Sit-and-wait pollination in the spring flowering woodland plant, &lt;i&gt;Trillium grandiflorum&lt;/i&gt;

Emily S Darling,Spencer C.H Barrett

doi:10.26786/1920-7603(2011)13

Abstract

In animal-pollinated plants, reproductive success is commonly limited by pollen availability, which can occur in environments where pollinator activity is scarce or variable. Extended floral longevity to maximize a plant’s access to pollinators may be an adaptation to such uncertain pollination environments. Here, we investigated the effects of flower exposure time to pollinators on female fertility (fruit and seed set) in the bee-pollinated woodland herb Trillium grandiflorum, a species with long-lived flowers (~17-21 d) that blooms in early spring when pollinator activity is often variable. We experimentally exposed flowers to pollinators for different amounts of time to determine the extent to which floral longevity influenced reproductive success. The amount of time that flowers were exposed to pollinators significantly increased fruit set and seed set per flower, but not seed set per fruit. Our results provide experimental evidence that long floral life spans may function as a ‘sit-and-wait’ pollination strategy to increase the amount of exposure time to pollinators and promote seed set in the unpredictable pollination environments often experienced by early spring ephemerals. In large populations with infrequent pollinator visitation, as commonly occurs in T. grandiflorum, pollination may be a largely stochastic process.

Highlights

The reproductive success of plants is often limited by pollen availability
We evaluated the relationship between exposure time and three measures of female reproductive success, fruit set, seed set per fruit and seed set per flower using linear relationships; exposure time was log-transformed prior to analysis
We evaluated the effect of plant height, neighbourhood flower density, pollinator availability and exposure time on seed set per flower using a general linear model (GLM) with a quasibinomial distribution to account for overdispersion (Zuur et al 2009)

Summary

Introduction

The reproductive success of plants is often limited by pollen availability (reviewed in Burd 1994; Larson & Barrett 2000; Ashman et al 2004; Knight et al 2005) This occurs when the quantity and/or quality of pollen that a plant receives during pollination is insufficient to fertilize available ovules, resulting in a reduction in fruit and/or seed production (Burd 1995; Aizen & Harder 2007). The fitness consequences of pollen limitation in variable pollination environments are diverse, and chronic pollen limitation can affect the evolution of a range of life history and reproductive traits in plant populations (Haig & Westoby 1988; Ashman et al 2004; Morgan et al 2005; Porcher & Lande 2005; Harder & Aizen 2010). Theoretical models have examined the factors influencing optimal floral longevities (Ashman & Schoen 1994; Schoen & Ashman 1995), but relatively few studies have experimentally examined the direct consequences of floral longevities on female reproductive success (but see Ashman & Schoen 1997; Rathcke 2003; Alonso 2004).

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