Scale-dependent effects of conspecific flower availability on pollination quantity and quality in an invasive shrub.

Pollen limitation can strongly influence reproduction of pollinator-dependent plants. Flower abundance can affect pollination 'quantity' and 'quality' due to its influence on pollen availability and foraging patterns of pollinators, ultimately impacting on seed production. We complemented individual-based measurements with landscape-level metrics to assess the influence of conspecific flower availability at different spatial scales on the quantity and quality components of pollination, and their impact on seed production in the invasive shrub Cytisus scoparius. In 2013-2014, we sampled 40 C. scoparius populations in Nahuel Huapi National Park, Patagonia (Argentina). In each population, we estimated the proportion of tripped flowers, fruit- and seed-set in five randomly selected individuals. The proportion of tripped flowers and the proportion of them setting fruit were used as proxies of pollination quantity and quality, respectively. Conspecific flower availability at distinct spatial scales (5-1000 m) was estimated as the area covered by flowering C. scoparius from color aerial photographs. Flower availability influenced seed output due to contrasting scale-dependent effects on pollination quantity and quality. Increasing flower availability at the landscape-scale reduced pollination quantity, whereas at the neighborhood-scale it increased pollination quality. The overall positive effect of flower availability on seed output at the neighborhood scale was slightly higher than the overall negative effect at the landscape scale. Moreover, pollination quality had a higher positive effect on seed output than pollination quantity. Our results demonstrate that pollination quality may severely limit plant reproduction. Pollination quality limitation can act independently of pollination quantity limitation because these factors operate at different spatial scales.

Evaluating the effects of pollinator-mediated interactions using pollen transfer networks: evidence of widespread facilitation in south Andean plant communities.

Information about the relative importance of competitive or facilitative pollinator-mediated interactions in a multi-species context is limited. We studied interspecific pollen transfer (IPT) networks to evaluate quantity and quality effects of pollinator sharing among plant species on three high-Andean communities at 1600, 1800 and 2000 m a.s.l. To estimate the sign of the effects (positive, neutral or negative), the relation between conspecific and heterospecific pollen deposited on stigmas was analysed with GLMMs. Network analyses showed that communities were characterised by the presence of pollen hub-donors and receptors. We inferred that facilitative and neutral pollinator-mediated interactions among plants prevailed over competition. Thus, the benefits from pollinator sharing seem to outweigh the costs (i.e. heterospecific deposition and conspecific pollen loss). The largest proportion of facilitated species was found at the highest elevation community, suggesting that under unfavourable conditions for the pollination service and at lower plant densities facilitation can be more common.

Phenological match drives pollen-mediated gene flow in a temporally dimorphic tree.

Variation in flowering phenology is common in natural populations, and is expected to be, together with inter-mate distance, an important driver of effective pollen dispersal. In populations composed of plants with temporally separated sexual phases (i.e. dichogamous or heterodichogamous populations), pollen-mediated gene flow is assumed to reflect phenological overlap between complementary sexual phases. In this study, we conducted paternity analyses to test this hypothesis in the temporally dimorphic tree Acer opalus. We performed spatially explicit analyses based on categorical and fractional paternity assignment, and included tree size, pair-wise genetic relatedness and morph type as additional predictors. Because differences between morphs in flowering phenology may also influence pollination distances, we modelled separate pollen dispersal kernels for the two morphs. Extended phenological overlap between male and female phases (mainly associated with inter-morph crosses) resulted in higher siring success after accounting for the effects of genetic relatedness, morph type and tree size, while reduced phenological overlap (mainly associated with intra-morph crosses) resulted in longer pollination distances achieved. Siring success also increased in larger trees. Mating patterns could not be predicted by phenology alone. However, as heterogeneity in flowering phenology was the single morph-specific predictor of siring success, it is expected to be key in maintaining the temporal dimorphism in A. opalus, by promoting not only a prevalent pattern of inter-morph mating, but also long-distance pollination resulting from intra-morph mating events.