Many eastern North American oak forests and woodlands are declining, in part, due to altered fire regimes. Fire-tolerant, shade-intolerant oaks are being replaced by fire-intolerant, shade-tolerant, mesophytic species -- a process called “mesophication.” However, it is hypothesized that oak ecosystems on xeric sites may be stable even with altered fire regimes because many mesophytic tree species tolerate dry conditions poorly. On sites with limited mesophication, canopy gap regime will likely determine opportunities for oak regeneration. Further, canopy gap regime may reflect stand age because of age-related differences in tree sizes and canopy height heterogeneity. We quantified the canopy gap regime in four older and four younger Cross Timbers oak woodlands in Kansas USA, at the xeric edge of eastern deciduous forest. We addressed 1) what is the canopy gap regime of these woodlands?, 2) how does the canopy gap regime vary with stand age? and 3) is formation and filling of gaps producing a change in tree species composition? At each site, we sampled expanded canopy gaps using the line-intercept method. We recorded gap frequency, expanded gap area, shape, cause of the gap, species and diameter at breast height (dbh) of the gap-forming tree(s), and species of replacement canopy tree(s).
Results/Conclusions
Woodland age did not affect any gap characteristics that we quantified. Canopy gaps in Kansas Cross Timbers woodlands occur less frequently (mean = 4.12 gaps / km), are smaller in expanded gap area (mean = 142.5 m2) and are less often caused by tree uprooting (6% of gaps) than in oak forests further east. We hypothesize that stressful abiotic conditions make tree age weakly correlated with size producing similar gap regimes in young and old stands. Small canopy trees, in both height and dbh, may explain differences in gap regime as compared to more mesic forests. In regard to the stability of oak dominance, we found significant differences between the species compositions of the current canopy, trees that formed canopy gaps and replacement trees in gaps. Post oak (Quercus stellata) was under-represented as a gap replacement species as compared to its abundance in the canopy and its representation as a gap-former. Eastern red cedar (Juniperus virginiana) and more mesic oaks (principally Q. rubra) were over-represented as gap replacements as compared to their representation among gap-formers and in the canopy. Through canopy gap formation and filling, there appears to be a gradual shift toward less post oak dominance in these woodlands.