Grazing hinders seed dispersal during crop failure in a declining oak woodland

Abstract

Masting, the synchronized production of variable quantities of seeds, occurs in many grazed systems and can be interspersed with years of extreme crop failure, whose frequency and unpredictability are increasing with climate change. Yet, the combined impact of crop failure and grazing on seed dispersal and seed-to-seedling transition remains poorly understood. To address this concern, we investigated rodent-mediated cork-oak (Quercus suber) acorn predation, dispersal, and seedling emergence in cattle grazed and non-grazed areas in central Portugal during years with contrasting masting seasons. We found that extreme crop failure led to six times longer and faster acorn dispersal, with 83% more dispersal events than during a year of reproductive success. The percentage of predated acorns also increased by 84%. However, the higher acorn predation was offset by a 2.4-fold higher percentage of unpredated dispersed acorns recruiting into seedlings. Both years ended up recruiting a similar number of seedlings. Acorns emerged seedlings 3.4 times farther in the crop failure year than in the crop success year. Cattle grazing was the main constraint on seed dispersal distance, reducing it by 51% during the extreme crop failure year, while having no noticeable effect during the successful crop year. Our study provides empirical evidence that cattle grazing modulates how an extreme crop failure year can surprisingly be an opportunity for trees remaining fecund to have seedlings established farther apart than in a crop success year. If we are to better manage and preserve the high conservation and socio-economic value of Mediterranean cork oak woodlands in the face of climate change, we must prioritize fecund trees and carefully manage seed dispersal factors such as cattle grazing, particularly during years of crop failure.