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I initially went out to my field site with the hopes of finding patterns in the way shrub species such as Amelanchier alnifolia and Salix were responding to browse pressure by moose. Looking at percent cover of biomass, horizontal growth patterns, or length of new shoots, I wasn’t able to imagine a study design that could answer specific questions, or maybe I just couldn’t formulate questions that lended themselves to good scientific study.
So I moved on to several places around my lake where Aspen stands have sent out suckers following beaver disturbance that ended some time ago, observing the gradient between the older stand and the younger individuals that have come up.
I also noticed other places where aspen were encroaching on fields or human-cleared sites, and some of these had distinctive successional gradients, too. And now to come up with a study design:
1.) Identify the organism or biological attribute that you plan to study.
I plan to study Aspen trees in the process of succession.
2.) Use your field journal to document observations of your organism or biological attribute along an environmental gradient. Choose at least three locations along the gradient and observe and record any changes in the distribution, abundance, or character of your object of study.
One location is inside the parent clone of the aspen organism, where the individuals are mature with similar ages (presumably) and similar sizes (heights and diameters). Also noteworthy may be variables such as density, basal area, and incidence of health or pathological indicators such as conk.
A second location along the gradient is where there is an obvious shift in the clone’s attributes such as size, age, and density; i.e. where the aspen clone initially sent out suckers in response to the disturbance from years ago, and there is a higher abundance of individuals.
The third location along the gradient is the point at which there are no longer any suckers coming up, or where the suckers are obviously young or new clones, and maybe more abundant. In some places, shrub species are present in varying numbers/densities at this point on the gradient with varying levels of browse, and some of the aspen suckers are also being affected by browse pressure. Could the aspen be responding to browse pressure by continually putting out new suckers?
3.)Think about underlying processes that may cause any patterns that you have observed. Postulate one hypothesis and make one formal prediction based on that hypothesis. Your hypothesis may include the environmental gradient; however, if you come up with a hypothesis that you want to pursue within one part of the gradient or one site, that is acceptable as well.
Several processes are likely acting on the aspen trees to cause differences in the way they expand and fill the spaces they occupy following disturbances. Aspect, slope, and time are definite variables influencing the rate of aspen expansion I would think. I’m also curious about how the age and density of the parent stand at the time of clonal expansion influences how the younger clone individuals proliferate/spread. Many potential hypotheses to pursue…this project may incorporate the Hypothetico-Deductive Method approach.
I think I’ll start with age of the parent stand influencing how the younger clones develop. I hypothesize that the older the parent stand, the less its offspring will spread, or the less biomass will be produced by offspring. Framed another way, I hypothesize that the younger and “more vigourous” the parent stand is, the denser (or taller or further-travelled) the offspring will be.
4.) Based on your hypothesis and prediction, list one potential response variable and one potential explanatory variable and whether they would be categorical or continuous. Use the experimental design tutorial to help you with this.
Response variable: Density of the younger aspen. Or height to age ratio of younger aspen. This is continuous.
Predictor variable: Age of the parent stand. Though ascertaining the age of multiple trees would provide a continuous dataset, in this case (because aspen are clones) the parent stand’s age may be categorical, e.g. “veteran, mature, older immature” etc.
If I were to predict that offspring density or site index (a continuous variable) would be affected by the parent’s age as categorical, the statistical design would be ANOVA.
If I were to predict that offspring density or site index (a continuous variable) would be affected by the parent’s age in years (or density, basal area, site index – continuous variables), the design would be a regression analysis.
