Would You Trust a Bryophyte for Directions? A Field Exercise for Determining the Distribution of Moss on Trees

Abstract

Every introductory biology course preaches the importance of scientific method, but often students do not have the opportunity to develop and test scientific questions in those very courses. For example, published lab manuals for the traditional survey of the kingdoms course rarely recommend little more than that students observe live and preserved materials and prepared slides in the lab. I designed the following field exercise both to illustrate the ecological requirements of a group of organisms (the mosses) and to allow students to test a hypothesis so popular that it has made its way into folklore: Moss only grows on the north side of The bryophytes, plants such as mosses and liverworts that lack vascular tissue and depend upon water for fertilization, are an excellent subject for study in the field because they are abundant and they are present yearround. One might predict that mosses would be more abundant in moist, shady locations than in dry, sunny habitats because they are tied to water for survival and reproduction. The physiological dependence of mosses on water, coupled with the inclination of the sun in the northern hemisphere underlies the biological explanation for the folklore belief that moss grows only on the north side of trees. In the northern hemisphere, solar radiation strikes south-facing objects directly, while the north-facing objects are shaded from the sun throughout the day. This phenomenon results in hotter, drier microclimates on southfacing structures, and wetter, cooler microclimates on north-facing structures. It has consequences for everything from the location of ski slopes (north facing slopes to avoid snow melt) to the best windows for house plants (south facing because the sun shines on them all day). Does aspect (north or south-facing) also dictate the distribution of moss on trees? In other words, would you trust a bryophyte for directions? Once students have been introduced to key characteristics of a particular group of organisms, it becomes possible to develop hypotheses about the ecological consequences of these characteristics. With rare exceptions, the distribution of a species in nature is inextricably tied to its physiological limitations. In order to train good scientists, we must teach our students to ask research questions and give them practice testing those questions and analyzing the results. In this field exercise, students are challenged to test the hypothesis that moss grows preferentially on the north side of trees. The exercise clearly accomplishes two goals: