Environmental Variables
A tree’s ability to access water and nutrients at CCESR would be expected to influence its vigor and potentially its vulnerability to fires. Due to the Anoka Sandplain’s slight relief, CCESR has an elevation gradient of only ten meters. Because the water table at CCESR is quite flat and located at the lower end of the site’s elevation gradient (Basiletti 2018), small differences in elevation in this sandy soil environment likely affect a tree’s access to water. Similarly, tree hardiness is likely to be affected any spatial variation in soil nutrient levels.
Nitrogen . Nine soil samples were collected from each grid cell in 1989, mixed, dried, and analyzed for total N using alkaline persulfate digestion followed by NO3 measurement on a Technicon II Autoanalyzer.
Elevation . In 1989, Mark Hurd Corp. (no longer extant) collaborated with a local surveying company and produced a set of highly accurate and high-resolution topographic data for the site, consisting of a grid of elevation data (m above sea level) at points 3 m apart (meaning 9 points within each 10 x 10 m cell). The elevation designated for each cell consisted of the mean of all elevation points within the cell. Elevation ranged from a low of 278.1 m and a high of 285.6 m. Elevation was standardized by subtracting 278 m, the minimum, and dividing by 7.5, the range. This meant that the unit of elevation, as for neighbor density, represented the entire range across the plot.
For each tree, a value for soil nitrogen and elevation was assigned from the closest grid cell center. The soil N and elevation variables turned out to be highly correlated (r = -0.552, p<0.0001), and preliminary models suggested elevation was a better predictor of growth than soil N. The growth and survival models thus included elevation, but not N, as a predictor.