Flowering behavior of clones in a Norway maple (Acer platanoides) seed orchard and mating system analysis using nuclear SSR markers

Norway maple (Acer platanoides L.) is a tree species native to Central Europe and occurs in scattered or in small populations mixed with other tree species. Since Norway maple is considered to be adaptable to climate change, it has increasingly become a focus of forestry as one of the so-called alternative tree species to diversify species composition in forests. However, little knowledge exists on the phenotypic variation in the natural range, and no studies on the reproductive behavior of this monoecious and insect-pollinated tree species. The sexual system of Norway maple is known as heterodichogamous, with male-first and female-first flowering individuals mixed in a population. In a first step, we conducted a mating system analysis in a seed orchard. We used a recently developed set of species-specific SSR markers to genotype the parental clones and respective seed samples. The total seed had proportions of 68% outcrossed offspring between clones of the seed orchard, 11% selfing and 20% external pollination. Four flower types were observed, with protogynous and hermaphrodite types contributing more female gametes, while the protandrous type is highly variable in its female or male contributions. The number of ramets per clone in the seed set has a significant impact on the proportion of genetic contribution per clone to total seed yield. Conclusions are drawn for the establishment of new seed orchards and for further research.