Variability of physiological traits and growth performance in aspen assemblages differing in genetic relatedness

Abstract

Due to the increasing demand for wood and renewable energy sources, short-rotation forestry with its reliance on highly productive Populus species is in the focus of current ecological research. In order to optimize biomass gain, Populus research has in recent decades mainly centred on productivity and related traits for poplar species which could be proven to be highyielding. Aspen (Populus tremula and Populus tremuloides), however, have for a long time been neglected, because their highest annual increment occurs later than for e.g. black poplar and balsam poplar and they therefore need longer rotation times. However, in this thesis we concentrated on aspen (P. tremula and P. tremuloides) as study objects because they cope better with drought on poor soils and have the lowest demand regarding habitat conditions when compared to other poplar species. These features make aspen a promising alternative for short-rotation forestry in the face of the future climate scenarios, which include increasing temperatures and decreasing summer precipitation. Therefore, we established a common-garden experiment in 2008 with closely related aspen full-sib families (P. tremula: 2-30% genetic distance) as study objects. In the subsequent year, we established a field experiment with more distantly related aspen collectives (P. tremula: 20-40% genetic distance) originating from Central Europe as well as two different aspen species (P. tremula with German origin and P. tremuloides with American background differing 77% in their genetic distance) as study objects. Hence, we worked with three aspen assemblages along a gradient of genetic relatedness. In both experiments we investigated more than 30 phenotypic traits with the aim 1) to identify from this pool of phenological, morphological and plant physiological traits, the best biomass predictors and controlling factors and 2) to reveal their contribution to successful plant growth for each study assemblage and 3) to determine their dependency on genetic constitution. The ultimate goal of this study was to use our results to provide advice for plant breeding and cultivation programmes in the context of short-rotation forestry. The results of the common-garden experiment showed that despite genetic distances of 2 to 30%, the aspen full-sib families had no significant differences in photosynthesis related traits, even though productivity differed up to twofold between the families. Growth rate was related to several morphological traits, most closely to leaf number and total leaf area. The start of bud burst correlated with the leaf number (early-starting families produced more leaves), and was significantly influenced by the genetic constitution. The more distantly related aspen collectives studied in the field experiment differed by more than 30% in productivity with a large genotype effect, while assimilation rate and most photosynthetic and water status traits showed a relatively small intraspecific variation with no significant influence on productivity. The timing of the beginning of net leaf loss (leaf abscission > leaf production) in early and mid-summer differed between the studied aspen collectives and resulted in different maximum leaf areas and ratios of leaves lost to leaves produced, which were identified as key factors controlling productivity. The comparison between the two aspen species showed a 20% higher productivity in American aspen than European aspen, which was nearly entirely caused by a larger mean leaf area of P. tremuloides, while 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 productivity. This study showed that most plant physiological parameters were not suitable for selection or breeding programmes due to their low phenotypic variation, but should not be neglected in growth experiments because their impact on productivity might increase under non-optimal habitat conditions. Therefore we conclude that selection for high-yielding aspen genotypes should focus on leaf phenology and total leaf area associated traits, because they are stable and have a great impact on yield irrespective of the variability in the plant material.