Natvik and Henry_Differential effects of the grasses Festuca rubra, Andropogon gerardii, and Elymus virginicus on the growth of Eurasian versus North American oak seedlings
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OVERVIEW OF RESEARCH: Research was conducted to compare how the origin of grass cover effects the growth of oak seedlings. Our hypothesis was that that grass species origin may influence oak recruitment. The experiment included 3 grasses species consisting of 2 species native to North America (Andropogon gerardii and Elymus virginicus) and 1 species native to Eurasia (Festuca rubra var. rubra). Comparisons were made between 6 oak species consisting of 3 species native to North America (Quercus alba, Q. macrocarpa, and Q. rubra) and 3 species native to Eurasia (Quercus acutissima, Q. cerris, and Q. robur). We predicted that oak seedlings would demonstrate greater growth when growing in grasses that they share a co-evolutionary history with. EXPERIMENTAL DESIGN: The 3 grass species were seeded into single-species plots measuring 6 x 7 metres. There were 8 plots seeded for each of the grass species (for a total of 24 grass plots). All 6 oak species were seeded into the plots as acorns. Each of the 24 plots contained 1 oak seedling of each species, with a total of 144 oak seedlings in the entire experiment (6 seedlings x 24 plots). DESCRIPTION OF DATA: 1. OAK SEEDLING MEASURMENTS: After 2 growing seasons, all oak seedlings were harvested. Measurements of each oak seedling were taken including stem height, stem diameter, total biomass (separated into stem biomass and root biomass), and root-to-shoot ratios. This data was analysed to compare the growth of all 6 oak species between the 3 species of grasses. 2. LIGHT READINGS: Photosynthetically Active Radiation (PAR) was measured from all of the grass plots at 15 cm above the soil surface during spring summer, and fall. These measurements indicate how much exposure to light the oak seedlings received across the growing season within the 3 different species of grasses. 3. SOIL TESTS: A range of soil parameters were measured from all grass plots at the end of the experiment (3 years after the grass plots were established). The purpose of the soil tests was to see if soil parameters were impacted in any way by the 3 different species of grasses. SUMMARY OF RESULTS: Our results demonstrate that grass cover has a significant but species-specific impact on the growth of two-year-old native and non-native oak seedlings. The variation in seedling growth in response to variation in grass cover was, in part, consistent with the hypothesis that grass species origin may influence oak recruitment, but there were nevertheless exceptions that could be explained in part by shading effects.
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DATA ANALYSIS: Response variables were analyzed using split plot ANOVAs, testing the main effects of oak species, grass species and their interaction. Significant differences among grass treatments within each oak species were analyzed using Tukey’s HSD tests. OAK SEEDLING DATA: The main data set (see sheet 1 of data file) includes measurements taken from oak seedlings after years of growth in the grass plots. Oak seedlings were harvested in October 2017. Seedlings were dug out using shovels, ensuring that entire root systems were extracted from the soil. Stem height and stem diameter were measured immediately after seedlings were harvested. Stem height was measured from the soil line to the terminal bud using a metric ruler with an accuracy of 0.0cm. Stem diameter was measured at the base of the stem, just above the stem flare using a digital meter with an accuracy of 0.00 mm. Oak seedlings were then dried in a drying oven until completely dry. Each oak seedling was cut at the crown to separate the root from the stem. Roots and stems were weighed on a digital scale with an accuracy of 0.00 grams. Total biomass and root-to-shoot data was calculated based on dry biomass measurements taken from root and stems. LIGHT MEAUREMENTS: Photosynthetically active radiation (PAR) transmission through the grass canopy was taken during spring, summer, and fall (see sheet 2 of data file). Light measurements were taken 15cm above the soil surface using an ML3 ThetaProbe Soil Moisture Sensor, Delta-T Devices, Cambridge, U.K. We performed the light level measurements on days without any cloud cover between 11:45 and 12:15 UTC-5, and each measurement was standardized relative to a second measurement taken above the plant canopy. SOIL TESTS: We collected the soil cores using a 2 cm diameter by 15 cm deep corer. These samples were analyzed by A & L Canada Laboratories Inc. in London, Ontario, Canada. Relevant measures included organic matter, phosphorus, potassium, and pH (see sheet 3 of data file).