Pinyon pine cover quite a bit of the west. Economically, they’re good for pine nuts and fence posts. Ecologically, of course, they’re a lot more important than that. They’re also very important in figuring out what the climate was like a long time ago since their needles can be found in packrat middens dating back around 40,000 years.

There isn’t just one kind of pinyon. For a while taxonomists thought there were two kinds: Pinus edulis and P. monophylla. P. edulis has two needles in a fascicle and P. monophylla has one needle in a fascicle. Fascicles are the little clusters pine needles grow in.

Then two other pinyon with only one needle per fascicle were identified, one as P. edulis var fallax and the other as Pinus californiarum. If that’s not confusing enough, there are regions where pinyon have one AND two needle fascicles on the same tree.

Zavarin et al. looked at monoterpene content to classify all these pinyon (don’t plants make cool chemicals?), and concluded that all of the single needle pinyon were subspecies of P. monophylla. But then a study on chloroplast DNA completely contradicted that.

So pretty much everyone disagrees on how to separate these trees. Everyone does agree, however, that these trees are all really closely related. Several people have also noticed that the trees with two fascicle types often have more one needle fascicles in dry growth years.

The authors of this study wanted to know where each kind of pinyon grows and if the climate is different. Since nobody can decide how to separate the trees, the authors came up with their own classification based on needle type that worked out really well. Between all these tree types, there are only 4 kinds of needles. The needles were classified by number of needles in a fascicle, shape, thickness, number of resin ducts, and stomatal lines. Resin ducts carry resin (which bugs don’t like to eat) throughout the needle and stomatal lines are lines of stomata (which I described in this post).

There’s one two needle pinyon (P. edulis) and three single needle pinyon. The fallax type needles are thin and the P. monophylla and californiarium-type needles are fat. The californiarium type needles have way more resin ducts and fewer stomatal lines than P. monophylla, though.

So the authors looked at a ton of needles from all over the southwest and separated them into these four types. They also checked out needles from dry and wet years in one of the overlap zones. Then they mapped that info out and matched locations with climate data.

Interestingly, even in the overlap zones, the four needle types don’t change. This is important since a lot of people think that these trees are hybrids

If hybridizatization of two species is occurring, it seems to only affect the tree as a whole. Individual needles on the trees usually fit as one of these four needle types, regardless of their frequency on the tree.

The needle types only seem to mix in two ways: P. edulis and fallax type or P. edulis and P. monophylla. On trees with more than one needle type, the single needle types are more common in dry years, though there is a lot of variation between trees.

The unmixed trees had ranges that matched specific climate types. P. monophylla and californiarum type grow where there is more winter precipication, but P. edulis and fallax type grow where the precipitation is biseasonal. Both the fallax and californiarum type are prevalent where early summers are very dry.

So what does this all mean?

First, the biggest thing separating needle types is climate. It also looks like P. edulis, P. monophylla, and fallax type might all be the same species responding to water availability: P. edulis needles looks like P. monophylla needles cut in half. Fallax type needles look like P. edulis where one needle in the fascicle was aborted. Trees that grow in “in between” climates might be choosing which needle type to have based on the water availability that year. These scenarios are not only drought adaptive, they also fit well with the chloroplast DNA study.

The way the needle types are separated by climate is really cool because it could allow us to get at the seasonality of precipitation in climate reconstructions. It will also help us get better at predicting how pinyon will respond to climate in the future.

Cole, K.L., Fisher, J., Arundel, S.T., Cannella, J., Swift, S. (2007). Geographical and climatic limits of needle types of one- and two-needled pinyon pines. Journal of Biogeography DOI: 10.1111/j.1365-2699.2007.01786.x

Zavarin, E., Snajberk, K., Cool, L. (1990). Chemical differentiation in relation to the morphology of the single-needle pinyons. Biochemical Systematics and Ecology, 18(2-3), 125-137.