Sexual systems in the New Zealand angiosperm flora

ABSTRACT

We present data on sexual systems and associated traits in the New Zealand angiosperm flora and discuss reasons for the anomalously high levels of gender dimorphism in the flora, and the low levels of monoecy in woody species. Along with Hawai'i and New Caledonia, New Zealand has exceptionally high levels of gender dimorphism (19.5% of angiosperm species). The plant traits associated with gender dimorphism (woody growth, small, unspecialised white to yellow-green flowers, abiotic pollination, fleshy fruit) are the same as those in other regions and most gender dimorphic species belong to lineages that were already gender dimorphic on arrival in New Zealand. We attribute the high levels of gender dimorphism to several distinct factors. New Zealand’s evergreen forests have many small trees and understorey shrubs with fleshy fruit and small, open, inconspicuous flowers, a combination characterised by high levels of gender dimorphism elsewhere. Many of these species belong to lineages that migrated from the tropical north, a region with high levels of gender dimorphism. In comparison with many other regions, the New Zealand angiosperm flora has few annuals, and few plants with large, specialised flowers or pollinated by birds, traits elsewhere associated with exceedingly low levels of gender dimorphism. Finally, chance may have played a role through the association of gender dimorphism with rapidly radiating lineages. While the New Zealand angiosperm flora has similar levels of monoecy (14.2%) to other comparable regions, monoecy is exceptionally uncommon in the tree flora (3.4% for strictly monoecious species). However, the endemic Nothofagaceae and introduced woody monoecious species thrive in New Zealand. We suggest it is the lack of temperate sources for monoecious tree species, combined with the difficulty large-fruited monoecious tropical species have in crossing ocean gaps that may be ultimate reason for their failure to establish in greater numbers.

KEYWORDS:

• Dioecy
• cenozoic flora
• evergreen broadleaved temperate forest
• fleshy fruit
• flower colour and size
• gender dimorphism
• immigrant selection
• maximum height
• monoecy
• pollination mode

Data availability

A full copy of the data are available from the Manaaki Whenua – Landcare Research Datastore website: https://doi.org/10.7931/qzf7-9q70.

Acknowledgements

Eric Godley was a pioneer in studies of sexual system in the New Zealand flora and David Lloyd advanced theoretical understanding of sexual system in a sequence of ground-breaking papers. We wish to acknowledge their contributions here, along with those of their students and colleagues Colin Webb, Lynda Delph-Lively, Linda Newstrom-Lloyd, and Alastair Robertson. We thank Angela Brandt for a detailed review of a draft of this manuscript. We acknowledge the use of data drawn from the Natural Forest plot data collected between 2009 and 2014 by the LUCAS programme for the Ministry for the Environment and the New Zealand Department of Conservation. Data were acquired using the National Vegetation Survey (NVS) databank (https://nvs.landcareresearch.co.nz/). Work was funded by Strategic Science Investment Funding for Crown Research Institutes from the Ministry of Business, Innovation and Employment. Photos in Figure 6 that are not by the authors: C, J. Barkla, https://inaturalist.nz/observations/107088651; E, A. Fergus, https://inaturalist.nz/observations/16682611; (f) https://inaturalist.nz/observations/111145836; H, T. Park, https://inaturalist.nz/observations/66972497.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by Ministry for Business Innovation and Employment [Strategic Science Investment Fund to Manaaki Whenua].