Growth responses of selected clones of birch betula pendula betula pubescens and willow salix caprea salix cinerea to nitrogen in solution culture
Good, J.E.G.; Williams, T.G.
Plant and Soil 92(2): 209-222
Growth of selected clones of birch and willow obtained from nitrogen deficient soils was compared with that of unselected controls in pot experiments using three levels of nitrogen. Unselected controls of both genera continued shoot growth, albeit very slowly, with a very low level of nitrogen (5 ppm), while selected clones of birch grew significantly more than the controls. Selected clones of willow, on the other hand, ceased shoot growth after 10 weeks with this low nitrogen treatment. Nevertheless, they remained healthy, their leaves containing similar concentrations of nitrogen to those of the plants which continued stem growth throughout the experiment. Furthermore, they had very high root:shoot ratios compared with those of control willows and both selected and unselected birch. The two genera may have developed different mechanisms for tolerating low nitrogen, birch producing a small, relatively efficient root system; willow a larger but less efficient one. Both appear equally effective in ensuring survival on low-nitrogen sites in the field since all the selected clones were obtained from such sites and have survived well in field trials on similar sites. Both birch and willow responded to increased nitrogen availability with increasing shoot growth and a relative decline in root growth. However, whereas in willow the unselected plants responded significantly more than selected clones, a similar but less marked difference was found in birch. It appears that in both genera, as in herbaceous plants originating from nitrogen deficient sites, selected clones are less able to respond to increasing nitrogen supply than control plants from more fertile habitats. Attempts to correlate the response of the selected clones to nitrogen in this experiment with that to added nitrogen fertilizer in field trials has been unsucessful. Further work is required to determine the importance of the many interacting factors which influence the response of young trees to nitrogen under the unusual field conditions associated with restored mineral workings.