One thing to tell nigra apart from most other pines are the insanely huge needles and nigra being bigger in everything compared to scots pines, ponderosa, jack pine, JBP, JRP, or any pine I've ever grown for that matter. In the seedling stage at least. Once they go adult, I find the resemblance to ponderosa pretty striking. Yet the buds of nigra are some of the biggest I've ever seen.
You have probably read about colchicine as a chemical to produce polyploid plants, in essence it halts cell division at a certain stage and causes plants to quadriple (or more) their nucleus. This leads to an increase in plant size. Very popular in violets, that go tiny again if they reproduce asexually for a few years.
When looking at nigra in the wild, there are types that are close to scots pines but coarser and generally larger, and there are types that go the extra mile; needles the size of fore-arms and cones the size of softballs. I always get the feeling they're treated with colchicine. Maybe for timber production after WWII. I might whip out the microscope to check if that's the case in the near future.
The only other European pines that come close when it comes to foliar, bud and cone size are Italian stone pines (P. pinea).
I'm thinking that, at least here in the Netherlands where most P. nigra stock was imported from the Mediterranean "Corsican pines", they might have been hybridized with pinea back in the mediterranean. It would explain both the vigor as well as the variety in size. Since there's natural backcrossing of hybrids taking place, we now get three varieties: Nigra x Nigra, Nigra x Pinea and Pinea x Pinea. This is speculation though, don't pin me down on it. I don't know the compatibility of the two families. The typical pinea juvenile needles are something I've never seen in the wild here, but I would expect them to occur if there are backcrosses that would yield pure pinea. It could very well be that we have different varieties of nigra in the wild, one being regular nigra and the other being the corsican nigra. Or the colchicine option of course.
Now I know this'll spark interest of some of the readers. Even though colchicine is sold medically to treat gout, it's still a carcinogen and it will screw up your cells processes. When I worked with the stuff in the lab, we had to wear extra protection on top of our regular lab gear and even create a separate waste-flow specifically for colchicine. It's not stuff to mess with for fun. And besides that, it makes things bigger.. In general we'd want things in bonsai to be smaller.
