Table 1 Variables used in mixed logistic regression models of tree mortality, supporting literature, and notes on choice of metric
Variable | Negative correlation with oak mortality | Positive correlation with oak mortality | Notes |
|---|---|---|---|
Tree basal area (m2∙ha− 1) | Greenberg et al. 2011 (Erythrobalanus), Voelker et al. 2008 (in low mort areas), Fan et al. 2012 (Leucobalanus), Wood et al. 2018 (white oak), Yaussy et al. 2013 (including managed stands) | Fan et al. 2011 (black and scarlet oaks), Kabrick et al. 2004 (black and scarlet oaks, dominant/codominant crowns) | Basal area of individual tree chosen because it scales more linearly with sapwood area than diameter (Meinzer et al. 2005) |
Total basal area of plot (m2∙ha−1) | Competition of all species chosen rather than competition of Quercus as some other studies have, because of the diversity of our study area and the lack of a major oak decline event | ||
Stand age (years) | Greenberg et al. 2011 (tree age, Leucobalanus), Oak et al. 1991 (tree age, non-linear), Wang et al. 2008 | Age is stand age, not individual tree age, and was determined from Forest Service Vegetation Management Information System | |
Solum depth (centimeters) | Depth of solum chosen rather than depth of entire soil profile, because some soil pits did not reach the depth to bedrock, and because most fine roots in forest ecosystems are found in upper layers of the soil (Schenk and Jackson 2002) | ||
B horizon acidity (pH) | Demchik and Sharpe 2000 (northern red oak) | B horizon chosen because it had greater significance than A horizon in a study of sugar maple mortality (Bailey et al. 2004), displayed different nutrient concentrations with different levels of oak mortality in (Demchik and Sharpe 2000) and because it is more resistant to erosion over time (Kreznor et al. 1989). | |
Slope percent | Oak et al. 1996 | ||
Slope aspect (cosine transformed [TASP]) | Slope aspect was cosine transformed (cos(45° – aspect) + 1)), to make the metric change continuously in a circular fashion. TASP maximizes at 2 for a 45° aspect, and minimizes at 0 for a 225° aspect (Beers et al. 1966). | ||
Slope position (proportion distance to ridge [PDR]) | Bendixsen et al. 2015 | Proportion distance to ridge is maximized at the bottom of a slope; positive correlation is higher mortality on lower slopes. | |
Slope aspect:slope position interaction (TASP:PDR) | High values of the interaction factor indicate upper slopes and northeast facing aspects. | ||