TOPICS: Plant structure and natural history, ecology, measurement
LEVEL: Middle and high school
TIME: One class period
ADVANCE PREPARATION: Collect twigs from several species of deciduous trees included in the eastern or western version of the "Key to Selected Deciduous Winter Twigs". Be sure each twig has a terminal bud and at least one full year's growth as indicated by terminal bud scale scars. Label the twigs with letters written on masking tape tabs
.
MATERIALS:
The twigs in the eastern and western versions of the "Key to Selected Deciduous Winter Twigs" have distinctive characteristics that students can easily observe. With older or more advanced students, the "Winter Key to the Genera of Northern Deciduous Trees" may be more appropriate
.
STEP A.
Students unfamiliar with dichotomous keys may require some help getting started. You may want to give all students the same kind of twig and do the first identification together
.
STEP B.
This activity presents an excellent opportunity to discuss measurement problems. Different twigs of the same type will show variability in their annual growth. Measurements of the same twig by different students will differ, permitting a discussion of the concepts of accuracy and precision of measurement.
QUESTIONS 4-8 require higher order thinking skills. We recommend that you discuss these questions in class before students write out their answers
.
1.
Distinctive characteristics of winter twigs include the arrangement of leaf scars, the shape of the leaf scars, the appearance of the nodes and bundle scars, and the color of the bark
.
2-3.
Answers will vary. There is a wide range of differences in growth rates among species. Willow and cottonwood are some of the fastest growing species; oak, maple, sycamore are among the slowest. The growth rate within a species is dependent on geographic location and environmental conditions. Reasonable answers range from 5-50 cm/yr.
.
4.
Growth stops in the autumn when the plant stops photosynthesizing and the leaves turn color
.
5.
Environmental conditions that affect growth rates include climate, weather, shading, and soil nutrient level. For example, willow trees in the arctic are very slow growing. Low rainfall, low temperatures and shading by other trees slow growth rates
.
6.
When damage to the terminal bud occurs, an axillary bud assumes the function of the terminal bud
.
7.
Deer, rabbits, hare, deer mice, voles, squirrels, moose, porcupines, and pocket gophers are mammals that feed on buds and twigs. Birds include the ruffed and spruce grouse, chickadees, robins, and blackbirds. Typically, beavers store summer twigs for winter use
.
8.
Wildlife biologists need to know the kind and amount of plants eaten by animals. They use this information to estimate and predict plant and animal population size.