One of the most important elements in a Greenstand seedling photo is the species of plant. Different species have different characteristics and are of different values for restoration programs. Labeling an image with its plant species adds information to the capture and builds training sets of images for machine learning. Eventually it is our hope that automated image analysis will be able to tag a photo with its species, but there is a lot of work to be done before then.
Recognizing species in photos is not easy, and requires learning. As during learning a language, you will be aware at first of very little that might help you to separate entities: you’ll see green color, leaves and stems. But over time you will learn to recognize increasingly subtle differences, and will come to recognize species you have seen before. This document is intended to guide you efficiently through the learning process.
A photo is a 2-dimensional representation of an organism in the field. That organism has characters that differ from organisms of other species (e.g., humans have opposable thumbs). In general, more closely related species have more similar sets of characters. Characters can be used to create identification keys: e.g., “Question 1: if hairs on leaf underside present, got to Question 2, if not, go to Question 3”, until a unique combination of characters indicates that the organism is the corresponding species. Many of the characters necessary for discriminating species will not be visible in photographs, but we can still search for some obvious characters in the image that will limit the set of species that it could be. The short descriptions in the ‘virtual herbarium’ may often explicitly mention some characters or combinations of characters.
Learning a core set of eight characters will help you a lot:
Please read through the details of these characters below, referring when directed to the virtual herbarium for Haiti. Note also that there are lots of specialized terms for plant characters. You will need to learn a few, but in this document and in the virtual herbarium we will try to keep specialized vocabulary to a minimum.
The stalks of a plant’s leaves may join the stem in one of two basic ways: they may meet separate from the stalks of other leaves, or they may meet at the same point as one or more other leaves. In the former situation we say the leaves are “alternate”, in the latter that they are “opposite”:
(Images taken with gratitude from from the online “Illustrated Glossary of Botanical Terms in the Flora of the Chicago Region” by Wilhelm & Rericha, 2017)
Often seeing the leaf arrangement is hard in an image crowded with leaves, but usually there is at least a single instance where it can be assessed. Some examples in Haiti are:
At the base of the leaf is the leaf stalk that connects the leaf to the stem. This stalk (“petiole”) can be long, relative to the length of the leaf, or very short (termed “sessile”). Check these plants out:
A leaf is a single structural unit, which will fall off as one when dead. This unit is sometimes a compound structure with its own stalk (“rachis”) and multiple leaf units called “leaflets”. It can be tricky to determine if what looks like a leaf is indeed a leaf, or just a leaflet, but the regular structure of a “compound” leaf becomes increasingly obvious as you get your eye in. The term “simple” is used for a ‘classic’ single leaf.
There are several types of compound leaf, with the main division being between “pinnate” leaves and “palmate” leaves. In the former, there is a single extended stalk with leaflets coming off of it at different points, while in the latter all the leaflets meet at a single point. A pinnate leaf may have a terminal leaflet (odd-pinnate, or “imparipinnate”), or be without a terminal leaflet (even-pinnate, or “paripinnate”). One further complexity: if there are two levels of division in a pinnate leaf, the leaf is “bipinnate”.
Some examples in Haiti:
The size of seedling leaves (and leaflets) can be remarkably variable, both on the same plant, and among plants of the same species in different growing conditions. The size of seedling leaves is also generally larger than the size of the leaves on an adult of the same species. For this reason, we generally do not use leaf size measurements as characters for these seedlings. However, as you can see from the images, there are some species that have strikingly larger leaves than others, and so some times a general “leaves large” or “leaves small” has been given. For example:
Most leaves are some variation on elliptically shaped: narrow at the base and narrow at the tip. However the position of the widest part of the leaf is an obvious and useful character, usually consistent within a species. There are many terms to describe fine variation in shape, but the terms that we will commonly use are: “elliptic” = widest in the middle, “ovate” (egg-shaped) = widest towards the leaf base, and “obovate” = widest towards the leaf tip. Palm leaves are shaped quite differently and often are referred to as “linear”.
Some examples in Haiti:
The edge of a leaf’s blade (its “margin”) can be smooth, i.e., without teeth, or can be broken up in a variety of ways. There are many terms to describe the details of the teeth or incisions, but three can be used to cover the majority of cases. An “entire” margin is plain and even, a “serrate” margin has teeth, and an “incised” margin has deep intrusions into the leaf. (Other terms you may read are related to these: “dentate” is similar to serrate, and “lobed” is similar to incised.) Sometimes the margin is wavy not in the 2-D plane of the leaf surface but in a plane perpendicular to the margin, i.e., not flat; this is termed “undulate”, and can be hard to see in a photo.
Most moist tropical species have entire leaves, with the incidence of serrate leaves increasing in dry habitats and in temperate floras. Some examples in Haiti:
The veins of leaves can be hard to see in photos taken from above - veins are usually clearer on the underside of the leaf. However, if you can see them, they provide another consistent and useful set of characters. ‘Classic’ leaves have “pinnate” venation, while maple-like leaves have “palmate” venation, with several main veins meeting together at the base of the leaf. Grasses and palms, and few other species, have veins that mainly run “parallel” to the main axis of the leaf:
Examples in Haiti:
Finally, leaf color can be a useful character. Under different lighting conditions the colors in photographs can be unreliable, and it is difficult to describe colors. However, subconsciously you will come to associate species with their color. Most leaves are obviously some shade of green, but watch out for dark greens, pale greens, bluish tinges, and grayish tinges. Yellowish tinges are more often a sign of nutrient deficiency than a characteristic of a species. Rarely a species will have consistent “variegation”, with pale or yellowish dots on a darker, greener surface.
Another feature of the leaf surface is its sheen. Some species have consistently “glossy” surfaces, some are consistently “matte”. This may be visible in photos, depending on the image’s lighting.
As you learn these characters you will see that most tropical tree species have simple, alternate, entire leaves with pinnate venation. However, many species have at least one character that is not the ‘usual’ one and this alone reduces the set of species you need to consider. And as you learn the species, and see them commonly, your subconscious will start to take over and you will not need to compare their characters one-by-one between your image and the reference set. Do however please always remember that the tropics are very diverse, and there are often rare species that look very similar to a common one you may know, differing only in minor ways, and so it is vital to express uncertainly if you are not totally sure.
Thanks for your work on this, and I hope you will find it rewarding to come to know tropical tree species.
(Cam Webb, 2021-10-17)