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Organism being studied: Alnus rubra (Red alder)
Gradient: The tree line of a forested ravine, the level top of of a slope before the shoreline, the shoreline itself which consists of boulders, small trees, mixed shrubs and perennials.
Alnus rubra is much more dominant along the shoreline and potentially non-existent along the tree line. This could be due to sunlight, a preference for more well-drained and less rich soil, or maybe Alnus rubra has a tolerance to the salt exposure (from the ocean) and has been able to outcompete other less tolerant species. It may also be that because its the only small tree species along the shore it’s much easier to spot than looking into a thick tree line.
The trees almost seem to have formed a natural spacing between individuals as well with none closer than roughly 20 meters. These specimens all seem quite mature considering the harsh environments in which they grow (taller than 20 feet with stems more than 8” in diameter). Even though these trees are well-spaced and have an abundance of flowering and fruiting bodies, I could not observe any seedlings or juvenile specimens in any part of the gradient.
Hypothesis: The mature Alnus rubra in this region, despite an abundance of flowers and fruit, can no longer reproduce in this location via seed.
Formal prediction: The number of Alnus rubra seedlings and/or juvenile specimens recorded will be very low or non-existent along the shoreline and at the tree line. Due to a change in some environmental factor (or factors) mature specimens have survived but their seed cannot germinate or the seedlings cannot survive.
Potential response variable: Continuous. The number of immature Alnus rubra along the gradient.
Potential predictor variable #1: Continuous. The number of mature, seed-bearing Alnus rubra along the gradient.
Other potential predictor variables:
Categorical- Soil type along gradient (sandy, loam, clay, etc)
Continuous- pH and/or nutrient composition along gradient.
Hello
You have clearly put a lot of time and thought into the details of your observations. It is interesting the pattern you have noticed with the distribution along your gradient! Your hypothesis though seems like a bit of a jump from the observations to say that they are not reproducing at all by seed because even if they are not present, that may not mean that it is because they are not reproducing. Not reproducing seems like a difficult idea to prove because of potential confounding variables such as both interspecific and intraspecific competition that may favour the mature trees over the seedlings. The way your hypothesis is worded it also seems that you may be proposing they are reproducing by other means other than seed? Is there observations you have made for this or is this just the way it is worded? Perhaps there is a way to rework your hypothesis into something that you can prove or disprove about the presence/absence of seedlings. I like the very precise details within your observations that you have noted though such as the natural spacing and distribution. Your variables are easily measurable and clear and both relevant to the hypothesis. I look forward to reading about what you find. It is an interesting topic. Best of luck to you!
Hi acunningham,
Thanks for the comment! It has helped me realize my hypothesis is not clear enough and I’ll need to re-visit it in the next step.
What I would like to prove is exactly what you said, “because of potential confounding variables such as both interspecific and intraspecific”. I believe the seed is viable but its the competition of other species (and potentially other alders) as well as the degraded quality of the shoreline that prevents them from growing.
In response to if they are reproducing by seed- yes the large alders have vegetative growth coming up from their roots in the ground within around 3 feet of the main stem. In my experience alder grows quite vigorously this way so I need to ensure I can confidently tell these vegetative stems apart from alders that have grown from seed falling and then germinating.
Please let me know if you have any other suggestions in the future!
-Brian