The effect of canopy openness on shrub and fern diversity on Burnaby Mountain, BC

Hanna Jackson

Department of Biological Sciences, Simon Fraser University

*This report is a class project NOT peer-reviewed science


Introduction

Competition is a major factor affecting the distribution of organisms. A finite supply of any resource means that only a finite number of individuals can survive that share that resource requirement, and that those that most effectively compete for it will be successful. This means there is a selective advantage for lineages that can diversity to occupy novel niches, ultimately increasing diversity. In the case of plants, a major limiting resource is sunlight. The highly evolutionarily conserved mechanism by which sunlight is translated into usable energy is not a plastic phenotype, and as a result all plants rely on sunlight for energy. This means that in areas with low sunlight there are more constraints on the strategies (niches) available for plants to live and reproduce due to lower energy availability. With less niches available, we would expect that diversity would be decreased. Because of this, we hypothesize that in areas with more canopy cover, shrub and fern diversity will be decreased.


Methods

We collected out data on Burnaby Mountain in the British Columbia, Canada. It is in the Coastal Western Hemlock Biogeoclimactic Zone and consists largely of deciduous forest the early successional stage. Since logging in the 1940s it has been relatively undisturbed with the exception of the addition of a university at the mountain’s top in the 1960s.


We sampled at two sites on the mountain, one on the north and one on the south face, to capture as much ecological variation of the mountain as was feasible. We chose these sites along hiking trails that were easily accessible. We then set up a row of 12 20x20-m quadrant plots along the side of the trail, starting at our initial chosen sampling site, and another row of 12 20x20-m plots were sampled directly behind those. Within each of the 20x20-m plots, we took our measurements at one randomly decided point per plot. We did not sample less than 5-m away from the trail to account for any edge effects and we did not go further than 45-m away from the trail for logistic and safety reasons.


At the point in each plot, we took our measurements. We determined canopy cover via a computer program that estimates openness from a photo of the canopy. We took the photos from our points at 1.3-m off the ground, the program then converts it into a purely black and white image and determines percent of white pixels. We chose to measure number of shrub and fern species as our response variable because we hypothesized that tree cover would affect the number of species below, so we did not include trees of any size, the source of this shade, into our measurements. To determine the number of shrub and fern species per plot we employed the Line Intercept Method with a 5-m long transect running along the trail in a predetermined direction away from our point and counted any shrub or fern that crossed the transect line.


Results were analyzed in Rstudio using a regression analysis.


Results

We sampled at two sites on the mountain, one on the north on one on the south side. The north site had an average tree density of 153.5 trees per hectare and the south site had an average density of 196.1 trees per hectare. The mean canopy openness on the north side was 23.75% and for the south side it was 25%, with an overall mean over both sites of 24.38%.


We sampled at randomly chosen points within quadrat plots for canopy openness and number of shrub and fern species that crossed each 5-m transect (see methods). We found that as plot canopy openness increased, the number of shrub and fern species directly under significantly increased (R2=0.2318, p=0.00053, F=13.88) (see Figure 1).


Figure 1. We sampled at two sites on opposite sides of Burnaby Mountain, BC in the Pacific Northwest. We set up 20x20-m quadrats along trails and within each plot we randomly chose points from which to take measurements of canopy openness and number of shrub and fern species that crossed a 5-m long transect line. This figure shows that as canopy openness increases, the number of species of shrubs and ferns that intercepted our transect increased significantly (R2=0.2318, p=0.00053, F=13.88). Each data point is a measurement from one plot, and the regression line is shown.

Figure 1. We sampled at two sites on opposite sides of Burnaby Mountain, BC in the Pacific Northwest. We set up 20x20-m quadrats along trails and within each plot we randomly chose points from which to take measurements of canopy openness and number of shrub and fern species that crossed a 5-m long transect line. This figure shows that as canopy openness increases, the number of species of shrubs and ferns that intercepted our transect increased significantly (R2=0.2318, p=0.00053, F=13.88). Each data point is a measurement from one plot, and the regression line is shown.