A resource budget model to explain intraspecific variation in mast reproductive dynamics

Rowan, Sorbus aucuparia L., exhibits conspicuous geographic variation in its inter-annual patterns of reproduction across southern Norway. Along the southern west coast, trees flower and produce clusters of succulent berries every other year, while in the east, trees mast every 3 years. We investigate two hypotheses for this variation: local adaptation to environmental conditions that favors different reproductive schedules at different locations or variation arising from single stereotyped reproductive strategy in the face of a geographic variation in productivity. To assess the theoretical plausibility of each of these hypotheses, we develop a refined resource budget model for mast reproduction. The refined model assumes that there is a lower bound on the amount of resource that may be allocated to reproduction, which allows the prediction of the relation between reproductive schedules and productivity. From the analysis of the model, we find that the observed geographic transition in rowan mast is caused if either productivity in the east is lower than that in the west or if plants in the eastern populations have evolved to pause reproduction until energy reserves are replenished to higher levels. The prediction by the productivity gradient hypothesis match with the empirical finding that the productivity is, in fact, likely to be lower in the east than the west, although we lack empirical data to test the likelihood of the local adaptation hypothesis. There is a need for experimental studies to clarify the validity of the local adaptation hypothesis.