Evaluating the Wind Pollination Benefits of Mast Seeding

Abstract

We developed a conceptual model for evaluating the benefits of wind pollination to mast-flowering species. The benefit that a plant population gains from mast flowering via increased wind pollination efficiency was predicted from how far pollination efficiency at mean seed crop size falls below the maximum. Species were most likely to benefit from mast seeding if mean reproductive effort in the field gave an intermediate level of pollination efficiency, regardless of the cost of unpollinated female structures. To quantify the benefits of different degrees of mast flowering, a simulation model was used to alter the seed production coefficient of variation (cv) and to calculate its effects on weighted mean pollination efficiency. The model was applied to seven real data sets for five species with pollination benefits from masting that ranged from relatively small (Chionochloa pallens), to moderate (Dacrydium cupressinum, Betula alleghaniensis), to large (Nothofagus solandri, N. menziesii). Many studies have reported higher seed production coefficients of variation at higher altitudes and latitudes within a species. Our model showed that higher coefficients of variation are favored by reduced mean seed output per plant at higher altitudes. Data for N. solandri at three altitudes in one site showed much higher pollination benefits from masting at higher altitudes. Reduced plant density (e.g., through fragmentation), which also lowers mean flowering effort per unit area, resulted in large increases in masting benefits in N. solandri, but only small increases in C. pallens. These contrasting results were primarily due to differences between the two species in mean reproductive effort vs. wind pollination efficiency, rather than to differences in the effects of fragmentation and altitude. The relative effects of masting on pollination, insect seed predation, and bird seed predation were modeled in B. alleghaniensis. Masting produced a small economy of scale from insect predator satiation, but an almost equivalent diseconomy of scale resulted from increased levels of bird seed predation. Efficiency of wind pollination improved moderately with increasing cv, providing some overall benefits from masting in this species. Accordingly, we propose that masting can be favored by either one dominant economy of scale (such as wind pollination efficiency in N. solandri or predator satiation in C. pallens), or a balance among several factors (such as pollination, predator satiation, and predator attraction in B. alleghaniensis). We predict that, in the absence of any selective benefits or disadvantages of masting, plants would be expected to have coefficients of variation in the range 0.85–1.35.