Experimental test of the role of mammalian herbivores on old field succession: Community structure and seedling survival

When elucidating plant community dynamics, investigators have focused on plant-plant and plant-environment interactions and on the characteristics of individual plants. The role of animals has been relatively neglected. Herbivory may affect vegetation dynamics by altering community composition and structure and by influencing the performance of species. Therefore, herbivory can be an important factor in succession. We experimentally tested the role of mammalian herbivores in two fields at the Hutcheson Memorial Forest Center in central New Jersey. Both fields were released from active cultivation, one at the beginning of the experiment and the other 17 years earlier. Large exclosures (5 X 5 m) were used to exclude mammalian herbivores, and open units of the same size were used as controls. The effect of herbivore exclusion on the structure and composition of the plant community was analyzed. In addition, tree seedlings of three species Acer rubrum, Cornus florida, and Juniperus virginiana-were planted in the experimental units in both fields. These species typically invade early- to mid-successional old fields. The survival of these individuals was calculated and, using height as an indicator, their performance assessed. Herbivore exclusion primarily influenced the structure of the plant community rather than its composition. Percent cover of species and species richness of all plants were not affected by herbivory, but in one field the relative cover of exotics was greater in the exclosures. The height profile of the plant community in the exclosures was significantly taller than the community in the open plots. The survival and growth of planted A. rubrum and C. florida seedlings was significantly greater when protected from herbivores. In contrast, the survival and growth of J. virginiana seedlings was not affected by herbivores. Growth and survival of J. virginiana did differ by the field they were planted in, suggesting that physiological constraints may be more important than herbivory for this species. These results reenforce that herbivorous mammals play a key role in old field succession, particularly in the pivotal shift of dominance from herbaceous to woody cover.