A Comprehensive Study of Orchid Seed Production Relative to Pollination Traits, Plant Density and Climate in an Urban Reserve in Western Australia

Mark C Brundrett

doi:10.3390/d11080123

Abstract

The pollination of 20 common terrestrial orchids was studied in a 60-ha urban banksia and eucalypt dominated woodland in Western Australia. Five years of data (24,000 flowers, 6800 plants) measured fruit set relative to floral areas, capsule volumes, climate, phenology, pollination mechanisms, disturbance tolerance and demography. Pollination varied from 0–95% of flowers, floral displays from 90–3300 mm2 and capsules from 15–1300 mm3 per spike. Pollination traits strongly influenced outcomes, with self-pollination highest (59—95%), followed by sexually deceptive autumn or winter-flowering (18–39%), visual deception (0–48%) and sexually deceptive spring-flowering (13–16%). Pollination was limited by drought in autumn or spring and cool winter temperatures. Some orchids were resilient to drought and one formed seed after the leaves withered. Plant density had the greatest impact on fruit set for orchids forming large groups, especially for sexually deceptive pollination. Consequently, small group average (SGA) pollination was up to 4× greater than overall averages and peak seed production occurred in the best locations for genetic exchange and dispersal. SGA rates and seedpod volumes were strongly linked to clonality, but not to demographic trends. Resource competition limited flowering at higher plant densities and competition within spikes resulted in smaller, later-forming seedpods. Pollination data from co-occurring common orchids identified five evolutionary trade-offs linked to pollination, provided baseline data for rare species and revealed impacts of changing climate.

Highlights

Orchid pollination and the subsequent dispersal and germination of seeds are key limiting factors that determine the persistence and spread of orchids in natural habitats [1,2,3,4,5]
There was resource competition within large groups (Figure 7A) and within flower spikes where upper seed pods were smaller than earlier-formed seedpods (Figure 8A)
The use pollination metrics that incorporate plant density, such as small group average (SGA), provide powerful new tools for understanding insect abundance and other factors limiting orchid pollination. They should allow for orchid dispersal, recruitment and demographics to be investigated more efficiently, as well as genetic patterns in orchid populations

Summary

Introduction

Orchid pollination and the subsequent dispersal and germination of seeds are key limiting factors that determine the persistence and spread of orchids in natural habitats [1,2,3,4,5]. Deceptive means of attracting insects without food rewards are more common, occurring in an estimated one-third of all orchid species [10,12,13,14]. Orchids that attract insects by visual deception, exploit insect color perception biases and often resemble other flowers that contain nectar or pollen rewards and are scented [17,18,19,20]. In these orchids, some insect specificity can occur because the mimicked flowers are preferred by particular pollinators, but these usually include multiple categories of insects (Table A1). Detailed studies of Australian visually deceptive orchids, such as Thelymitra and Diuris species, found them to attract a suite of Diversity 2019, 11, 123; doi:10.3390/d11080123 www.mdpi.com/journal/diversity

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