Belowground inter-ramet water transport capacity in Populus euphratica, a Central Asian desert phreatophyte

Populus euphratica Oliv. is a widespread phreatophytic tree species that forms riparian forests in (hyper-)arid regions of Central Asia. Its recruitment strongly relies on vegetative propagation from 'root suckers' that emerge from underground root spacers. The water transport through the spacers, although decisive for emerging ramets, has only rarely been quantified, but is crucial for the vegetative regeneration of the forests. In root spacers with different diameters collected from a mature poplar forest in northwest China, we calculated the hydraulic conductivity (k_c ) from anatomical investigations on the basis of a modified Hagen-Poiseuille equation and measured it (k_m ) with a perfusion solution in the laboratory. The k_m values were compared with the water use by young and mature P. euphratica trees determined in previous studies. We obtained a significant correlation between k_m and k_c (which, however, was higher by at least one order of magnitude). Due to the extensive occurrence of tyloses, particularly in older conduits and thicker spacers, and because the conduit area did not increase with spacer diameter, neither k_c nor k_m increased with an increase in spacer diameter. The water supply through the spacers would be sufficient to meet the water demand even of mature trees. Our results provide a mechanistic explanation for the observed occurrence of P. euphratica clones across large areas and, provided that they are also valid for stands with larger distances to the water table, for the sustained growth and vegetative reproduction of P. euphratica stands growing at larger distances from the groundwater.