Causes and consequences of frequent flowering on edges in the mast-seeding genus Chionochloa (Poaceae)

Recent work has shown that resource accumulation is important in allowing mast-seeding plants to display occasional intense reproductive efforts. Anecdotal reports suggest that Chionochloa tussocks (bunchgrasses) on patch edges flower more frequently, and it has been proposed that this is due to greater resource availability. This study aimed to quantify any edge effect in flowering effort in Chionochloa populations at Mt Hutt in the Southern Alps of New Zealand, and to look for correlations with available soil nutrients. It also focused on how higher flowering along edges might affect seed predation rates in these plants, since seed predation is recognised as the likely cause of masting in this genus. As predicted, in a generally low-flowering year flowering was found to be most prolific on upslope edges across a range of altitudes with at least 43% of the flowering effort in the upslope plots being concentrated in the 20% of plot area within 2 m of the upper edge. Upslope edges also exhibited higher nitrate availability than did patch interiors or downslope edges. However, other measured nutrients did not show such a clear pattern. The elevated flowering effort on upslope edges does support the importance of a resource accumulation component in flowering effort. Higher edge-flowering did not significantly affect the distribution of the insect seed predators, suggesting the scale at which these insects move in search of oviposition sites is greater than the scale of the vegetation patches studied.