Photosynthetic characteristics and growth performance of closely related aspen taxa: On the systematic relatedness of the Eurasian Populus tremula and the North American P. tremuloides

The European and American aspen species Populus tremula and P. tremuloides are closely related taxa with very large distribution ranges and high economic importance. Genetic and morphological data are not fully congruent with respect to the question of the systematic relatedness of these sister taxa, pointing either at separate species on the two continents or a single aggregate species with circumarctic distribution. In a replicated growth trial with 1-year-old saplings, we compared about 30 morphological (leaf size, leaf area, leaf numbers, leaf growth, leaf phenology and the ratio of leaves lost to leaves produced) and physiological traits (Amax, quantum yield, carboxylation efficiency, maximum rates of carboxylation and electron transport, leaf dark respiration, leaf conductance, leaf water potential and WUE) with the aim to obtain evidence in support of or against the one-species hypothesis and to identify key determinants of growth in the two aspen taxa.American aspen showed a by 20% higher canopy carbon gain than European aspen which was nearly entirely caused by a larger mean leaf area of P. tremuloides, while differences in mean assimilation rate and the length of the leafy period were of minor importance. Species-specific differences in the onset of leaf abscission in early autumn were identified as main determinants of the size of mean plant leaf area and thus of canopy carbon gain. American aspen with a larger leaf area and C gain possessed by far fewer leaves and side branches than European aspen, but had considerably larger leaves. These morphological and phenological contrasts support the view that the two aspen collectives are indeed different species, what has been questioned recently. However, the large majority of physiological traits is not significantly different between the two species and they are much less variable among the individuals of a species than the morphological traits indicating a conservative inheritance of physiological traits.