Extended phenotypes in the canopies of Norway spruce

Abstract: The extended phenotype may help us understand how genetically determined trait variation in foundation species can influence associated organisms. Today, there is no existing demonstration of how genetically determined trait variation in Norway spruce (Picea abies) may affect associated organisms in tree canopies. As a majority of all planted Norway spruce seedlings used, in Swedish forestry today, originates from breeding programs it is interesting to know how the genetic material used in these programs may affect associated organisms. For example, according to the growth rate hypothesis the nutritional status of a plant should correspond negatively with tree growth, and the nutritional status of the plant should affect associated organisms. In this study, I aim to evaluate if the genetic origin of Norway spruce affect plant growth and nutritional status of needles (in this case the C:N ratio) and in extension associated communities (abundance of aphids) in the tree canopies. More specifically I will test the following six hypotheses; (1) there is a genetic influence on growth so that breast height diameter (DBH) differ among full-sib families, (2) the growth rate of the trees expressed as their DBH have a significant negative effect on needle C and positive effect on N and consequently negative effect on the C:N ratio, (3) there is a genetic component influencing C:N ratio of the needles so that the ratio differs between full-sib families, (4) growth rate of different full-sib families effect the C:N ratio of needles, (5) the abundance of aphids is influenced by full-sib family and the C:N ratio of needles and (6) growth rate of different full-sib families effect the abundance of aphids. To address these hypotheses twig and insect samples was sampled from Norway spruce trees in 90 two-by-two tree plots from 9 full-sib families. Aphids were determined to species level and needles was analyzed for C and N content with an elemental analyzer. The data on tree growth, needle chemistry, and aphid abundance were used in statistical tests using the statistical software JMP PRO 12.1.0. I found a clear genetic influence on growth in Norway spruce; fast growing full-sib families reached a DBH almost three times larger than that of slow growing full-sib families after 37 years (3.8 cm vs. 9.8 cm). In line with the growth rate hypothesis differences in growth rate also corresponded negatively with the C:N ratio of needles suggesting that needles of fast growing trees are more nutritious than slow growing trees. Despite this relation fast growing full-sib families did not necessarily have the highest nutritional status suggesting that the growth rate – nutrition relationship follow different developmental trajectories in different families. Further, aphid abundance in the canopy of Norway spruce was significantly influenced by full-sib family, but this effect could not be related to tree growth nor C:N ratio of the needles of the different full-sub families. Given these results, it seems possible that selection of plant material of certain genetic origin (e.g. fast growing plants) such as currently conducted in Sweden can influence how other plant traits are expressed in populations. If these other traits are of ecological significance we would also expect them to influence ecological processes.