Viewing Plant Systematics through a Lens of Plant Compensatory Growth

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An important objective of most college and university plant systematics courses, and of thematic lesson units on plants in some high school science classes, is to teach students how to effectively use dichotomous keys for the purpose of identifying plants. The language of most keys is quantitatively descriptive and, as such, provides students the explanatory terms and measurements required to determine the fit of their "plant-in-hand" in the order, family, genus, and finally species to which it belongs.

Dichotomous keys are generally designed as a series or sequence of paired questions that, when answered, allow the student to move to the next set of paired questions. Answering questions in sequence ultimately enables students to identify the organism of interest. These questions are often framed in a way that requires students to determine answers and make decisions based on the presence or absence of structures or on some measurement (or range of measurements) for certain plant components, such as leaf blade length or width, thorn or petiole length, or plant height (e.g., Figure 1). Although dichotomous keys provide a useful range of measurements and attributes for each species in question, these measurements are often regional and derived from averages. They do not always account for the large plasticity exhibited by plants.

The term plant compensatory growth refers to exaggerated vegetative growth that results from mechanical damage to plants (e.g., cutting, animal browsing, or breakage from snow) as a physiological consequence of an increase in the root-to-shoot ratio following the loss of aboveground biomass (McNaughton, 1983). When aboveground tissues are damaged or removed, more root reserves are allocated to relatively fewer buds, which produce fewer but larger shoots and leaves (Millington, 1963; Danell et al., 1997).

All of the examples of plant compensatory growth that we have documented (see Figure 2) exceed (sometimes by one or two orders of magnitude) the size ranges described for the species in dichotomous keys. Consequently, it has been our experience that students who are attempting to use dichotomous keys are often confused when they encounter samples of exaggerated leaf and shoot growth; they may simply disregard these atypical forms or may confront the instructor, asking why their "plant-in-hand" does not fit the key Seeking a simple explanation, instructors may answer that "Plants are variable and no keys are perfect" or "Botany is a science of exceptions to the rule," instead of proposing a response that helps students to understand why such variability can occur in nature. Here, we present several ideas for how instructors can turn these seeming conundrums into teachable moments.

At the University of Northern British Columbia (UNBC), we began to realize that our Plant Systems (Forestry 201) students were challenged each fall when attempting to key out plants during their laboratory exercises. Plants often came from areas such as river banks, where river-ice scouring sheared plants; ungulate winter ranges, where plants were heavily browsed by moose; road and trail sides, where plants were compensating from brush-cutting; and parks and city streets, where ornamental hardwoods are routinely pruned.

To help students understand ranges of variability in nature and concepts such as phenotypic plasticity (the ability of an organism to modify its morphology in response to a changing environment), and to prepare them for discrepancies between key descriptions and what they might observe in the field and in class, we now initiate discussions early in the semester on the concept (and consequences) of plant compensatory growth. We choose striking examples from the field that visually illustrate how imbalances in the root:shoot ratios can change plant morphology and phenology (e.g., how events such as bud burst or time of flowering are affected by climate). …


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