Synchronized and quasi-periodic production of seeds by plant populations, known as masting, is implicated in many ecological processes, but how it arises remains poorly understood. Flowering and pollination dynamics are hypothesized to provide the mechanistic link for the observed relationship between weather and population‐level seed production. We report the first experimental test of the phenological synchrony hypotheses as a driver of pollen limitation in mast seeding oaks (Quercus ilex). Higher flowering synchrony yielded greater pollination efficiency, which resulted in 2-fold greater seed set in highly synchronized oaks compared to asynchronous individuals. Pollen addition removed the negative effect of asynchronous flowering on seed set. Because phenological synchrony operates through environmental variation, this result suggests that oak masting is synchronized by exogenous rather than endogenous factors. It also points to a mechanism by which changes in flowering phenology can affect plant reproduction of mast-seeding plants, with subsequent implications for community dynamics.

We conducted the pollen supplementation experiment with a population of 81 marked Q. ilex trees in 2018 and 2019 (Fig. S1 & Fig. S2). At each plant, we selected two branches facing south, and haphazardly assigned the branch to control or pollen-supplementation treatment. Previous work has shown that within-plant comparisons may lead to overestimation of pollen limitation when pollen-supplemented flowers compete for resources with control flowers (Knight et al. 2006). We attempted to minimize this effect by choosing branches that were separated by at least 5 m of vascular tissue. Because trees are often modular in their carbohydrate use (Hoch 2005; Han & Kabeya 2017), these distant branches are less likely to compete for resources than neighboring flowers on smaller plants (Pearse et al. 2015b). Experimental studies performing defoliation, branch girdling and stable isotope analysis have reported that separate branches behave as if autonomous for fruit maturation in at least some trees species (Obeso 1998; Hasegawa et al. 2003; Sala et al. 2012). On pollen supplementation branches, we individually marked 3 shoots and hand-pollinated flowers (~2000 over the course of two years, median per shoot = 3, min = 1, max = 39) with a mix of conspecific pollen collected from at least five local trees from outside the experimental population. We added pollen when flowers were receptive, i.e. their stigmas were swelled and yellowish. Flowers at each shoot received pollen supplementation twice with a 2-3-day break between additions. On control branches, we marked three shoots and handled them in similar ways, but did not apply hand-pollination. We visited all trees every 2-3 days, starting at the first week of flowering. At each visit, we counted all female flowers at marked shoots, and scored the tree flowering phenology by examining the catkin stage (0 – not active; 1– up to 50% of the crown active; 3 – over 50% of the crown active; 4 – spent). We used male flowering as the measure of phenology because it was relatively easy to quantify objectively on a large scale. The surveys were continued until all trees finished shedding pollen. We evaluated seed set on the marked shoots twice: in early June to evaluate the effects of pollen addition before drought-induced acorn abortion, and in mid-September, to evaluate the cumulative effects of pollen addition and drought on seed set.