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Gene flow and sex ratio of Populus euphratica Oliv. floodplain forests at the Tarim River, Xinjiang, NW China

Subject Area Forestry
Term from 2010 to 2012
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 187252343
 
Final Report Year 2013

Final Report Abstract

We investigated stand structure and development of Tugai forests (Populus euphratica) along the Tarim river in western China using molecular methods, to explain how a contiguous stand develops from discrete germination rows, how large clones can grow and if they can be detected by spatial mapping alone, how landscape shapes the genetic structure of these forests and what are the reasons for the observed male-biased sex ratio. 1. We adapted microsatellite primers from related poplar species, and developed a high-throughput protocol including two multiplex PCR reactions. For paternity analysis, one marker was omitted due to frequent occurrence of null alleles. 2. We used the data set obtained from seven plots of P. euphratica to test how frequent genotyping errors occurred and what are the consequences of these errors for the estimation of stand clonality. 3. The mechanism of root-suckering was elucidated with a combined mapping / excavation / molecular study. Surprisingly, we found that root suckers of all ages (even as adult trees) show a different root architecture allowing to tell them reliably apart from generatively grown trees. Trees start to develop root suckers in an age between 11 and 15 years, bridging distances up to 40 meters, thus closing quickly gaps between germination rows. 4. Clonal diversity within stands varies strongly, and the most likely reason is the history of past river movements and the connected changes in ground water levels. In areas where riverbed move quickly (or fell dry in recent decades due to increasing water demands for crop irrigation), virtually non-clonal stands develop, where trees can nevertheless become very old. In areas with continuous ground water supply (slower movements of rivers) rather large clones may develop. 5. Gene flow between stands is significant, and consequently most of the genetic diversity is within stands. 6. The apparent (flowering ramets) sex ratio of the stands is clearly male-biased, less so the intrinsic (all ramets) sex ratio, whereas the genet sex ratio is nearly even. In spite of comparable age of first bloom, especially under harsh conditions male trees seem to flower more readily and tend to develop larger clones. 7. Bilancing resource allocation into sexual reproduction (here measured as nitrogen content of the respective structures) for flowering alone, male trees invest more since they develop nearly twice the number of catkins compared to female trees. However, if all female trees would set fruit, their total investment into sexual reproduction would be three times higher. In reality, a high proportion of trees abandoned fruit during the summer, and this constitutes a powerful compensation mechanisms, avoiding a strongly biased sex ratio. 8. Pollen dispersal curves are highly leptocurtic for the rather heavy (diam. about 25 µm) grains, and most female trees should be pollinated by its next male neighbour. In contrast, paternity analysis showed that most of putative fathers for female trees are not the closest male neighbours. We investigated synchronisation of flowering times, which is high but not complete, and hypothesize that a slight desynchronisation in flowering times will give male trees a chance to pollinate not only their closest, but also more distant female counterparts, thus increasing the genetic diversity of the offspring generation.

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