Competitive interactions between the vines Lonicera japonica or Parthenocissus quinquefolia and the tree host Liquidambar styraciflua: Effects and mechanisms

Detrimental effects of vines on the growth of trees in successional environments have been frequently reported. However, the competitive mechanisms involved in these interactions are poorly understood. The objectives of this study were to quantify the growth responses of trees to below and/or aboveground competition with vines, measure the effects of vines on availability of resources to trees, describe some of the physiological and morphological bases of the growth responses of trees, and compare the competitive abilities of different vines in relation to a common host. The vines selected for this study were the exotic, evergreen Lonicera japonica (Japanese honeysuckle), and the native, deciduous Parthenocissus quinquefolia (Virginia creeper). The pioneer tree Liquidambar styraciflua (sweetgum) was chosen as the target host. Soil trenching and vine-training away from the tree were used to reduce below- and aboveground competition, respectively. A total of 10 treatment combinations, resulting from a 2 x 3 x 2 incomplete factorial design, were represented in field plots at the Smithsonian Environmental Research Center, Edgewater, MD. The 3 factors and their levels were: vine species (L. japonica or P. quinquefolia), presence of vine on host (on, off, absent), and soil trenching (trenched or not). The experimental units were planted in a randomized complete block design, with 6 replicates of each treatment. Growth, physiological, and morphological characteristics of trees were compared among treatments. Effects of vines on the availability of light, water, and nutrients were measured. Competitive abilities of the two vine species were compared in terms of key physiological and morphological characteristics. The experiment lasted two growing seasons. L. japonica had a greater effect on the growth of L. styraciflua than P. quinquefolia. This effect was largely due to root competition for soil nitrogen. Canopy competition only resulted in growth suppression when combined with root competition. Changes in biomass allocation (e.g., root:leaf biomass ratio) of trees were more pronounced than leaf-level physiological responses (e.g., photosynthesis). The competitive superiority of L. japonica was attributed to a more prolific water use pattern (greater stomatal conductances), greater photosynthetic capacity and nitrogen use efficiency, and greater production of fine root biomass.