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For my study, I will be looking at western red cedars (Thuja plicata). For the environmental gradient, I focused on a gradient going from no trees of any species to abundant cedar trees.
The first location had no trees of any species. It was a rocky meadow, where there is very little soil on the surface of the bedrock. The bedrock is mostly covered in a layer of moss that is about 3.7 cm thick with occasional patches of completely exposed bedrock (see photos for data and species – this data will not be used for my study as the thickness of the moss is dependent on its species but I thought it would be interesting to include it in my description here). The most common moss present was Oligotrichum parallelum. This location had the most moisture of the three locations I observed on the gradient. Water pooled around my feet as I stepped on the moss and there were small puddles in the depressions of the rock.
The second location was on soil with a layer of moss about 6.6 cm thick. The most common species of moss was Kindbergia oregana and there is not a single patch of exposed soil. This location had lots of other low-growing plants, arbutus trees and Douglas fir trees, but no cedar trees. This location was less moist than the last, but still moist enough that my feet would’ve gotten wet if they were not already.
The third location had thinner, more patchy moss and there isn’t a dominant species. There are many sections of exposed soil and lots of debris from fallen branches. I observed 10 living cedar trees from where I was standing and many more that were dead. Moss did not grow at the base of the living cedars but it was much more common at the base of dead cedars. This location was the driest, as it didn’t have as much moss to hold water. My feet would’ve stayed dry if I had stayed in this location!
My hypothesis is “The distribution of Thuja plicata is affected by the biomass of moss present.” and I predict that “Both Thuja plicata and a large amount of moss will not be present in the same location.” The response variable for this study is the presence of cedar trees and the explanatory variable is the biomass of moss. The response variable is categorical and the explanatory variable is continuous.
Link for the photos (I apologize for the slightly blurry parts):
What other underlying processes could be influencing this pattern, you talk about water present and soil depth and soil type as well. Do you think the plant species distributions are more strongly influenced by these types of environmental variables or by competition with each other? Something to think about as you continue to develop your variables and design your field data collection.
The study you are investigating – interaction between plants of different species and evolutionary histories – is fascinating and one in which observations seem to show correlation. I too have noticed how moss does not seem to grow around the base of cedar trees. Your piece and pattern are clearly stated, as are your response and predictor variables. Your prediction is fairly clear, but I’m not sure how falsifiable it is – how much moss is a large enough amount to predict the exclusion of the cedar trees? Also, since cedar are such long-lived species, it seems like many lifetimes of moss may come and go during a cedar’s life-span, and that maybe the biomass or % cover of moss would be a better candidate for the response variable, and the presence of cedar would be more of an explanatory variable. It could be a pH or nutrient contribution produced by the cedar that limits moss ability to grow. Whatever the case, you are on to a good idea and I encourage you to figure out the best way to study this.