Although some bamboos flower every year, most species flower infrequently. In fact, many bamboos only flower at intervals as long as 60 or 120 years. These taxa exhibit mass flowering (or gregarious flowering), with all plants in the population flowering simultaneously. The longest mass flowering interval known is 130 years, and is found for all the species Phyllostachys bambusoides (Sieb. & Zucc.). In this species, all plants of the same stock flower at the same time, regardless of differences in geographic locations or climatic conditions, then the bamboo dies. The lack of environmental impact on the time of flowering indicates the presence of some sort of “alarm clock” in each cell of the plant which signals the diversion of all energy to flower production and the cessation of vegetative growth. This mechanism, as well as the evolutionary cause behind it, is still largely a mystery.
One theory to explain the evolution of this semelparous mass flowering is the predator satiation hypothesis. This theory argues that by fruiting at the same time, a population increases the survival rate of their seeds by flooding the area with fruit so that even if predators eat their fill, there will still be seeds left over. By having a flowering cycle longer than the lifespan of the rodent predators, bamboos can regulate animal populations by causing starvation during the period between flowering events. Thus, according to this hypothesis, the death of the adult clone is due to resource exhaustion, as it would be more effective for parent plants to devote all resources to creating a large seed crop than to hold back energy for their own regeneration.
A second theory, the fire cycle hypothesis, argues that periodic flowering followed by death of the adult plants has evolved as a mechanism to create disturbance in the habitat, thus providing the seedlings with a gap in which to grow. This hypothesis argues that the dead culms create a large fuel load, and also a large target for lightning strikes, increasing the likelihood of wildfire. Because bamboos are very aggressive as early successional plants, the seedlings would be able to outstrip other plants and take over the space left by their parents.
However, both have been disputed for different reasons. The predator satiation theory does not explain why the flowering cycle is 10 times longer than the lifespan of the local rodents, something not predicted by the theory. The bamboo fire cycle theory is considered by a few scientists to be unreasonable; they argue that fires only result from humans and there is no natural fire in India. This notion is considered wrong based on distribution of lightning strike data during the dry season throughout India.
The mass fruiting also has direct economic and ecological consequences, however. The huge increase in available fruit in the forests often causes a boom in rodent populations, leading to increases in disease and famine in nearby human populations. For example, there are devastating consequences when the Melocanna bambusoides population flowers and fruits once every 30–35 years around the Bay of Bengal. The death of the bamboo plants following their fruiting means the local people lose their building material, and the large increase in bamboo fruit leads to a rapid increase in rodent populations. As the number of rodents increase, they consume all available food, including grain fields and stored food, sometimes leading to famine. These rats can also carry dangerous diseases such as typhus, typhoid, and bubonic plague, which can reach epidemic proportions as the rodents increase in number.
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