Swiss Stone Pine and European Nutcracker - a close relationship with big effects?

Do spatial genetic structures of adult and juvenile Swiss stone pines indicate local and regional gene flow through wind pollination and seed dispersal by nutcracker?

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Swiss stone pines on "Rautialp", Kt.Glarus Photo: S. Brodbeck (WSL)

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Measuring of young stone pines. Foto: S. Brodbeck (WSL)

The interdependence of the Swiss stone pine (Pinus cembra) and the European nutcracker (Nucifraga caryocatactes) as its main seed disperser has been extensively observed in the Alps. The birds bury the pine seeds in caches, using them as winter food source, which in turn places pine seeds into microsites favorable for germination and seedling establishment.

This type of seed dispersal implies a strong directional effect on the small-scale genetic structure of pine populations, whereas wind-pollination and the overlapping of many generations likely counteract this trend. We aim at elucidating the process of bird-mediated seed dispersal and its effect on genetic patterns of future pine generations. We hypothesize that the seed dispersal by the nutcracker generates a distribution pattern that results in the aggregation of closely related seed genotypes, which should remain detectable also in the extant population structure.
In a model population, we map all adult stone pine individuals and assess their genotypes for molecular markers with contrasting inheritance. Genotyping adult trees for bi-parentally inherited nuclear microsatellites and paternally inherited chloroplast microsatellites will provide an insight into the spatial genetic pattern of the extant population, reflecting historical pollen and seed gene flow over several generations. Recent gene flow, both via pollen and seed, will be studied by genetically analyzing juvenile trees that are likely to have germinated from caches. The same molecular markers as in the adults will be applied to the saplings, allowing us to perform parentage analyses with a high likelihood of identifying both parents. Moreover, we can identify saplings originating from seeds that were brought into the study stand by the nutcracker.

We hypothesize that the spatial aggregation of closely related trees favors inbreeding and, consequently, leads to inbreeding depression in subsequent generations. This would be the case if nearby and, thus, closely related trees tend to pollinate each other, which is more likely if populations are small and isolated. To test this assumption, we performed germination experiments with open-pollinated seeds that were collected in various Swiss stone pine populations differing in size and degree of isolation (Salzer et al. 2012). We found that seed number and weight, but also germination rate, were lower in small and isolated populations as compared to large populations. However, mortality and growth in the first year did not reveal any differences between population types. We thus presume that inbreeding effects are largely expressed in the earliest life stages and, as expected, are particularly manifest in small stands.

The genetic structure in the adult stand in comparison with that found in the juvenile generation will help us to elucidate the relative effects of pollen and seed gene flow towards the overall genetic structure within the extant and future generations of the pine stand. Our study should provide the basis to better understand population genetic processes at the local scale and to elucidate the co-evolutionary interaction between species acting at two trophic levels.

• Felix Gugerli Künzle