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.