Rocky Mountain juniper (Juniperus scopulorum) and one-seeded juniper (J. monosperma) have overlapping ranges. Pollen grains (which contain sperm cells) from one species are unable to germinate and make pollen tubes on female ovules (which contain egg cells) of the other species. Are these two types of juniper considered reproductively isolated from one another?

Reproductive Isolation can be defined as the mechanism adopted by different species during the course of their evolution, due to which the individuals belonging to different species are incapable of mating with each other and producing viable offspring.
The given question is an example of Mechanical, Pre-zygotic Reproductive Isolation.
Prezygotic Isolation refers to the mechanism adopted by individuals of different species during the course of evolution such that successful fertilisation of gametes from individuals of different species never occur, hence a viable zygote is never formed.
Mechanical Isolation refers to the mechanism adopted by individuals of different species during the course of evolution such that their reproductive organs or the organs that assist in the process of reproduction are incompatible to each other. Hence, the gametes of each species are unable to come in vicinity of each other.
Here, the pollen grain from the flower of one species of juniper is unable to form the pollen tube when its sits on the stigma of the flower of another species of juniper. This is an example of Mechanical Isolation.