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I have decided to look at the effect of site moisture on the abundance of Hedera helix.
I am interested in studying the effect of invasive species on the abundance of native species but had a hard time finding an observable gradient between the two categories of plants. Upon observation of the ivy in the area, I began to notice a potential link between site moisture and proliferation of ivy. Since the presence of ivy can almost always be attributed to a reduction in native ground cover species, I decided to narrow down my observations to simply abundance of ivy. While I could have compared species all of the ground cover species in a given quadrat, including other invasives like Daphne laureola and Ilex aquafolium, H. helix is having a markedly more destructive effect on native species abundance.
I looked at three different moisture gradients, using tree species as a proxy for soil moisture in lieu of specialized equipment. I’ve classified the three different points on the gradient as zones:
Douglas-fir/ grand fir zone.
-characterized by heavy shade and mesic soil. The highest elevation of the three zones.
Arbutus/ Garry oak/ douglas-fir zone.
-Mesic-dry/ approaching xeric. Along the edge of the river, roughly 2m above the water level. I imagine the soil near the surface is quite dry, and the tree species composition is indicative of such.
Red alder zone.
-hydric/ probably seasonally mesic. Ground is visibly saturated and has been for many months. The only tree species that are able to grow here are red alder, with a few doug-fir on the margins where the soil moisture is starting to drop off.
I hypothesize that soil moisture levels affect the ability of H. helix to proliferate and out-compete native ground cover. I predict that abundance of H. helix will decrease with decreasing site moisture levels, and native species abundance will be higher on drier sites.
A response variable would be % ground cover ivy. This is a continuous variable.
An explanatory variable would be site moisture (determined by tree species composition). Since I have designated three “categories” by tree species composition, this variable is discrete.
Overall sounds like a good question and start to your experimental design. Is each area big enough to have independent samples? For your final paper it would be good to have a map of each zone which can be done in google earth.
I believe so! The Garry oak/ arbutus area is definitely the smallest of the bunch. Not sure how this would effect my sample size, but the next time I make it out to the park I will take some GPS coordinates and try to draw out each area on QGIS