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Demography of the giant monocarpic herb Rheum nobile in the Himalayas and the effect of disturbances by grazing.

BACKGROUND
Perennity of giant rosette species in combination with a single "big bang" reproduction followed by death of the genetic individual is relatively rare among plants. Such long-lived monocarpic plants are usually slow growing and can be found in deserts, bogs or in alpine regions of the tropics or subtropics. Due to their longevity monocarpic perennials risk to lose everything before reproduction, which make them particularly susceptible to disturbances. Because of the inherent difficulties to assess if long-lived populations are growing or declining, usually neither their demography nor the consequences of increasing grazing pressure are known.


METHODS
We used integral projection modelling (IPM) to measure growth rate and passage time to flowering of Rheum nobile, a monocarpic perennial, and one of the most striking alpine plants from the high Himalayas. We estimated the impact of grazing cattle (yaks) by including in the models those rosettes, which were not found any more due to disturbances or grazing by yaks, either as missing or dead. Data was collected from plants at 4500 m a.s.l. in Shangri-la County, Yunnan Province, southwest China. In four consecutive years (2011-14) and in two populations, 372 and 369 individuals were measured, respectively, and size-dependent growth, survival and fecundity parameters were estimated. In addition, germination percentage, seedling survival, and establishment probability were assessed.


KEY RESULTS
The probability of survival, of flowering and fecundity were strongly size-dependent. Time to reach flowering size was 33.5 years (95% CI from 21.9 to 43.3, stochastic estimate from pooled transitions and populations). The stochastic population growth rate (λs) of Rheum nobile was 1.013 (95% CI from 1.010 to 1.017). When disturbance by grazing cattle (yaks) was accounted for in the model, λs dropped to values below one (0.940, 95% CI from 0.938 to 0.943).


CONCLUSION
We conclude that natural populations of this unique species are viable, but that conservation efforts should be taken to minimise disturbances by grazing and to protect this slow-growing flagship plant from the high Himalayas.