This paper analyzes the vegetation found on a 20-by-100-meter east-facing forest slope plot located on a university campus in Kelowna, British Columbia's Okanagan Valley. The study investigates whether an environmental gradient — defined by differences in moisture, soil nutrients, soil depth, and sunlight exposure — controls vegetation growth and species diversity across the slope. Three hypotheses predict greater plant diversity and density at the basal end of the slope. Field methods include slope profiling with clinometers, tree tagging, diameter at breast height (DBH) measurements, and diversity indices such as Simpson's Index and the Shannon-Wiener Index. Results confirm that Ponderosa Pine and Douglas Fir are most abundant at lower elevations, while shrub patterns are less consistent, with one notable exception in Shepherdia canadensis.
The purpose of this analysis is to describe the vegetation in a forest plot and to note the gradation of vegetation across the slope of the observed area. Gradation is influenced by variables such as available light, rainfall and ambient moisture, the depth and quality of soil, and environmental disturbances such as soil runoff, pollution, or fire.
Observation was directed at a particular plot measuring 20 meters by 100 meters, consisting of an east-facing slope set aside for study in 2007. Conditions suitable for plant life exist across the plot, but not uniformly. Given the slope of the observed plot, rainwater and snowmelt will soak into the soil and will also travel across the surface given sufficient saturation. This means that nutrients in the soil will be carried along with the moving water, both across the surface and penetrating into deeper soil layers. From this dynamic, it can be seen that nutrients will accumulate at the lower bounds of the slope, as will moisture on an intermittent basis. Given the overall arid conditions of the land in and around Kelowna, the presence or absence of moisture is a primary variable in the growth and reproduction of vegetation on the observed plot.
Soil also accumulates at the bottom of the slope, having been propelled by moving water, pulled by gravity, or disturbed and loosened by birds or animals. The eastern exposure of the slope permits more shade at the base during the noontime and afternoon hours. Sunlight exposure is a factor in evaporation levels across the face of the slope, with higher levels of evaporation occurring at the upper reaches, where sunlight is more direct and exposure lasts longer than at the lower levels.
Given the descriptions of the environmental dynamics found on the observed plot, the following research questions are posed:
The environmental dynamics on the slope suggest that more favorable growing conditions are established along the base of the slope. Considering the key variables discussed above, the following hypotheses appear relevant and reasonable:
The geology of the Okanagan Valley is characterized by basaltic lava, carbonaceous sedimentary rock, foliated gneiss, and granitic rock (Meidinger & Pojar, 1991). Erosion in the area has been sufficiently substantial to result in the deposition of a valley floor consisting of a mixture of clay, gravel, sand, and silt (Meidinger & Pojar, 1991). The soils in the area include brunisol and chernozem, with the area surrounding Kelowna predominantly chernozem soil (Pidwirny, 2006). Geologists mark the formation of the Okanagan Valley to the Pleistocene Age, noting that it was a river valley further eroded by the Cordilleran glacier during the ice age (Pidwirny, 2006).
Visual observation of the forest in the location of the observed campus plot suggests a preponderance of Ponderosa Pine (Pinus ponderosa), lesser quantities of Douglas Fir (Pseudotsuga menziesii), and several varieties of shrubs. Moreover, the location of the plot on the campus has resulted in a human-induced edge running along the Bunch Grass zone that occurs in the hottest and driest interior valleys, which transitions into stands of Ponderosa Pine.
The climate of the Okanagan Valley is substantially impacted by its proximity to the Pacific Ocean and the nearby Cascade and Coastal mountain ranges. As wind moves inland from the sea, orographic uplift causes the wind to deposit moisture primarily on the windward side of the mountains, creating a moderate rain shadow and drier winds on the Okanagan Valley side. The fact that dry winds prevail in the Okanagan Valley during the summer months means there is always a danger of forest and brush fires. Some biological plant adaptation can be seen in plants that have developed resistance to forest fires. The diversity of vegetation in the Okanagan Valley is fundamentally shaped by these climate-controlling conditions, combined with a temperature range averaging from roughly 27°C in summer to 5°C in winter.
Previous cycles of studies of the vegetation on the observed plot have identified and mapped the vegetation to within 0.1 meter. In addition, measures of the diameter at breast height (DBH) and overall height of the trees in the plot have been recorded. All trees have been tagged and cored for age at DBH, and a few trees were cored at the base to ascertain their true ages.
To construct a slope profile of the study site, measurements of slope distance and slope angle were taken between points along the slope. These points were selected where there was an observed change in slope along the transect. Slope distance was estimated within each interval using existing survey pins placed 2 meters apart, and the slope angle was measured within each interval using a clinometer. Using the slope distance and slope angle, trigonometry was then applied to calculate both the vertical and horizontal distance within each interval.
A base datum and top datum elevation were used to ensure that all 11 transects measured the same total distance. The elevation was calculated at each point of slope change along the transect by cumulatively adding the vertical distance from the base point. The total change in elevation from the base datum to the top datum was measured using an altimeter and compared to the value obtained by summing the vertical distances.
To quantify vegetation diversity across subsections of the plot, field data were compared against Simpson's Diversity Index (Magurran, 1988) and the Shannon-Wiener Index. These indices allowed for a standardized comparison of species richness and evenness both across the slope as a whole and within the basal subsection specifically.
"Abundance, density, and biomass findings across slope subsections"
"Gradient effects on trees and shrubs, root system differences"
"Slope gradient confirmed as driver of vegetation density and diversity"
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