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Special Topics #18

Pseudotsuga macrocarpa and Abies concolor

Until recently I had always thought that white fir (Abies concolor) and bigcone spruce (Pseudotsuga macrocarpa) were fairly easy to tell apart. But on a hike in the San Jacintos, I encountered a tree that I was convinced was the spruce. It was only when Tom Chester analyzed my photographs that I realized I had been fooled. These first two pictures show what was my impression of the general appearance of these two species, erect needles for the Abies on the left and spirally arranged needles for the Pseudotsuga on the right.


But as the picture below left, also of Abies concolor, shows, the two can be more similar than I had ever imagined. And on the right below, another species that might at first be mistaken for white fir is singleleaf pinyon pine (Pinus monophylla).


Tom's first theory was that the variation is caused by the amount of available sunlight, with a tree in dense shade orienting its needles to capture light from all directions and a tree in more direct sunlight orienting its needles to capture light coming basically from one direction. The Jepson Manual gives the following description: "leaves ± two-ranked on lower [shadier] branches, twisted upwards on higher [sunnier] branches." However I have seen many lower branches with erect needles. In fact, the photo above left was from the lowest branch of a tree in a very shady location, so something else must be going on here. After thinking about it, Tom suggested that this erect orientation may avoid overheating their needles in strong sunlight or restrict water loss, or reduce the photosynthetic rate to match available water. My admittedly limited observations have been that in younger trees essentially all the needles are erect, and that it is only with the more mature trees that you find these flattened sprays of needles. And since younger trees do have all the branches in the light and it is only the older, bigger trees that create shade for the lower branches, a light variation may indeed be a major factor.

I also thought that white fir was more bluish-green or glaucous in appearance and bigcone spruce was much greener, as in the following two pictures, with Abies on the left and Pseudotsuga on the right.


But the next two pictures show not only how similar in overall appearance the two species can be, but also how green white fir can be, again with Abies on the left and Pseudotsuga on the right. Apparently the newer growth on Abies is glaucous, but that wears off as the leaves age, leaving a much greener surface, and the subsequent photo shows new glaucous needles with older, greener needles on the same branch.


So if we can't depend on the orientation of the needles or the color, what are the distinguishing characteristics? White fir needles are stiffer and more resistant to being moved when you handle a branchlet (at least this has been true for those individuals that I have examined), whereas bigcone spruce needles have a much softer and more flexible feeling. But a better discriminant is the base of the needle. The following two photographs show that in Abies concolor, the leaf tapers smoothly to the base, lacking a real petiole, whereas in Pseudotsuga there is a definite petiole which attaches the needle to the stem. If you pull off an Abies needle, the scar that is left is just about flush with the stem surface, but if you pull of a Pseudotsuga needle, the remaining scar is raised above the stem surface. This is shown somewhat in the subsequent two photographs, although the difference is not as striking as I had expected.



The following lefthand photograph shows the upper (L) and lower (R) surfaces of a white fir needle. The middle and righthand photos show the upper and lower surfaces of a bigcone spruce needle. In general, I would say that the fir needles are somewhat longer and wider, the Jepson Manual gives 3-9 cm, which seems hard to believe since I don't think I've ever seen needles longer than perhaps 3-4 cm, but apparently in wetter parts of its range, its needles are longer. The Jepson Manual gives 2-4.5 cm for bigcone spruce. Peterson's Native Trees of Southern California gives the following description of the Abies needles: "...bluish-green, two whitish bands on under surface separated by greenish keel." Munz says: "...bluish-green and stomatiferous above, with pale stomatiferous bands beneath separated by a median keel." With bigcone spruce needles, the stomatal bloom appears only on the lower surface in two bands on either side of a keel which is reflected by a groove on the upper surface.


There's not much to distinguish between the bark of white fir and bigcone spruce, with the young bark of both being basically smooth with resin blisters, and the mature bark being deeply furrowed and grayish-brown, as shown in the following four photographs, Abies on the left and Pseudotsuga on the right. White fir bark is white when young and ages to gray and darker.



The cones are a dead giveaway, with the seed cones of bigcone spruce being pendulous with prominent three-lobed bracts and which fall as a unit, and the seed cones of white fir being ± erect with the scales falling separately leaving an axis attached to the branch. This latter is why you cannot collect white fir cones around the base of the tree. I am also including a line drawing of each from Stuart and Sawyer's Trees and Shrubs of California.



It always interests me to see how the shapes of various tree species are described, as though they are all shaped the same, and not affected by age, weather conditions, surrounding trees, slope aspect and many other factors. However, both white fir (left) and bigcone spruce (right) are generally pyramidal, especially in younger specimens, both can develop more rounded crowns and irregular shapes as they age, and both are species that can grow to well over 100' tall, although mature bigcone spruces are on average quite a bit shorter.


Another key discriminant is where these species are found. In general bigcone spruce is a lower elevation tree, in Southern California reaching its highest limit around 6000', which is about the elevation at which white fir begins. And white fir is a species that has a much greater geographic range, extending both to the desert mountains and the high Sierra Nevadas.

I assume that white fir is so-called because until the tree reaches a mature stage, the bark is whitish, and even in older trees there is probably some white bark toward the top. The name concolor refers to the leaves which are uniformly colored, despite the stomatiferous bands showing up under magnification. This species was first described in George Gordon's The Pinetum: being a Synopsis of all the Coniferous Plants at present known, with Descriptions, History and Synonyms, 1st edition 1858, and named Picea concolor. Included in that description was the following: "Leaves closely spaced, somewhat two-rowed, and the same colour on both surfaces."" Bigcone spruce is interestingly named because it is not a spruce but is evocative of that species with its drooping lower branches. Sometimes called bigcone Douglas fir because of its relationship with Pseudotsuga menziesii, it has at various times been put in the genera Abies and Tsuga as well as Pseudotsuga, which in turn means "false hemlock."