Michael Jeschke

Stand structure and productivity of Populus euphratica along a gradient of groundwater distances at the Tarim River (NW China)

Aims Along central-Asian rivers in arid regions, lowering of the groundwater level constitutes a major threat to the riparian forests, whose tree layers are dominated by Euphrates poplar (Populus euphratica). Some of these fragile ecosystems are supplied with additional (‘ecological’) water for protection and conservation. We investigated interrelationships among groundwater distance, stand structure and above-ground wood production (at the tree and the stand level) in P. euphratica stands along a groundwater gradient (distances of 2.0–12.0 m) that also included a plot supplied with ‘ecological water’. Methods We measured the height, diameter at breast height and basal area of poplar trees and determined the annual above-ground wood production for the three most recent years on the basis of tree-ring analyses and allometric regressions at three sites at the upper, middle and lower reaches of the tarim River, Xinjiang, NW china. Important Findings the distribution of age classes differed significantly among the plots: in accordance with their life histories, stands growing at a larger distance to the groundwater harboured a larger fraction of old trees (>80 years) and, in most cases, a smaller fraction of young trees (≤20 years). the annual above-ground wood production per ha was highest on a plot with a short groundwater distance (2.0 m; maximum of the 3-year investigation period: 3.0 t ha−1 a−1) and lowest, at a plot with a large distance to the water table (6.6 m; minimum: 0.23 t ha−1 a−1). However, the plot located close to the groundwater (and to the river) at the middle reaches exhibited a relatively large interannual variation in above-ground wood production, which can be attributed to interannual variations in the river discharge. At the middle and the lower reaches, these interannual variations on plots with the most favourable water supply were even more obvious at the tree level. For the fraction of mature trees (60–99 years old), no significant differences in above-ground wood production were found between the plots with the most and the least favourable water supply. Overall, the productivity at the stand level was most closely correlated with the stand density (number of trees per ha, tree cover percentage). Productivity was negatively related to tree age, whereas groundwater distance alone was not a good predictor of above-ground wood production. In conclusion, vigorous growth of P. euphratica is possible at sites with groundwater distances of up to 12 m. Supply with ‘ecological water’ may be beneficial to trees growing at groundwater distances not larger than ~6 m. However, allocation of water should focus on stands with a short distance to the groundwater because only under these conditions, natural generative reproduction of the poplars is possible.

Clones or no clones: genetic structure of riparian Populus euphratica forests in Central Asia

Many riparian (Tugai) forests growing along rivers in arid and hyper-arid regions of Central Asia are dominated by the Euphrates poplar (Populus euphratica). Besides generative reproduction, which is only possible upon flooding events and at a distance to the groundwater of less than 2 m, this phreatophytic tree species also reproduces vegetatively by forming clones that can cover land surface areas of several hectares. Along a gradient of groundwater distances, we investigated whether the fraction of clones in P. euphratica stands (1) increases with increasing distance to the water table; (2) is higher if supplied with water via river cut-offs; and (3) approaches 100% at a short distance to the groundwater, but at high salt concentrations in the upper soil layers, which would prevent germination and establishment of seedlings. AFLP (Amplified Fragment Length Polymorphism) analyses were conducted on leaf samples taken from mature P. euphratica trees growing at the fringes of the Taklimakan Desert in stands with different distances (2–12 m) to the groundwater at two plots at the middle and the lower reaches of the Tarim River and in a stand close to Ebinur Lake, Xinjiang, China. Genetic diversity was large among plots, but considerably smaller within plots. We found the highest genetic diversity (caused by regeneration from seeds) at plots that have a short distance to the groundwater or are supplied with additional water. There was no significant relationship between groundwater distance and clonal fraction. All investigated trees at the saline Ebinur Lake site belonged to one single clone. Our results demonstrate that the genetic pattern of this widespread species is not easily predictable even over small distances as it is a result of a complex interplay of stand history and dispersal of propagules (pollen, seeds, and vegetative diaspores) by wind and water. In conservation and restoration schemes, P. euphratica stands with a high genetic diversity and stands that grow at short distances to the water table and are regularly subjected to flooding (which favors generative over clonal reproduction) should be prioritized.