Introductory Key
A dichotomous key is a way to identify organism in a systematic either-or way. For each step (numbered), you chose the either a or b for what matches your specimen the best. Sometimes, you may incorrectly proceed through a step, in this case you go back. You keep going through a or b until you find an acceptable genus and then species. I find that sometimes I have to go backwards, because I mis-classified a characteristic (character). Don't worry! It's normal!
McCune et al. (2009) designed this dichotomous key based on their research in the Pacific Northwest of the US. I have used it successfully for Northern California.
How to use this website: Read both options for each level. Click to see pictures of examples. When you think you know which option it is, you can click on the light blue box of the option (a or b). Either the next step or the key will pop up.
KEY A - BASIDIOMYCETES
Most lichens don't belong in this category. These tend to look a lot more like mushrooms than the rest of the lichens, because they have basidia, which are reproductive structures that look like mushrooms or clubs.
Overall, we did not go through this category, so I mostly skipped over it, but the important difference between lichen basidiomycetes and non-lichen basidiomycetes is that lichen basidiomycetes have a photobiont partner (either green algal or blue-green cyanobacteria). You're not going to find these organisms in complete darkness (like non-lichen fungi can grow).
This is your most likely choice for identifying a macrolichen. This category are defined by the fact that they have asci which hold the spores, instead of the basidia. Asci are sacs that usually have an opening at the tip.
Ascomycete lichens are a big category, and they have a huge range of looks and colors. They can have a leafy thallus or almost look like a tangle of branches. Here are some pictures:
KEY B - BLACKISH, GELATINOUS
Thallus gelatinous, non-stratified; non-filamentous; black to browm or dark gray, photobiont mostly blue-green
These lichens are jelly-like sometimes and look less leafy. If you break off a piece and look at it from the side, you're not going to see layers of different colors. Additionally, if you spray it with water, it can look more glossy than stratified, but when it's dry, it looks very dull and matte.
In the field, I did not find a lot of these, and whenever I did, they tended to be black or very dark colored. Because these mostly have the blue-green photobiont cyanobacteria, you're generally not going to find these with a bright green color when wet.
These lichens have layers (stratified) when you break off a piece and look at it from the side. Most lichens I encountered in Northern California were stratified, probably because these lichens have a cortex which are almost like a lichen's skin.
Here I tore a piece of the Lobaria genus and shown it wet (left) and dry (right) from the side view. Take note of the medulla (the inside white part), the cortex (skin on the top), and the photobiont layer right below the cortex. This specimen also has cephalodia, which are compartments that hold blue-green photobiont, in addition to the algal layer.
Source: Mathieu, 2023
KEY C - YELLOW, ORANGE, RED
These lichens are bright and often found in more populated areas. This is because yellow-orange colored lichens are often nitrophiles. Nitrogen is more plentiful in more heavily-polluted areas. This one is the one category that I readily found in my suburb. Often the thallus (the leafy body) is small and very closely appressed (attached) to the bark of a tree.
The non-bright colors of lichens are most likely in this category. The interesting this about this category is that lichens look so different when they are wet vs dry. The colors in this dichotomous key generally describe what the lichen looks like dry.
Foliose basically means leafy, as opposed to stick-like. It's more of a two-dimensional body. Additionaly, it has a clear upper side and lower side. They could be different colors or have different textures. They could also look somewhate similar.
KEY D - UMBILICATE, FOLIOSE
This category is very specific. It needs to have a single attachment point, and it must attach to a rock substrate, instead of another substrate.
For this category, the lichen can attach to its substrate at a single point (but not rock) or it can be more broadly attached. If it's not easy to pull off the substrate or hard to see the underside, it is probably more broadly attached.
KEY E - STRATIFIED FOLIOSE CYANOLICHENS
Cyanolichens tend to look darker, but there are some dark lichens that aren't cyanolichens. I spray the lichen and see what the color looks like. Another helpful thing is to break off a piece and look at it from the side. I think distinguishing between blue-green photobiont and algal photobiont is easier when you have them side by side.
KEY F - GREEN ALGAL FOLIOSE
Thallus stratified, not umbilicate
If the lichen has an algal photobiont, it tends to look brighter green when wet. Like with cyanolichens, the color of the photobiont is a lot more vibrant and obvious when it is wet. It's important to know that a lichen with an algal photobiont can still look brown. This took me a while to understand.
Fruticose means that it is three-dimensional, like branches rather than leaves. What's important here is that there is usually not a top or bottom.
KEY G - FRUTICOSE, BLACKISH FILAMENTOUS
This category is fairly specific. If the lichen has fruticose filaments (string-like) that are black, choose this category.
KEY H - FRUTICOSE, HOLLOW STALKS
Podetia are hollow stalks that rise above a lichen's prostrate (flattened) thallus. The key to identifying this characteristic is to cut open a stalk and look if it is hollow.
KEY I - FRUTICOSE, NOT HOLLOW-STALKED
Congratulations, this is the last option! If your specimen is fruticose with non-hollow stalks, you're likely in the right place. If not, retrace your steps to find where you might have mistakenly classified a character.
