Over the past few weeks, I have returned to Nose Hill Park a few times, trying to narrow in on a biological attribute to study. I have finally pinned it down on my most recent trip on February 10, 2022. While my first blog post focused more on animal observation, I have become more interested in how the vegetation changes over different slope aspects. In particular, (from my notes):
- Clear distinction between vegetative structure and composition between north-facing and south-facing slopes.
- North-facing aspects consisted of primarily woody cover – mostly trees and shrubs
- South-facing aspects consisted primarily of grasses and low shrubs.
- Shrubs, although present on both slope aspects, the species composition appears to differ.
- Trees were not notably observed on south-facing slopes, accept in some cases at the very base of the slope were the slope changes from north-facing to south-facing (in the valley between the two slopes)
- On steeper north-facing slopes, trees were more abundant than shrubs.
- Tree species mainly included aspen and poplars (deciduous), only 1 or 2 coniferous trees observed.
- There were more shrubs on the south-facing aspects than originally thought.
- Shrubs at the base of the slope differed from the species at mid and upper south facing slopes.
For the purpose of this study, I would like to focus on vegetative structural layers (1. moss/lichen, 2. grass/grass-like//forbes, 3. Shrubs, 4. trees) and how they vary across the slope-aspect gradient.
Three locations across my gradient that I have looked into thus far include mid-slope north-facing, mid-slope south-facing, and the valley bottom between the two slopes. I have noticed that tree and shrub species are mainly confined to the north-facing slopes and the valley bottom, while grass and shrubs are mostly prevalent on the south-facing slopes. An underlying process that could be influencing my observation is that of sun exposure; typically south-facing slopes are drier and obtain more sunlight, while north-facing slopes are generally more moist and receive less direct sunlight. The dry vs. moist aspect was even observed today where the snow on the south-facing slopes was virtually non-existent but patches still remained on north-facing slopes and down in the valley bottom.
Because the valley bottoms in Nose Hill Park mostly seem to contain pathways, I don’t feel that studying the very bottom between the 2 slopes will provide accurate results (some trails are wide and graveled, others are compacted foot and bike trails devoid of vegetation. Therefore, I plan to adjust my gradient slightly going forward so that I can observe the structural layers in less disturbed/non-disturbed areas.
Hypothesis: Slope aspect and position (lower, mid and upper) influences vegetative structural layers.
Prediction: North-facing slopes will have one more structural layer than the south-facing slope
Response Variable: Vegetative Structural Layers; Continuous because it will be measured on a continuous numerical scale (1, 2, 3, or 4 structural layers).
Explanatory Variable: Slope position and aspect; Categorical because it will be either north-facing or south-facing and lower, mid or upper slope.
I was really interested to hear that shrub species may vary between north and south aspect slopes and that south slopes had more shrubs than you initially thought. I think you have a good start to your study design, though you could consider altering your prediction slightly. Do you want to limit your self to the number of structural layers when you already have observed that north slopes have trees and south do not, or look at something that might be a bit more interesting (shrub species diversity) or even both! Also, your hypothesis mentions slope position but your prediction is only about aspect. You don’t have to do both slope position and aspect though, I also think you could get overly complicated with measurements if you are not careful as there are lots of things you could measure.
Thanks for the insight. I had contemplated looking strictly at shrubs, so I’m open to looking into the shrub species diversity between slope aspects!
Would it still be worth collecting data on the vegetative structural layers? It is possible that areas on the North-facing slope may have fewer to no shrubs where tree density is higher resulting in the North-facing slopes having one less structural layer than I anticipate. But maybe I’ll drop the structural layers and instead look at shrub density as well? So I would measure the number of different shrub species and their %cover per plot along a transect.
I see what you mean about how this could get overly complicated quickly!
I think I would need to collect data via transects that run parallel to the slope so that I can truly capture the diversity of shrub species (As I mentioned, I think I observed different species at the base of the slope vs the mid slope, so if I were to say run a transect perpendicular to the slope at say mid-slope, then I would miss the diversity at the base and top of the slope).
If I alter my study then my new hypothesis, predictions and variables would be as follows:
Hypothesis: Slope aspect influences shrub density (% cover) and species composition (shrub diversity).
Predictions:
1. On average, shrub density will be greater on the south-facing slope.
2. Shrub diversity will be greater on the south-facing slopes than the north-facing slopes.
Response Variable:
1. Shrub density; Continuous because it will be measured on a continuous numerical scale (% cover).
2. Shrub diversity; Continuous as I will be measure the number of different shrub species.
Explanatory Variable: Slope aspect; Categorical because it will be either north-facing or south-facing slope.
Nice job pulling together some ideas for your project! I admire you for taking on something I had considered starting myself and then realized it would be challenging. Your predictions are clear and I believe you will be able to test them both to determine if correct or not, and I agree with you your response and explanatory variables. Have you given any consideration to if you will be able to identify the shrub species? I found in my first attempt that a lot of the ground cover had decaying foliage from last fall, and it was too early for new buds/leaves to be emerging. Without nice leaves or flower to look at, it was a struggle. This is something you may want to consider, but at the same time, if you are simply able to count “different” plants without definitive identification you would still be able to estimate some sort of diversity. Also, for density, have you considered standard to estimate ground cover? The one below is for BC (pp 22-23) but I am sure there would be something comparable for Alberta. It may help you better think about what you’d like to use to estimate for % coverage and what range(s) might be helpful.
https://www.for.gov.bc.ca/hfd/pubs/docs/Lmh/Lmh28.pdf
Lastly, when thinking about slope aspect, will you be touching on what influences the differing conditions between the north and south facing? I am thinking differences in light, wind, moisture etc. Each of the abiotic conditions is not something you’d need to take a deep dive into, but it may be useful to think about them and how they could influence you predictions. Maybe you want to considering taking the temperature on each aspect when you are sampling as well?
Best of luck on your project!
Hi Erika,
Thanks for you comments. Yes, I have considered the challenges of vegetation ID in the winter, however, I am comfortable with shrub winter ID, I was trained in my diploma and I do it for a living. If there happens to be a shrub I cannot ID but I can tell it is different then I will just have to call it “Unknown A, Unknown B, etc). Grasses and forbs on the other hand are very difficult in the winter, which is part of the reason I won’t be studying them.
% cover I’m still working on, and I have (hopefully) devised a couple plans to attack this. One would be to use the % cover classes from one of our tutorials to estimate the shrub crown cover, the other would be stem/plant counts (# of individuals/unit area) to get an estimate of the shrub density. I have created my data collection sheets for both approaches and will evaluate which is best in the field (I haven’t collected my assignment 1 data yet, so that will be the test).
From my schooling and research, I know there are a lot of variables that will change from the N to S facing slopes, and like you said, it’s something I will most likely discuss as I know they influence the vegetation but it’s not something I’m going to measure in the field. No, I wasn’t planning on taking the temperature of each aspect, as this is something I would need a tool for to get an accurate reading and it’s not something I have access too. I also don’t feel it’s necessary of the purpose of my project. The temperature per slope would change daily and there is lots of research out there that will tell you the south-facing slopes are typically warmer and drier than the north-facing slopes due to hours of sunlight, etc.