Many plant species reproduce synchronously at supra-annual intervals, a phenomenon known as “mast-seeding”. In many plant species, synchronous mast-seeding increases fitness by reducing seed predation, and increasing pollination efficiency (especially in wind-pollinated plants). Although these ultimate fitness benefits are well documented, the proximate factors that synchronize reproduction among individual plants remain mysterious. Theories for how reproduction become synchronized fall broadly in three categories: (1) simple environmental cues (for example, seed production is cued by fire, drought, or spring frost), (2) differences in environmental conditions between successive years (for example, seed production is high when a year is warmer than the previous one), and (3) dynamics of stored resources within individual plants (for example, plants cannot produce high seed crops for two years in a row due to depletion of stored carbohydrates).
In this talk, we evaluate both proximate and ultimate causes of mast-seeding in sugar maples, Acer saccharum, using monitoring data collected at the Harvard Forest (Petersham MA USA). In the context of mast-seeding, sugar maple is an interesting species because it is both insect and wind pollinated, and because sap sugar, a possible metric of carbohydrate stores, is easily monitored during spring. We monitored flowering and seed production from 2011-2022, with ongoing data collection in 2023. We monitored seed predation and pollination, and used pollen exclusion bags to quantify the importance of insect pollination in mast- and non-mast years from 2012-2017. We monitored sap sugar concentration and sap yield from 2012-2022, with ongoing data collection in 2023, based on the hypothesis that sap sugar might reflect stored resources within plants, leading to low maple syrup yields after high seed years.
Over this time period, sugar maples flowered and produced seeds synchronously in approximately alternate years. As expected, seed predation was lower and pollination success was higher in high-flowering/high-seed years. Contrary to some hypotheses, insect pollination was equally important in high- and low-flowering years. Sap sugar did not show obvious correlations with seed production, which was also contrary to the hypothesis that mast years deplete stored carbohydrates. Analyses of weather data are in progress. Results to date recapitulate the pattern that fitness advantages of mast-seeding are common, but proximate causes are often elusive. They also emphasize the importance of mast-flowering for increasing seed production, even in insect pollinated species.
