Encroachment of Utah juniper has been a problem in the Great Basin region of the western US. Management strategies such as burning and chaining have been used to control encroaching populations of this species. However, little is known regarding effects of these actions on the fitness of remaining, isolated populations. At Dugway Proving Ground (DPG), a U.S. Army facility in western Utah, there has been a significant loss of the juniper trees due to fires started from accidental ignitions during army training missions. Burned juniper populations, at DPG, are not recovering as would be naturally expected for this encroaching species. Therefore, we initiated a study to examine reproductive success or fitness in isolated populations of juniper at DPG. We also examined the relationship between genetic variability and reproductive success in these populations. Reproductive success, or fitness, was determined by examination of fruits for seed abortion and presence of pre-dispersal seed parasites. Genetic variability of the isolated populations was determined using enzyme electrophoretic techniques (allozymes). Isolated populations of Utah juniper on DPG had significantly lower fecundity than non-isolated encroaching populations. These populations exhibited either significantly higher embryo abortion or higher seed parasitism. The populations also showed less genetic variation. Seed set was significantly correlated with heterozygosity across study populations. Land managers making future decisions regarding Utah juniper populations should consider the effects of isolation on the loss of reproductive capacity and loss of genetic variation in this species.

Key words: allozymes, genetic variability, Juniperus osteosperma, population fragmentation., seed set