摘要：
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.
