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Wednesday, July 15, 2015

Stomatal Bloom

What is stomatal bloom?  And why do we care about it anyway?

Bloom on grand fir and western hemlock
The first question is easy. Stomatal bloom is a waxy white coating around stomata.  OK, but what are stomata? They are the tiny pores on conifer needles that enable them to breathe. That is, the stomata are openings used to exchange gases, taking in carbon dioxide and exhaling, as it were, oxygen and water vapor. Each stoma (pl. stomata) has two guard cells that control the size of the opening, like tiny lips. When it rains, the lips close to keep the water out of the internal part of the leaf. And the bloom? It aids in water proofing the needles, both keeping out rain water and helping to retain water when it is dry.

Rows of stomatal bloom on grand fir
When you view the underside of the needles of a grand fir or western hemlock, you can clearly see two white lines of bloom on each needle. Each line or band of stomatal bloom contains several rows of stomata. You can see these individual rows with a good hand lens.

Pacific yew
Now, why do we care about all this?  We can use the differences in the presence of stomatal bloom to help identify our native conifers. For example, you can distinguish Pacific yew from other native conifers by looking at the lower surface of the needles. The needles on grand fir and western hemlock have two bands of stomatal bloom on the lower surface, but the yew has no distinct bands of bloom. The lower surfaces of yew needles just appear to be a lighter shade of green.  That’s not to say that there is no stomatal bloom on Pacific yew  needles. It’s just that the color is muted compared to the striking white color found on grand fir and western hemlock.

Pacific silver fir and subalpine fir
It is often difficult to distinguish firs found near the timberline in the Cascades. Pacific silver fir and subalpine fir look similar. Both have stomatal bloom on the lower surface of their needles. However, if you look carefully at the needles of subalpine fir, you will see that they also have bloom on the upper surface. On the other hand, the upper surfaces of silver fir needles are dark green.

You can distinguish western white pine from sugar pine by looking at the stomatal bloom on their three-sided needles. Sugar pine has bloom on all three sides, while western white pine has bloom on just two.

Western red cedar and Port Orford cedar
Stomatal bloom sometimes appears in different patterns. The bloom on the lower surface of western red cedar leaves resembles the shape of a butterfly, while the bloom on Port Orford cedar leaves forms an x-shaped pattern.

Shade-tolerant conifers in the Pacific Northwest usually have bloom only on the lower surfaces of the needles. This is particularly noticeable in the firs and hemlocks. Natives with bloom on the lower and upper surfaces are generally more intolerant of shade, and often grow in the higher elevations. This seems reasonable. The bloom on the upper surfaces would block the sun. Shade-tolerant trees need to have clear upper surfaces to capture what sunlight they can under the forest canopy. On the other hand, the bloom on the upper surface of needles gives trees growing in direct sunlight protection from the sun’s ultra-violet rays, especially in higher elevations where sunlight is more intense. This may explain why you find subalpine firs growing in open meadows near the timberline, while Pacific silver firs compete better in shady woods.

These relationships between stomatal bloom and the distribution of conifers are bound to be complex. In any case, some familiarity of the differences in the presence of stomatal bloom on conifer needles can help us identify conifer species in the wild.
See also


Air Pollutants and the Leaf Cuticle edited by Kevin E. Percy, John Cape, Richard Jagels, Caroline J. Simpson, p. 90.

Subalpine Fir in Silvics of North America


  1. Thank you for an excellent overview.

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