Mast seeding, predator satiation, and temperature cues in Chionochloa (Poaceae)

Snow tussocks (Chionochloa spp.) in New Zealand exhibit extreme mast seeding which is synchronised within and among populations and species over large spatial scales. This masting behaviour satiates three endemic insect seed predators, and the weather cue that triggers heavy flowering has been reported as a very warm austral summer in the year before flowering. Here we elucidate the details of flowering, predator satiation, and temperature cues for Chionochloa. A 22-year observational data set for Chionochloa pallens from 1,070 m elevation on Mt Hutt, Canterbury, indicates that flowering was highly variable (CV = 1.79) which was effective in predator satiation, and the key cue for heavy flowering was warm temperatures between 1 January and 7 February the year before flowering. Both the number of inflorescences per tussock (the most variable component of reproductive output) and the number of florets per inflorescence were increased. Surprisingly, there were also same-season effects of temperatures between October and December on the number of inflorescences per tussock produced in late December through February. Experimental warming of tussocks using open-topped clear plastic enclosure tubes verified the same-season observations by significantly increasing flowering in the current season, but much less often increased flowering in the subsequent season. A comparison of masting patterns 1990-2008 with altitude in two tussock species, C. pallens and C. macra, indicates that at higher altitudes (ca. 1,580 m vs. 1,070 m) the temperature thresholds for a given flowering effort were lowered. The lowering of thresholds (2.1 degrees C in C. pallens and 1.5 degrees C in C. macra) was similar to the reduction in mean growing-season air temperatures (2.0 degrees C) found between the sites. Thus, despite different local temperatures at high and low altitude sites, the different populations flowered with a similar intensity each year. The primary benefit of mast seeding in Chionochloa appears to be predator satiation, and the key predator (the cecidomyiid Eucalyptodiplosis chionochloae) has extended diapause. If diapause can be influenced by climate cues, the exact nature of the climate cues will be important to the evolutionary interaction between the plant and its seed predators.