Effects of aspect and slope position on Quercus alba growth rates

Background/Question/Methods Differences in topography encompass light, moisture, and soil gradients important to tree growth and ultimately stand composition and structure. We examined the effects of aspect and slope position on decadal growth rates over time for Quercus alba in the western edge of the eastern deciduous forest. We established six transects for a total of 23 sites. Three to five trees were cored at site center along three slope positions (upper back slope, midslope, and lower slope) on north-northeast (NNE) (22.5o-112.5o) and south-southwest (SSW) (157.5o-292.5o) facing slopes in the Thomas S. Baskett Wildlife Research and Education Area near Ashland, Missouri. Individual trees cored ranged in age from 88-198 years. Decadal growth rates were organized by calendar year and analyzed using repeated measures ANOVA.

Results/Conclusions

Q. alba growing on slopes with NNE aspect had greater decadal growth rate (mm decade-1) than those growing on SSW aspects (P=.0026). There was no significant time x aspect interaction. Slope position alone did not affect decadal growth rates (P=0.27) but SSW midslopes had lower decadal growth rates than all other slope and aspect combinations (P=0.022). The mean decadal growth rates ranged from 5- 31 mm decade-1. The time period with the slowest growth appears to be 1810-1900 while 1900-1960 was the period with the fastest growth. Patterns in growth over the past two centuries suggested that, as expected, NNE sites entered the self-thinning stage and subsequent growth phase sooner the SSW aspects. Greater soil moisture, less solar radiation, and decreased wind are characteristics associated with NNE aspects in contrast to SSW aspects. Although slope position typically represents an important influence on tree growth, in this transition zone of the eastern deciduous forest, aspect is the only variable that affects Q. alba growth.