Rates of spontaneous hybridization and hybrid recruitment in co-existing exotic and native mature larch populations

Exotic larch (Larix decidua Mill., Larix kaempferi (Lamb.) Carr. and Larix sibirica Ledeb.) and their hybrids have been introduced into Canada to test adaptation and growth for reforestation purposes. For tree breeding opera- tions, provenance trials and seed orchards of exotic larches and their hybrids have been established in arboreta adjacent to natural forest stands of the indigenous species Larix laricina (Du Roi) K. Koch. This proximity offers an opportunity to measure spontaneous hybridization and the recruitment of hybrids. Using a combination of two chloroplast and three mitochondrial DNA markers, we estimated the rate of spon- taneous hybridization taking place between native (L. laricina) and exotic (L. decidua, L. kaempferi, L. sibirica) species of larch in two arboreta. More than 5,000 seeds and 1,000 recruits from the two experimental sites were sampled and genetically identified. The occurrence of sponta- neous hybridization between L. laricina and the exotics was found both in seeds and from recruits freely established near the plantations. A low hybridization rate (3 % or less) was observed among the seeds of L. laricina maternal trees. Spontaneous native-exotic hybrids were also observed to es- tablish and survive in the environment under the current climatic conditions at a similarly low rate. However, a much higher and variable rate of establishment was observed for recruits with exotic components (up to 34.6 % at one site). These results indicate that the spontaneous hybridization and the recruitment of hybrid and exotic recruits do occur and should be taken into consideration in the management and establishment of plantations of fast-growing species with ex- otic components.