The Chionochloa Story
Chionochloa species have some of the most extreme mast seeding seen in any plants worldwide, as shown in some very long datasets from New Zealand. For example in a global review of 210 datasets by Kelly et al. (2000), the two most variable datasets were C. crassiuscula and C. teretifolia from Fiordland, New Zealand. Chionochloa datasets made up 5 of the most variable 20 datasets worldwide. The reasons for this derive partly from the details of their biology and ecology, which is reviewed in detail in Rees et al. (2002).
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snow tussocks at Mt Hutt and Lake Wanaka |
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The genus Chionochloa includes 22 species endemic to New Zealand, most of which are alpine tussocks (bunchgrasses), which variously dominate the vegetation in mountainous regions above the tree line (Connor 1991). Most species, including C. pallens, grow as individual tufts around 0.5-1.5 m tall and 15-50 cm in basal diameter and are very long-lived (>100 yr). Each plant consists of hundreds to thousands of long-lived tillers, in a heavy flowering year; a single plant can have hundreds of flowering tillers.
Levels of flowering at the population level vary extremely widely from year to year (chaotic flowering behaviour), with 16 published data sets from 11 Chionochloa species having CVp (using sample standard deviation/mean) between 3.08 and 1.51 with a mean of 1.90, which are the highest reported worldwide. Flowering is synchronous among plants at a site and among sites, both within and between Chionochloa species (Kelly et al. 2000). Flower bud initiation is triggered by warm temperatures in the austral summer (January or January–February) the year before flowering. Like other grasses, Chionochloa species are wind pollinated, and all eight tested species (including C. pallens) are self-compatible (Connor 1967; McKone et al. 1997).
Chaotic behaviour is almost unknown in any aspect of plant population dynamics (Rees and Crawley 1989, 1991). It seems significant that C. pallens' flowering behaviour is determined by a value of a depletion coefficient. If the depletion coefficient is reduced, the variation among years for individual plants goes down (plants have smaller, more frequent flowering episodes), so population variability decreases. If the depletion coefficient is increased, the dynamics become so chaotic that synchrony declines and the population-level variability decreases. Therefore, to get the maximum benefits from satiating seed predators, individuals have to have just the right level of chaotic (but statistically periodic) dynamics built into their flowering responses. The fitted resource-based model, in the absence of environmental forcing, has chaotic (but often statistically periodic) dynamics. Environmental forcing synchronizes individual reproduction, and the models predict highly variable seed production in close agreement with the data. An evolutionary model shows that the chaotic internal resource dynamics, as predicted by the fitted model, is selectively advantageous provided that adult mortality is low and seeds survive for more than 1 yr, both of which are true for C. pallens. Therefore highly variable masting and chaotic dynamics appear to be advantageous in this case because they reduce seed losses to specialist seed predators, while balancing the costs of missed reproductive events.
Relationship between the number of inflorescences produced and the reconstructed deviations from the equilibrium energy budget. The open circles are from years when the January temperature was above the threshold (11.5 degrees C). The filled circles are from years when the January temperature was below the threshold.
Relationship between CVp (the variation in population-level reproduction), synchrony, and resource depletion (A), the smaller the value of A the greater the resource depletion.
Flowchart illustrating the decisions for individual Chionochloa plant’s that influence whether or not the individual therefore the population decide to flower.
Seedlings can often be found where there has been little disturbance, but growth rates are very slow, with 6-yr-old seedlings being <5 cm tall and having less than seven tillers. Seedling establishment in Chionochloa is inhibited by competition with established vegetation (including adults) and by introduced mammalian herbivores. As well as the native insects which feed on Chionochloa seeds, large seed crops are important to birds such as the endangered takahe or notornis (Porphyrio mantelli). Removal of the vegetation cover by fire can enhance establishment.
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takahe feeding on Chionochloa seeds |
seed with cecid fly larva |
