Increasing Liana Abundance and Basal Area in a Tropical Forest: The Contribution of Long-distance Clonal Colonization

Recent evidence suggests that liana abundance and biomass are increasing in Neotropical forests, representing a major structural change to tropical ecosystems. Explanations for these increases, however, remain largely untested. Over an 8-yr period (1999—2007), we censused lianas in nine, 24 × 36 m permanent plots in old-growth and selectively logged forest at La Selva Biological Station, Costa Rica to test whether: (1) liana abundance and basal area are increasing in this forest; (2) the increase is being driven by increased recruitment, decreased mortality, or both; and (3) long-distance clonal colonization explains the increase in liana abundance and basal area. We defined long-distance clonal colonization as lianas that entered and rooted in the plots as vegetative propagules of stems that originated from outside or above the plot, and were present in 2007, but not in 1999 or 2002. Our hypotheses were supported in the old-growth forest: mean liana abundance and BA (≥ 1 cm diameter) increased 15 and 20 percent, respectively, and clonal colonization from outside of the plots contributed 19 and 60 percent (respectively) to these increases. Lianas colonized clonally by falling vertically from the forest canopy above or growing horizontally along the forest floor and re-rooting—common forms of colonization for many liana species. In the selectively logged forest, liana abundance and BA did not change, and thus the pattern of increasing lianas may be restricted to old-growth forests. In summary, our data support the hypothesis that lianas are increasing in old-growth forests, and that long-distance clonal colonization is a major contributor.