Elk, Fire and Aspen – Quaking aspen (Populus tremuloides) is oneof the most widespread tree species in North America. Althoughquaking aspen has been a key component of forest ecosystems formore than ten thousand years, it is currently in decline acrossbroad portions of its range. Historically, aspen recruitment hasbeen favored by the occurrence of low intensity fires, which createopenings that allow young aspens to grow and eventually reproduce.In recent decades, however, the number of fires per year and thearea burned per fire has increased; these increases in thefrequency and magnitude of fires are thought to be caused by the“hotter droughts” that have resulted from climate change and byprevious fire suppression policies. Some recent fires havedestroyed more than 400 km2 of forest; such fires are referred toas “mega-fires.” In addition to fire, browsing by elk can preventyoung aspen trees from becoming large enough to reproduce. FieldExperiment – Suppose that researchers wanted to examine thecombined effects of a mega- fire and browsing by elk (Cervuselaphus) as factors that may be affecting the decline of quakingaspen. Immediately after a mega-fire, the researchers establishedfenced-in plots that prevented by browsing by elk (the “Elk absent”treatment) along with nearby plots from which elk were not excluded(the “Elk present” treatment). Five fenced-in plots and fiveunfenced plots were established in each of two areas: A section offorest that was burned in the mega-fire (the “burned” treatment),and a nearby section of forest that was not burned (the “unburned”treatment). After 6 years, the number and mean height of youngaspen trees in each plot are shown in table

1. What is the total number of plots that were established inthis experiment? How many of these plots were burned? How many wereunburned?

2. The aspen height data were used draw the bar graph shown inFig. 1. Summarize how elk and fire affect the height of young aspentrees by answering the following questions: 2.1. What are theoverall effects of elk and fire on aspen height? 2.2. Does theimpact of fire on aspen height depend on whether elk werepresent?

3. Summarize how elk and fire affect the number of young aspentrees by answering the following questions: What are the overalleffects of fire and elk on the number of aspen? Does the impact ofelk on aspen number depend on whether the plots were burned? Doesthe impact of burning on aspen depend on whether elk werepresent?

Treatment	Number of Trees	Mean Height
Elk absent, burned	2058	2.8
Elk absent, unburned	738	1.2
Elk present, burned	91	0.3
Elk present, unburned	753	0.4