Wk 3: Dichotomous keys & winter twigs

Overview & Introduction

The goal of today's laboratory exercises is to learn to recognize traits of twigs and to use those traits to identify trees even when they don't have any leaves.

Watch narrated 24-minute lecture. Powerpoint slides for lecture material are provided. Note: watch video up to 21:30, there is no Microsoft Word assignment that is mentioned in the video, instead you will complete quizzes.

Print the woody twig key, or open it on a second device so you can refer to it while taking the Blackboard quizzes. You can also refer to the Digital Twig Images document at the bottom of this folder to look at traits and compare with your twigs.

Find the twigs in your box of supplies. The twigs are labeled A - G (except there is no F twig). Each Blackboard twig quiz corresponds to the twig with the same letter. Use the quiz to answer questions step-by-step in the dichotomous key until you have identified the twig. If you choose the correct answer, the quiz will lead you to the next question in the key. However, even if you choose a wrong answer in the quiz, the quiz will tell you what the correct answer to the previous question was, and where in the key the correct answer would lead next. Thus, even if you get wrong answers during the quiz, you will be directed towards the correct species identification.

Even though the quiz questions are nearly identical to the questions in the key, it is best to follow along in the key because the key has diagrams in the margin that will help you to understand what some of the terminology means. If you don't see a term illustrated next to its use in the key, look back over earlier pages in the key - the terms are illustrated next to their first appearance.

Identify woody twigs with winter twig key.

• • Complete Woody twig identification quizzes for twigs A-E and G

  • Save twig H for part of next week's lab (not used in Spring 2020)

Link to Narrated Lecture about Dichotomous Keys & Keying Out Twigs

This link is for a YouTube lecture about how to use dichotomous key for keying out twigs.

https://www.youtube.com/watch?v=1bn3Y8lg9p8&feature=youtu.be

Linked here are Lecture notes (pdf of PowerPoint slides) for narrated lecture about ow to use dichotomous key for keying out twigs

The linked file is an abridged, modifed version of a dichotomous key of winter twigs from the Stucky (2003) (used with permission)

Stucky, J. M. 2003. Winter twig keys to common, native, fully deciduous trees

and phanerophyte shrubs of the North Carolina Eastern Piedmont. Vulpia 2: 23-42

Notes for instructors

For this exercise, I collect twigs over the winter and freeze them in ziploc bags for summer or fall courses (or substitute other exercises). I mail students twigs, each labeled with letters A - G on a piece of lab tape at their base. I slide twig bases to expose the pith so students do not need to do so.

I supplement the twigs I mail students with some images of details of each twig, which I share over our course management system (I can't share the images I've used here as I wasn't careful about noting attributing sources). This way, students can handle the real thing while I can also be confident that they all have access to the same information (because some individual twigs don't always show features well).

I have found the best way to grade twig key-outs is to set up each step in the key as a multiple choice question in a quiz (each answer is one of the two alternatives in the key; each twig is a different quiz on Blackboard). The advantage to this is that if a student makes a wrong choice, the next question automatically gets them back on the right track - each question starts with the answer to the previous step and then proceeds to the next couplet. Otherwise, students typically make a wrong choice early in the key and are then presented with a series of choices that aren't meaningful for the specimen they are considering - this is frustrating for students and impossible to grade. I grade key-outs leniently - I set the course management system to give bonus points. I do this because it's a hard skill, even for advanced students, and it's harder when students are working alone.

Labelling of twig

As a follow-up exercise, each student is responsible for documenting traits of an unknown twig. Students do not key this twig out, but they do need to photograph each characteristic and submit the photograph along with a description of each trait. I provide a PowerPoint template students may use to facilitate this.

For this assignment, you should document the traits of Twig H. Take close-up photos using the zoom function on your camera/device. Then label each of the following traits using arrows and text:

1. twig diameter (near the base),

2. twig color,

3. appearance of lenticels (color; are they conspicuous or hard to see; are they raised and bumpy or smooth) 4. pith (color; is it homogenous, diaphragmed, or chambered),

5. leaf arrangement (opposite, alternate, whorled),

6. leaf scar with axillary bud

• leaf scar shape: triangular, oval, horseshoe-shaped, or some other shape?;

• number and arrangement of bundle scars in each leaf scar;

• position of axillary bud: surrounded by leaf scar, partially surrounded, above leaf scar, etc.;

7. number of bud scales on terminal bud: none, 1, 2, 3, .... many?

Characteristics of good photographs:

• Take photos with good light, and on a plain background so the twig stands out.

• Include a ruler with metric distances in as many of your photographs as possible so the observe has a sense of scale.

• Labels can be added on paper prior to photographing or digitally afterwards. Digital labels can be added with arrows and text boxes in Powerpoint, or on phone apps (Snapchat?).

• Make sure an identifiable part of a sticker is apparent in your photos (for close-up photos, it is okay if only a small part of the sticker is visible). Stickers are in your general supplies bag. You need to include stickers so I can be sure I am seeing your own work.

A powerpoint template has been provided on Blackboard - you can copy and paste your photographs into the template

Powerpoint template for labelled twig images linked here.

Note to instructors:

• I use stickers through this course to make sure that students are submitting their own work. I purchase sets of varied stickers and send each student a set of identifical stickers. I ask them to make sure the sticker is present in photos they submit, so I know they are not borrowing photos from other students or the web.