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.