1. Introduction
Pollen limitation (i.e. limitation of seed production by deposition of pollen grains) belongs among the key factors affecting the fitness of individual plants and consequently, population dynamics and species survival (Ashman et al. 2004). Therefore, with the global pollination crisis (Potts et al. 2010, Burkle et al. 2013), pollen limitation has become a key topic of ecology and conservation of plant communities (e.g. Alonso et al. 2010, Castro et al. 2015, Janečková et al. 2019). Despite several decades of research, there is still no consensus on how widespread pollen limitation is in plant communities. The optimality theory (Haig and Westoby 1988) and sexual selection theory (Charnov 1982, but see Wilson et al. 1994) predict that pollen limitation should be rare. However, numerous empirical studies showed pollen limitation as a relatively common phenomenon (Burd 1994, Larson and Barrett 2000). A review of 306 plant species found evidence of pollen limitation (within an individual site) in 73% of the studies (Knight et al. 2005). Consequently, this suggested insufficient pollen receipt to be the major cause of reduced fruit production (Knight et al. 2005). Nevertheless, the existing geographical bias of available detailed data (Bennett et al. 2018) limit any strong generalisations on the extent of pollen limitation, as well as causes and consequences in individual plant species and in communities.
Pollen supplementation experiments represent a standard method for pollen limitation quantification (Knight et al. 2006). Based on saturation by manually applied additional pollen to flowers, it allows a robust subsequent comparison of their fruit sets and/or seed sets with naturally pollinated flowers (Knight et al. 2006). Besides the effects of pollen saturation on the quantitative characteristics, possible trade-offs in resource allocation can be evaluated also by a qualitative comparison of seed or fruit sets (e.g. by their size or weight, Huang et al. 2017). Nevertheless, published results from pollen supplementation experiments are predominantly based on single-species case studies. Therefore, they may not be representative of the realized pollen limitation in communities (Vamosi et al. 2006). Plant species, as well as individuals in the population, may not be equally sensitive to changes in environmental and associated biotic conditions because the possible lack of pollination depends on the ecological context, plant life history, and type of breeding system (Ashman et al. 2004).
The shift of plant species to outcrossing can be caused by specific plant trait evolution regardless of the possible consequence of pollen limitation (Knight et al. 2005). However, the correlation of pollen limitation with various life-history and ecological traits was tested in only a few comparative studies (Larson and Barrett 2000, García-Camacho and Totland 2009). In 224 species from 64 families of flowering plants, Larson and Barrett (2000) revealed pollen limitation as less intense in species which are self-compatible, autogamous, monocarpic, herbaceous, nectariferous, and occurring in open habitats and temperate regions. Although self-incompatible plants are generally expected to be more pollen limited than self-compatible plants (Burd 1994), this assumption may not always be true (García-Camacho and Totland 2009). Furthermore, comparisons of pollen limitation between phenotypically specialized and generalized flowers reported ambiguous results. Larson and Barrett (2000) found that species with specialized floral morphology and less accessible nectar did not differ from those with generalized morphology in the level of pollen limitation. Contrarily, Lázaro et al. (2015) recorded that species with specialized flowers were more pollen limited than those with generalized flowers. Therefore, individual floral traits can explain only a small part of variation in pollen limitation (Larson and Barrett 2000).
New insights into the variation of pollen limitation causes could be provided by exploration of correlative effects between multiple reproductive and functional traits and pollen. For example, even key traits like dichogamy or clonality have not been thoroughly explored in this context. While dichogamy level has been suggested as ensuring higher autonomous seed set in plants exposed to outcross pollen limitation (Brys et al. 2013), clonality may provide reproductive advantage for obligate outcrossing species that are in the higher risk of pollen limitation (Vallejo‐Marín and O’Brien 2007).
In this study, we applied pollen supplementation to evaluate the level of pollen limitation in a community of flowering plant species in a wet meadow in a fragmented cultural landscape in Central Europe. Consequently, we correlated the pollen limitation with multiple functional traits of the plant species. We hypothesized that the degree of pollen limitation of plant species will be influenced by (i) a type of breeding system, (ii) floral traits important for pollinator attraction and foraging technique, and (iii) their degree of functional specialization on pollinators. We expected that plants visited by a broad spectrum of different insect functional groups (i.e. bees, flies, beetles, etc.) will be less pollen limited. We also predicted that the lower pollen limitation would occur in pollinator-attractive plants with abundant nectar rewards and/or more open flowers. Last but not least, we provided a comprehensive pollen limitation dataset from Central Europe, a region previously largely neglected in pollination networks and pollen limitation studies (Bennett et al. 2018).