The influence of grass competition and herbivory on native hardwood seedling establishment in the Inner Bluegrass of Kentucky

Background/Question/Methods

Plant communities emerge from two main forces, resource competition (for water, light, and/or nutrients) and disturbance (e.g., herbivory and/or fire). Savanna systems are particularly interesting as they represent assemblages of contrasting life forms (grasses and trees) and a delicate balance between competition and disturbance, of which the Kentucky Bluegrass savanna is unique. Anecdotal evidence suggests that during European settlement central Kentucky was primarily a savanna ecosystem, with bison and elk herds grazing in a grassland under large, open grown oak and blue ash trees.  However, with urbanization and agriculture, few remaining stands of Bluegrass savanna are left. 

 To understand how competition and disturbance influence survival and growth of native hardwood tree seedlings, a long-term project has been established at Griffith Woods in Harrison Co. KY (38^o19’48”N, 84^o21’01”W), one of the best and largest remnant stands of Bluegrass savanna.  In March 2011, fourteen species of native hardwood tree seedlings were planted with two experimental treatments applied in a factorial block-plot design.  Treatments include vegetation removal, herbivory removal, vegetation and herbivory removal, and a control.  Each block was replicated twelve times, and each treatment plot has fourteen rows of trees (one for each species) with ten trees per row, a total of 6148 trees (some species could not be fully replicated).   During the planting process, seedling protectors were installed to prevent herbivory. During summer 2011, mowing and herbicide regimes prevented vegetation competition. In summer 2011, trees were assessed for height, diameter, number of leaves, leaf length, and survival of planting. In September 2011, blocks were assessed for above and below vegetation light levels and biomass of surrounding vegetation. 

Results/Conclusions

All species showed low mortality (~10%) three months after planting, except for Ohio buckeye (Aesculus glabra), which showed significantly higher mortality.  Light levels were shown to be significantly higher above vegetation than under vegetation, which demonstrates the difference in competitive environments between vegetated and vegetation removal plots. Dried biomass varied between blocks but these were not found to be significantly different, indicating similar competitive environments between blocks in vegetated plots.  Some differences were noticed in growth responses between treatments, with protected seedlings having taller heights on average.  This is potentially from herbivory immediately after the pre-leaf out planting process.  Though somewhat preliminary in nature, initial results indicate that both vegetation competition and herbivory are influential factors in the survival and growth of native hardwood tree seedlings in the Kentucky Bluegrass savanna.