Symbolism, Imagery, Allegory
We know from the get-go in this book that Yuri Zhivago has a bit of a thing for nature, and especially for smells. But linden trees stand out in this category because of the way they always seem to come up when Zhivago is thinking about his feelings for Lara Antipova. Shortly after seeing Lara, for example, Zhivago finds that he can smell "the dusty, thicket fragrance of an old linden coming into bloom" (5.6.8).
Soon after this first encounter with the smell of lindens, Zhivago again smells them while he's thinking of Lara: "The ubiquitous wafting of this [linden] smell seemed to precede the northbound train, like a rumor spread to all junctions, watch houses, and little stations, which the travelers found everywhere, already established and confirmed" (5.13.10).
Now before you start oohing and ahhing about how romantic this is, there's something you should know about the smell of linden trees. They are famous for smelling like… well… semen. Hey, don't look at us; we didn't invent linden trees. But anyway, yeah, we know: that's part of romance, too. We'll just say that Pasternak has a fairly aggressive way of saying that Zhivago is sexually attracted to Lara and leave it at that.