Final answer:
Temporal isolation is the genetic isolating mechanism preventing interbreeding between Pinus radiata and Pinus muricota, since they release pollen at different times.
Step-by-step explanation:
The genetic isolating mechanism involved when two species of pines (Pinus radiata and Pinus muricota) do not interbreed despite living in the same region of California is temporal isolation. Temporal isolation occurs when two species breed at different times, thus preventing them from interbreeding. In this case, one species releases pollen in February and the other in April, which results in the two species not being able to reproduce together even under conditions that otherwise permit hybridization.