Termite mounds as islands: woody plant assemblages relative to termitarium size and soil properties

Questions We investigated whether soils of small mounds resembled large mound or matrix soils, whether changes in plant composition reflected changes in soils, and the sequence in which plants colonize and disappear from mounds of increasing size. Location Miombo woodland in northwest Zimbabwe. Methods Macrotermitinae termitaria vary in size and soil nutrient concentrations, harbouring distinct woody plant assemblages, making them foci for plant and animal diversity, and also influencing primary, secondary and tertiary productivity. In spite of the importance of termitaria to heterogeneity and diversity, no studies have investigated changes in plant species assemblages as mound surface area increases to the point where mound vegetation is distinct from that of the matrix. We compared woody plant assemblages on 43 matrix plots with 95 Macrotermes termitaria across a range of surface areas, using ANOSIM, cluster analysis and MDS ordination. We compared soil nutrients, pH and clay, from ten large and ten small termitaria, and ten matrix sites. We also assessed how relative representation of large mound or matrix indicator species changed with mound area. Results Change was apparent even at mound sizes of >10m2, where both soils and plant assemblages on mounds were significantly different to those of the matrix. Plant assemblages fell into two main groups at 20% similarity; the first comprised of matrix plots, mounds <10m2 and some mounds between 10 and 30m2; the second, the remainder of the mounds between 10 and 30m2 and all mounds >30m2. At 40% similarity, four groups emerged: matrix, mounds <10m2, mounds 10-30m2 and mounds >30m2. Woody plant composition changed gradually as mound area increased. On termitaria <10m2, only 25% of indicators were mound indicator species, but on mounds between 10 and 30m2 in size, 62.5% were mound indicators. On termitaria >30m2 in surface area, only mound indicator species were found. Conclusions Through termite activities in concentrating nutrients and clay, termitaria provide habitat for species usually excluded from the matrix. The process of mound building and the nature of the plants that establish on them seem to establish a positive feedback for establishment of other non-woodland matrix species.