IN THE COMPANY OF ALDERS

by Phoebe Goit
Summer 2020
Photo by John F. Williams
Photo by John F. Williams

IN THE COMPANY OF ALDERS

by Phoebe Goit, Summer 2020

[Some of the more technical terms in this article are indicated by orange text. If you hover your cursor over the orange text, an explanation will pop up (on touch screens, touch the orange text.)]

 

Lacking the magnificence of the cedar or the great presence of a Douglas fir or grand fir, the easily recognized alder tree is not considered impressive by most locals. It’s just an alder. With its splotchy white and gray bark, it’s really kind of messy looking. Some even call it a “weed tree.” Not worth keeping.

However, an alder is very impressive in the amount of life it supports. For example, there are dozens of species of epiphytes living on its surface – mosses, lichens, and liverworts. There is also a fern that seems to grow out of the bark, and a terrestrial alga! And then there are the critters, most of them small, that call these habitats home, plus the unseen microscopic world underground.

You can learn more about lichens in Issue 5’s article titled, Lichens
Photo by John F. Williams

What appears to be the alder’s splotchy-white bark is actually not bark at all. Alder bark is gray-brown! The splotches of white are lichens that cover the bark. Lichens that are part of the pattern on the bark are crustose lichens. Crustose lichens adhere to the surface so closely that they can’t be removed without causing damage to either the lichen or the bark or both, which is why they appear to be part of the bark. They take their sustenance directly from rain and humidity, so they don’t harm the tree.

Like any other life-form, these epiphytes are simply looking for a home where they can prosper. Crustose lichens prefer a substrate that is fairly smooth and not too acidic. Because of this they are seldom found on conifers, which have rough, acidic bark and produce dense shade all year round. Crustose lichens are found on other deciduous trees, but the alder seems to be a preferred residence. It has smooth bark that is not highly acidic, and it grows on wet ground where there is good humidity and lots of light. The shade beneath an alder in the summer is not dense, and the leaves fall early at summer’s end, providing several months of sunlight.

Bigleaf maples seem like a good choice for these lichens, and several are present on very young trees, but their corduroy-appearing bark is not as smooth, and it holds water. This makes it a perfect habitat for mosses, and as the maple gets older, the mosses become more and more dense, and the lichens can’t compete.

Crustose lichens are symbiotic, not parasitic, and basically just use the tree as a residence. Because they manufacture a host of defensive chemicals, they do not have a lot of predators. Slugs occasionally scrape off bits, and a few species of moths and beetles feed on these lichens.

some of the lichen community

If you take the time to look closely, you will see a complex mosaic of different colors and textures. Whites and grays and creams abound, but there are also greens and blacks. Between some species there is a black line, giving the viewer a stained glass window effect. Each of these different patches is a different lichen species!

(click on a thumbnail below to see the gallery full-size)

An easy-to-identify lichen in this maze of pattern and color, is the pencil-mark lichen (Graftis scripta). It looks as if someone took a fine-tipped black pen and made squiggle marks on the bark. They are very small, so you need to look closely.

Photo by Phoebe Goit
Photo by John F. Williams

The pencil-mark lichen leads us to another denizen of the bark, a terrestrial alga. Lichens are made up of a fungus that forms the structure and either an alga or a cyanobacterium that does the photosynthesizing for the lichen. The terrestrial alga that is common on the alder is Trentepohlia aurea. It belongs to the group of green algae, even though it is orange in color. Close-up it looks fuzzy, and it forms small to large patches of orange on the bark of many alders. It is this alga that is the photosynthetic partner in the pencil-mark lichen.

Photo by Phoebe Goit

There are also many of the larger, non-crustose lichens on alders. These are called macro-lichens but don’t be fooled. They are still mostly small. Macro-lichens are clearly three-dimensional in form, and there is an incredible variation in appearance. Many of these lichens can be seen just by looking up and down the trunk, and there are even more if you look at branches that fall after a wind storm.

An easy macro-lichen to identify is the beard lichen (Usnea sp.). It truly looks like a beard. Like beards on men, some are short and bushy, and some are very long and stringy. Alder bark usually sports the short ones.

Photo by Phoebe Goit

A large lichen that is not so common but worth looking for in riparian areas — and a real beauty — is Pseudocyphellaria anthrapis. Because lichens are ignored by so many people, often the common name is the same as its scientific name: Pseudocyphellaria lichen for this one.

mess and moss

Photo by Phoebe Goit

The messy look of alder trees comes partly from all the mosses that call it home. Young alders have only a few, but the moist environment encourages more with each added year until there are large bundles. Mosses are good at holding water, and where there are large masses of mosses, they support another life form–ferns. The licorice fern (Polypocium glycyrrhiza) finds the moist habitat of the mosses perfect for winter growth. Summer is too dry, and the fern goes dormant. Like the mosses, lichens, liverworts, and algae, this is an epiphytic life form, and it does no harm to the tree.

Back to the moss. Mosses are very small and form dense, mostly green mats. They are plants, but reproduce by spores rather than seeds. If you look closely again, you will see there are many different species. It’s amazing how much variety there is in what just looks like green moss. A moss that is fairly easy to identify is cat-tail moss (Isothecium myosuroides) because it is long and stringy and hangs down the bark.

It is the same moss that hangs from dead branches in the woods.

Photo by Phoebe Goit
Photo by Phoebe Goit
Photo by Phoebe Goit

Mixed in with the moss or separate on the bark are several species of liverwort. Many of them look like moss on first look, but there is one that grows on the bark and is quite distinct. Woodman’s eczema (Frullania nisquallensi) is the common name because for sensitive people it can cause a rash if handled too much. It forms a beautiful lacy pattern on the bark. Its black-purple or black-red colors are elegant when growing on white bark.

an even larger community

Other types of creatures also live on the tree or use it as a food source. In the clumps of moss and liverworts are micro-animals, e.g., tardigrades, springtails, protozoa, nematodes, mites, larvae of many insects, and many others. Other arthropods, such as insects and spiders, are regular visitors. On a larger scale, birds eat the alder seeds and also feed on the insects, spiders, and mites. Deer and elk browse the leaves and twigs. Even slugs scrape morsels off lichens.

Underground there is an entire community that is essential to the health and growth of the alder. This is the unseen world; not only is it underground, but most of the players are microscopic. The area around the roots is called the rhizosphere, a very active zone with roots exuding nutrients used by microbes and fungi, which in turn provide benefits to the roots and the tree. This zone supports a dense and diverse microfauna along with the very important fungi.

You can learn more about mycelium and the rhizosphere in Issue #5’s Survival of Our Woods Depends on Mushrooms

This bustling rhizosphere is teeming with vitally important nitrogen-fixing bacteria. These bacteria interact with alder roots to form nodules that provide nitrogen, an essential nutrient, in a useable form to the tree. The bacteria in turn receive a banquet of carbohydrates from the tree’s photosynthesis. This allows alders to grow in depleted soil, often in damp areas, and when the tree dies, nitrogen is left behind in the soil to be used by plants that do not have this relationship with nitrogen-fixing bacteria. In this way the alder is a first succession tree, and other tree species that follow are in some ways a part of the alder’s community.

You can learn more in Factories Underfoot: The Nitrogen Cycle
Photo by Phoebe Goit

Mycorrhizal fungi also dwell in the rhizosphere. The hyphae or mycelium of these fungi attach themselves to alder roots and become an extension of the root hairs. This greatly increases the surface area of the tree’s root system. The fungi extract macro- and micronutrients from the soil and send them to the alder. The alder in exchange provides the fungi with sugars and other nutrients. Some of the fungal species are specific to the alder and would not exist without it.

This is only a small fraction of the magnificent mass of microbes that live in the rhizosphere. And it reminds us that, just like the microbiome that covers our bodies inside and out, there are also the millions of microbes that cover the tree’s bark, leaves, and twigs, and exchange nutrients with the roots deep underground.

And you thought it was just an alder!

Phoebe grew up in Minnesota, so the move to the Pacific Northwest was like moving to gardening paradise and to a whole new crew of native plants to learn. She is a retired librarian, so her formal education is not in the science area, but since retirement she has had time to follow that path. She is a Master Gardener and Native Plant Advisor with a particular interest in lichens, moss and liverworts. She has given talks on those topics and led many walks as well.

Table of Contents, Issue #8, Summer 2020

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