Cross-timbers oak woodland regeneration as a function of woodland age, canopy gap size, and deer herbivory
Oak woodlands and forests in much of eastern North America are undergoing a decline as a result of changes in disturbance regimes and herbivore populations. This conservation concern has stimulated research focusing on oak woodland regeneration. To address possible changes in tree species composition in the Cross Timbers region, a post oak (Quercus stellata) / blackjack oak (Q. marilandica) dominated, xeric vegetation type extending from Texas into Kansas, I compared canopy gap size, frequency, cause, and identities of replacement species in gaps within five young woodlands and three old woodlands in southeastern Kansas. There was no difference in gap characteristics due to woodland age, but a weak difference due to site. There was a significant difference between gap replacement species and current canopy composition. Blackjack oak was over-represented as a replacement species in canopy gaps, but this may be due to its prolific resprouting. Eastern red cedar (Juniperus virginiana) was also over-represented in gaps, but this was due to over-representation at one site. Overall, there was no evidence of a trend to large imminent change in woodland composition. Large deer populations have been hypothesized to retard forest and woodland regeneration through intensive browsing of saplings. To address possible negative effects of white-tail deer (Odocoileus virginianus) browsing on oak regeneration in a Cross Timbers woodland, I used fencing to manipulate deer access to post oak saplings that occurred in varying sizes of canopy gaps. Light availability had a positive effect on sapling growth. Though intensity of deer browse was low, and all saplings survived, browse-excluded saplings showed greater growth than exposed saplings. Browse intensity and effect on saplings was independent of light availability. While current deer densities in Kansas Cross Timbers woodlands are not severely affecting juvenile oaks in the short term, further increases in densities should be avoided.