocene inhibited the growth of C4 plants. In middle latitudes, rising temperature in the Holocene promoted the C4 expansion. In high latitudes, temperature was too low to favor the growth of C4 plants and the biomass was predominated by C3 plants since the Last Glacial. Our review would benefit interpretation of newly gained records of C3/C4 relative abundance from different areas and different periods, and has its significance in the understanding of the driving mechanisms of C3/C4 variations on longer timescales (e.g., since the late Miocene) with reliable records of temperature and atmospheric CO2 concentration.
Highlights
The primary factor controlling C3/C4 relative abundance in terrestrial ecosystem since the Last Glacial has been widely debated
Records of C3/C4 relative abundance since the Last Glacial on a global scale have been reviewed, and we conclude that: except the Mediterranean climate zone, C3 plants predominated the high latitudes during both the Last Glacial and the Holocene; from the Last Glacial to the Holocene, C4 relative abundances increased in the middle latitudes, but decreased in the low latitudes
Synthesis of results from all over the world pointed out that C4 expansion during the late Miocene may have occurred on a global scale, and remarkable decline of atmospheric CO2 concentration was proposed as the main driving force [7,8]
Summary
Due to the influence of local factors, such as landform and physiognomy, natural vegetation shows high diversity. Our newly gained 13C data of both long-chain n-alkanes and total organic matter of surface soils [61,62] in eastern China with higher spatial resolution further confirm that of phytoliths in surface soils [60] Together, all these three datasets demonstrate that relatively higher C4 abundance in modern vegetation of the eastern China occurs in mid-latitudes spanning from ca. 14° to 25°S, and gradually decrease both southward and northward (Figure 3(c)) Our analysis of these evidences [62] indicates that, the specific latitudes are different in these three continents, a basic pattern is that the highest C4 abundance in modern vegetation occurs in the mid-latitudes. By tropical rainforest or sub-tropical seasonal rainforest while Savanna vegetation (typical C4 vegetation) occupies its two swings and no C4 plants dominate the high-latitudes [5], C3/C4 relative abundance in modern vegetation revealed by surface soil 13C values should be reliable
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