Factors limiting climber distribution and abundance in a southern African forest
Journal of Ecology
The distribution and abundance of climbing plants was surveyed over an altitudinal gradient in the Knysna Forest. S. Cape, South Africa. The abundance of climbers was surveyed in gaps standardized for light conditions. The results are compared with the abundance of climbers in the disjunct Alexandria Forest. Climber abundance was positively correlated with the altitudinal gradient as well as with the level of base nutrients in the soil, but not with the nitrogen concentration. However, these factors together with rainfall may co-vary and thus confound any explanation of the distribution of climbers. The hypothesis was tested that climber abundance is limited by trellis availability, i.e. the presence of small-diameter branches, and thus host tree architecture and crown characteristics. The results suggest that the presence of trellises is important in determining the distribution of climbers. Where the forest canopy is high, and small-diameter branches (trellises) are scarce, significantly more climbers enter their host trees horizontally from neighbouring trees than vertically, whereas when the forest is low more climbers enter their host trees vertically via the available trellises. The density of climbers per host tree was correlated with the height of the lowest branches of the tree canopy. The bole of trees was seldom used as an access route for climbers to the canopy of their host trees. It is concluded that the distribution and abundance of climbers is probably determined by biotic factors, in particular the architecture of host trees, rather than by climatic or soil factors.