Growth
Growth of both species was greater at lower elevations, likely because lower elevation sites were closer to the water table and were higher in soil nitrogen.
The overall negative association between growth and conspecific neighbors for both oak species of all size classes, except for largeQ. macrocarpa trees, is consistent with expected outcomes of intra-specific competition (Adler et al. 2018, Yang et al. 2019). That growth in large Q. macrocarpa trees was not affected by neighboring conspecifics is not surprising. They are unlikely to be overtopped by neighboring conspecifics, and the fact that large Q. macrocarpa trees produce some of the deepest roots of all oaks (Deitschmann 1965, Johnson 1990) means they may be able to escape much of the underground competition with smaller Q. macrocarpa trees. Although growth in large Q. macrocarpa trees was not affected by conspecifics, it was negatively affected by heterospecific neighbors. This is likely due to interspecific competition with the much faster growing Q. ellipsoidalis , which grows (increase in dbh) three times faster than Q. macrocarpa (Davis 2021). Also, at this siteQ. ellipsoidalis typically grows taller than Q. macrocarpa , meaning that even large Q. macrocarpa trees can end up growing in the shade of Q. ellipsoidalis .
Unlike Q. macrocarpa , Q. ellipsoidalis growth was not affected by neighboring heterospecifics. As emphasized above, by not serving as a good fuel, Q. macrocarpa is likely to impede the spread of fire. Since fire is known to reduce growth of surviving trees (Refsland et al. 2020), Q. macrocarpa would be expected to provide its neighbors some protection from growth reductions due to fire, thereby perhaps canceling out negative effects due to competition.