Patterns of liana abundance in oak-hickory forest in southwestern Illinois
Research in tropical forests has found that lianas (woody vines) are increasing in abundance. This phenomenon has been attributed to the rising atmospheric CO2 concentration, which enhances liana growth and increases their competitive advantage over trees. Some studies in temperate forests have also demonstrated increases in liana abundance but it has been suggested that this is a response to habitat fragmentation, with higher light levels at forest edges favoring liana growth. This study quantifies liana abundance in oak-hickory forest in southwestern Illinois and tests the hypotheses that liana abundance increases with proximity to the forest edge and decreases with forest age. In forest of three ages (40 yr, 70 yr, > 100 yr) three pairs of 0.1-ha edge plots (adjacent to the forest edge) and corresponding interior plots (≥ 45 m from the edge) were established. Each liana rooted within a plot was identified, tagged, and its diameter measured 1.4 m from the rooting point. Dominance and density of each liana species were calculated and generalized linear models with log link and quasi-Poisson errors were fitted to test for the effects of distance from edge and forest age on both total liana abundance and the abundances of individual species.
Eight species of lianas were recorded, including three exotics. For total liana density, the results supported both hypotheses. Total dominance decreased with forest age. Four species had sufficient occurrences to fit a model. Density and dominance of Lonicera japonica, the most abundant exotic liana, decreased with forest age. Parthenocissus quinquefolia declined in both density and dominance with forest age but its dominance increased with distance from edge. Toxicodendron radicans declined in density with distance from edge and its dominance decreased with forest age. The most abundant liana, Vitis spp., showed no systematic trends in abundance. Temperate forest is increasing in area worldwide, as previously cleared areas are reforested. Our results predict an increase in liana abundance during the initial stages of reforestation, followed by a decline as the forest matures. If reforestation proceeds as infilling among fragments, the decrease in the area of edge habitat should contribute to a decline in liana abundance. On the other hand, in regions where fragmentation is increasing, an increase in liana abundance is expected. More research in temperate forests is needed to determine whether the abundance of lianas is generally increasing and disentangle the roles of edge creation, forest age, and CO2 fertilization.