DOI: 10.5061/dryad.sn02v6x2b
title: Flowering synchrony drives reproductive success in a wind-pollinated tree

description: 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.
description: We conducted the pollen supplementation experiment with a
 population of 81 marked <i>Q. ilex </i>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 <i>et al.</i> 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 <i>et al.</i> 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 <i>et al.</i>
 2003; Sala <i>et al.</i> 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.