A growing threat to perennial herbs in the eastern deciduous forest is deer herbivory. Loss of leaf area may reduce seed production during the season in which the herbivory occurs, and it may also limit the storage of carbohydrate resources required to produce the aboveground plant tissues the following year. Next year’s plant consequently may exhibit smaller leaf area, fewer flowers or lower capacity for seed maturation. One factor which could influence the type of response is the timing of the herbivory relative to flowering, fruiting and storage phenology. We investigated the effect of herbivory timing in American ginseng (Panax quinquefolius).
To simulate deer herbivory we removed one-half of the area of each leaf. Three plants were assigned to each of 18 blocks and received one of three treatments: leaf removal at start of flower opening (‘Early removal’), leaf removal at start of fruit maturation (‘Late removal’), and no removal (‘Control’). We recorded initial leaf area and numbers of flower buds and final fruits and seeds in both the year of treatment and the following year. Our preliminary statistical analyses consisted of one-way analyses of variance.
Results/Conclusions
No significant differences were found among treatments in flower bud number or leaf area prior to imposition of the treatments. Although the absolute numbers of seeds and fruits did not vary in the year of treatment, the number of fruits produced per initial flower bud was lower in the early removal treatment (p = 0.04). The following year plants in the early removal treatment exhibited a marginally significant (p = 0.052) reduction in leaf area but there were no differences in # flower buds. In contrast, by the end of the season the number of fruits produced was significantly reduced, with early removal having the greater effect (p = 0.02). The same pattern was found for seed production (p=0.03). These results suggest that in P. quinquefolius 1) early herbivory by deer will have a greater effect than later herbivory, and 2) the immediate effects are primarily a reduction in carbohydrate storage. The implication is that allocation to reproduction is favored over storage when carbohydrate resources become more limiting after deer herbivory, giving rise to a year lag time in negative effects on plant fitness.