Amplifying a rather terse explanation: Hybrids generally have to be closely related. Breeding within a species is generally successful -- indeed, it's one of the definitions of species. (You do however get situations where a 'species' with a large range has situations where specimen B can breed with A, 50 miles to the north, and C 50 miles to the south, but not D, 100 miles to the south. C in turn can breed with B and D, but not A. This example came up in my zoology class with some frog in the Appalacian mountains. Breeding between species in the same genus sometimes works. Poplars interbreed frequently, as do willows. E.g. We have 3 hybrids -- hill, northwest, and brooks #6 that are all the results of crossing plains cottonwood P deltoides with a russian poplar P. petrovskyana (Say that fast after 3 beer...) In this case the hybrids are faster growing that either parent. They all have distinctive growth patterns too. A few pines can cross. I know jack and lodgepole can cross. But they are so similar to each other, and so variable in their growth habits depending on environment that just telling them apart can be tough. I would not expect you to be able to cross a scots pine and a white pine. To many aspects are different -- e.g. needles per bundle, needle diameter, needle cross section. And those are the visible signs. The underlying chemistry is also difirrent. Interspecies hybrids are often sterile, or have some non-competitive disadvantage. Breeding between species of different genuses seldom works.
For a conifer example [since this thread started with conifers], northern populations of Pinus muricata (Mendocino area, N California) can't hybridise with southern populations (San Luis Obispo area, S California). Yep; since Nootka Cypress was moved back to Cupressus with Leyland Cypress becoming Cupressus × leylandii, there are no intergeneric hybrid conifers. There are however plenty of hybrid conifers between species within a genus. Some orchids produce fertile intergeneric hybrids; perhaps that says more about the definitions of 'genus' used by orchidists, than it does about hybridisation . . .
