A multi-scale biogeographical analysis of Bambusa arnhemica, a bamboo from monsoonal northern Australia

Aims: To identify the edaphic, environmental and historical factors influencing the patchy distribution of the semelparous bamboo Bambusa arnhemica F. Muell. at global, catchment and streambank scales. Location: The entire range of B. arnhemica, a highly fire-prone savanna matrix with generally infertile soils in the north-west of the Northern Territory of Australia above the 1200 mm mean annual rainfall isohyet. Methods: Distribution surveys were conducted by air, boat and on the ground. Plot data were collected throughout the range of the species. Results: Bambusa arnhemica occurred predominantly in gallery forests on flood-prone but nevertheless well-drained and deep alluvial soils on sloping stream banks. It ranged widely along lentic watercourses from ephemeral headwater streams to the banks of major rivers and levees on the coastal floodplain. The species did not occur in savannas; savannas adjacent to B. arnhemica gallery forests were also flood-prone and on deep alluvial soils, but were upslope on level ground. Bambusa arnhemica's infrequent non-riparian occurrences were on a wide variety of substrates but generally on soils of moderate fertility and in coastal and/or rocky areas where at least partial topographic protection from fire is likely. Within and between catchments, the distribution of B. arnhemica was idiosyncratic, occurrence being almost always continuous downstream from highly variable 'starting' points to the poorly drained coastal floodplain. Main conclusions: At local scales, B. arnhemica appears constrained by poor drainage and high fire-frequencies. Enhanced soil fertility may increase its capacity to cope with fire. At the catchment and global scales, we propose that the distribution of B. arnhemica is the product of infrequent and as yet incomplete dispersal across and away from watercourses by seed that lacks specialized dispersal mechanisms, combined with passive dispersal along streams. From this we infer that B. arnhemica is neither a very recent, nor very ancient colonist from Asia. Bambusa arnhemica's circumscribed global distribution has no parallel amongst co-occurring rain forest plants and may be the product of poor dispersal capacity and a substantial rock and floodplain barrier to the east. Limited dispersal capacity may be inextricably linked to local domination of space and the subsequent creation of regeneration space by parental death.