John Timmer, writing for Ars Technica:
Unlike many groups that vary widely in the number of chromosomes their species carry, pretty much all the Gymnosperms have a dozen pairs of chromosomes. And pretty much all of these chromosomes are up in the area of two billion bases long, or a bit smaller than the human genome. That size is so consistent, in fact, that the authors think the trees might be pushing up against the limits of how much stuff you can put in a chromosome and still get it copied and shared between two cells when they divide. In other words, if firs wanted to carry any more DNA than they already do, they’d have to start making new chromosomes.
To put that in context, chromosomes in Norway spruce (Picea abies) have 2 billion or more base pairs. That’s nearly as large as the entire human genome, which contains about 3.2 billion base pairs spread out over 46 chromosomes.