Synchrony and asynchrony: observations and hypotheses for the flowering wave in a long-lived semelparous bamboo

Aim (1) To describe the spatio-temporal patterns of mass-flowering and die-off in a long-lived, semelparous, clumping bamboo, Bambusa arnhemica, at landscape and local scales. (2) To discuss causal processes in the flowering patterns of semelparous bamboos. Location The entire range of B. arnhemica, in the monsoonal, tropical, north-west of the Northern Territory of Australia, mostly along watercourses. Methods Landscape-scale flowering patterns were assessed by a combination of air, boat and ground survey in each year from 2000 to 2002. Areas that flowered prior to 2000, and those in which no flowering occurred, were also recorded, and historic records collated. At local scales, initiation of flowering, rates of die-off, and subsequent germination densities of seedlings were quantified by ground-based counts. Results After an estimated 40-50 years of vegetative development, B. arnhemica flowered, seeded prolifically, then died. Flowering occurred synchronously within patches ranging from 0.002 to 3200 km(2). One or more patches flowered in successive years from 1996 to 2002, forming a temporally-structured but spatially-chaotic flowering wave that affected c. 80% of the population. Synchronous flowering took the form of a flowering distribution in which over 95% of clumps within a patch initiated flowering in a central year, most of the remainder flowering the year before or after. Along the Daly River, an exception was observed in which 56% of clumps flowered in the peak year. Seedling densities were three orders of magnitude greater under clumps that flowered in the central rather than the leading year of the flowering distribution. Main conclusions Synchrony is argued to be the primal state in semelparous bamboos, promoted by intense selection acting on a endogenous (genetic or biological) clock whose influence largely overrides that of the environment. A flowering wave may develop within an initially synchronous population when stochastic events interact with the biological clock without permanently altering the clock setting, producing an off-set patch. Off-set groups may only survive if sufficient individuals are off-set by the same amount at the same time and in the same vicinity so as to produce a new synchronously-flowering patch. This could be driven by two processes. Inter-year climatic variation may alter the biological clock's perception of time, producing off-sets at local or regional scales or even affecting entire populations. Severe environmental pressures may also force one-off changes to flowering schedules, as suggested by a severe flood event prior to flowering on the Daly River. A dynamic hypothesis for a wider range of bamboo flowering patterns is proposed in which synchronous flowering is fragmented and disrupted over time but renewed by allochronic speciation and dispersal.