Three clones of black poplar and a hybrid poplar clone, “NE-42”, were trialled in a short rotation coppice system. The trial was replicated in three sites located in the Czech Republic with distinctly different soils and climates which were judged to be respectively favorable, unfavorable and marginal for growing black poplar. In the marginal and unfavorable sites the planting density was 2222 trees ha-1 and in optimal conditions 7407 trees ha-1. The aim of the trial was: (1) to test the performance of black poplar clones as a possible replacement for commercial hybrid poplars in areas where the genetic integrity of wild black poplar populations is under threat; (2) to test the range of conditions in which it is possible to grow black poplar and its suitability for utilizing marginal agricultural land; and (3) to measure the potential yield of black poplar clones grown in these conditions. During four harvests at three-year intervals, the following parameters were measured: plant mortality, number of shoots, thickness of shoots, the total cross-sectional area (TCA) of all shoots, the dry matter weight of individual plants (DMIP) and the dry matter yields per hectare. The differences observed between “NE-42” and the best black poplar clone decreased as conditions became more favorable. During the fourth harvest at the unfavorable site for growing poplars, the yield of dry biomass was 11.7 t ha-1 yr-1 for “NE-42” compared to 3.7 t ha-1 yr-1 for the best black poplar clone. In marginal conditions the yields were 11.8 and 9.9 t ha-1 yr-1 respectively, and in the favorable conditions there was no statistically significant difference, being 15.9 and 13.2 t ha-1 yr-1, respectively. The higher yield of “NE-42” was due to the higher proportion of thicker shoots or, alternatively, lower plant mortality. The observed TCA was highly correlated with DMIP (rs = 0.87) and dry biomass yield (rs = 0.48). It was demonstrated that black poplar can be successfully grown in marginal conditions on land which otherwise would not be especially suitable for agricultural production, and also in areas where the genetic purity of native populations of black poplar is threatened by the spread of commercially grown hybrid poplars.

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Differences in Populus nigra L. clone resistance to the rust fungus Melampsora larici-populina Kleb. (MLP) were studied in field trials where infected trees were compared with fungicide-protected trees. MLP rust infections were assessed using a 6-point scale. Four parameters of poplar growth were also measured to gauge host response to infection: shoot thickness, shoot number at the end of the growing season, individual plant dry weight, and dry matter yield per unit area. Five of eight known pathogen virulence types were detected. Cumulative growth in shoot thickness in sprayed and unsprayed plots was similar in clones with high rust pathogen resistance, but significantly different in clones with low resistance. Clones with low resistance also exhibited delayed growth initiation in the year following infection, an effect attributed to lower food storage accumulated during the previous year, reflected in a reduction in stem diameter. Based on stem thickness measurements, it was confirmed growth ceased at the end of August. Average rust severity symptoms ranged from 2.75 to 4.22 on the 6-point scale. The percentage reduction for the various growth parameters resulting from rust infection ranged as follows: individual plant dry weight 5-64%; dry matter yield 21-66%; shoot number 17-46%; and stem diameter 1-35%. Exclusive of stem diameter, these reductions correlated with severity in MLP rust infection. The one exception was tolerance to rust infection displayed by one of the clones. Clones with the lowest growth reductions were 97/152 and 97/157. Growth and yield parameter variation as a result MLP resistance difference was observed among tested P. nigra clones. This variation can be viewed as confirmation that resistance observed in this study and P. nigra clones is a suite of quantitative traits. These findings have important implications for MLP resistance breeding programs.