Effects of seed transfers on yield and stem defects of silver birch (Betula pendula Roth)

We studied the effect of seed transfers on survival, yield and stem defects of silver birch (Betula pendula Roth) of North European origin in five parallel provenance trials in Finland. The trials were located at Loppi (60°39′N) in southern Finland, at Kannonkoski (62°58′N), Ilomantsi (62°58′N) and Toholampi (63°47′N) in central Finland and at Rovaniemi (66°21′N) in northern Finland. Four trials were growing on moist upland forest sites and one on agricultural land. The material consisted of altogether 38 stand seed origins from Finland, Sweden, Estonia, Great Britain and Russia ranging between latitudes 53° and 67°N. Survival, height, diameter at breast height, stem volume/ha and number of stem defects per tree were assessed when the trees were 19 years old. Seed transfer distance was calculated either as latitudinal or temperature sum difference between the seed origin and the trial location. According to coefficients of determination (R2), the latitude-based transfer models gave a slightly better fit with the data than the models based on temperature sum differences. Significant differences were found among the origins regarding all studied traits in all trials. In all traits, the pattern of variation in relation to the transfer distance was curvilinear, and the effect of transfer distance was significant. A long transfer from both directions led to a reduction in all measured traits. The optimal transfer distance and direction varied depending on the trait and covariate used in the models. According to the latitudinal model, local or slightly more northern origins had highest survival and lowest number of stem defects. Height growth and yield was at highest with the local or slightly more southern origins. A northward transfer of ca. 2° of latitude increased yield, while a southward transfer or a longer northward transfer, decreased the yield. According to the temperature sum model, origins within the limits of approximately ±200 d.d. compared to the trial location produced the highest yield. Origins with similar or slightly lower temperature sums compared to the trial locations had highest survival and best stem quality. The current seed transfer guidelines for silver birch in Finland, which recommend transfer distances of 150 km or 150 d.d. at maximum in southward or northward direction, seem still valid. Because of the low gain in expected yield by even moderate northward transfers and the uncertainties in the success of more southern origins in varying future climate, we recommend no changes in seed transfer guidelines.

► We examine the effect of northward and southward seed transfers on silver birch.
► The effect of seed transfer distance is significant and curvilinear.
► Best growth and yield at local or slightly more southern origins.
► Best survival and stem quality at local or slightly more northern origins.
► Transfers longer than 2° of latitude are not recommended.