The impact of Microstegium vimineum, seed predation, and herbivory on the regeneration of a mesophytic and a xerophytic oak in a restored open-oak woodland

Background/Question/Methods

One of the most pressing issues facing oak-dominated ecosystems in eastern North America is the lack of natural regeneration of oaks. The “fire-oak hypothesis” states that modern oak regeneration failure is caused by canopy closure and increased competition from less fire-tolerant mesophytic trees as a result of prolonged fire exclusion. Although restoration of suitable fire regimes appears to favor oaks over mesophytes in many cases, effects on different species of oak have not received nearly as much attention. We studied differences in acorn supply and effects of invasive plant species and seed predation/seedling herbivory on seedling emergence, growth, and survival of the historically dominant post oak, Quercus stellata, and currently dominant white oak, Quercus alba, in a restored oak woodland in north Mississippi (USA) during the 2021 growing season. We used a split-plot design in which each of 20 patches dominated by Microstegium vimineum and 20 patches dominated by native vegetation (whole plots) were divided into four 0.25 m2 subplots to which we randomly assigned a 2 x 2 factorial arrangement of neighbor removal and exclusion cage split-plot treatments. Each subplot contained a Q. stellata and Q. alba acorn (for a total of 320 acorns in the 160 subplots).

Results/Conclusions

Despite overstory density of Q. stellata being greater than that of Q. alba in 2020, the supply of viable acorns was much greater for Q. alba at this site. In early 2021, the cage treatment significantly reduced acorn removal of both species. By October 2021, survival/presence of seedlings of both species combined was substantially lower in patches dominated by M. vimineum than in those dominated by natives. The positive effect of neighbor removal was modestly but significantly greater in M. vimineum-dominated patches than in native-dominated patches. When each species was analyzed separately, there was a trend of greater Q. alba seedling survival/presence in caged subplots than in uncaged subplots and the difference was greater in M. vimineum-dominated patches than in native-dominated patches. Overall, seedling survival/presence was slightly greater for Q. alba than for Q. stellata, but this difference was not statistically significant. These results suggest that a major contributor to interspecific differences in regeneration potential is differences in seed supply. Although legacy effects and possibly apparent competition from M. vimineum appeared to reduce seedling survival, these effects did not account for differences in regeneration potential between Q. alba and Q. stellata.