Quantifying safe seed transfer distance and impacts of tree breeding on adaptation

•We demonstrate a new approach to quantify critical (i.e., safe) seed transfer distance (CSTD).•Our approach produces logical transfer functions from which reliable CSTDs can be calculated.•Changes in adaptation due to artificial selection are observed only when seed is moved long distances.•Conservative (i.e., short) CSTDs are recommended when tests are young.•Provenance tests should be located in disparate climates.

Tree seed zones that are too large can compromise forest plantation health and productivity due to maladaptation arising from some populations being transferred long distances, whereas tree seed zones that are too small can result in unwarranted seed collections or excessive numbers of breeding zones. There has been little discussion, however, regarding the development and use of transfer functions for quantifying critical (i.e., safe) seed transfer distance (CSTD) or how artificial selection might affect CSTD. Using data from a large, multi-site provenance test for Douglas-fir and Interior spruce containing both genetically selected and wildstand seed sources, we illustrate an approach to quantify CSTD using Euclidean climate transfer distance modeled with a half-normal transfer function. A wide range of CSTDs was calculated and most transfer functions showed that selected seed sources were considerably taller than wildstand seed sources when transferred short or medium climate distances. Contrarily, selected seed sources were shorter than wildstand seed sources when transferred long climate distances. CSTDs were shorter for Douglas-fir than for spruce, and shorter when calculated using height rather than survival as the transfer function response variable.These findings suggest that (1) unfavorable changes in adaptation due to artificial selection will be observable only when seed is transferred considerably beyond the CSTD; (2) differences in adaptation between selected and wildstand seed classes may not warrant separate seed transfer guidelines for these seed classes; (3) British Columbia’s Douglas-fir and Interior spruce breeding programs are generating significant height gain; (4) methods presented here produce logical transfer functions that can be used to calculate reliable site-specific CSTDs; (5) use of conservative (short) CSTDs may be advisable when tests are young; and (6) provenance tests should be located in disparate climates.