Environmental and Management Effects on Demographic Processes in the U.S. Threatened Platanthera leucophaea (Nutt.) Lindl. (Orchidaceae).

Catherine A Pollack,Marlin L Bowles,James E Ibberson,Lawrence W Zettler,Timothy J Bell

doi:10.3390/plants10071308

Abstract

Populations of the U.S. threatened orchid, Platanthera leucophaea, are restricted to fragmented grassland and wetland habitats. We address the long-term (1998–2020) interactive effects of habitat (upland prairie vs. wetland), fire management (burned vs. unburned) and climatic variation, as well as pollination crossing effects, on population demography in 42 populations. Our analysis revealed the consistent interactive effects of habitat, dormant season burning, and climatic variation on flowering, reproduction, and survival. Burning increased flowering and population size under normal or greater than normal precipitation but may have a negative effect during drought years apparently if soil moisture stress reduces flowering and increases mortality. Trends in the number of flowering plants in populations also correspond to precipitation cycles. As with flowering and fecundity, survival is significantly affected by the interactive effects of habitat, fire, and climate. This study supports previous studies finding that P. leucophaea relies on a facultative outcrossing breeding system. Demographic modeling indicated that fire, normal precipitation, and outcrossing yielded greater population growth, and that greater fire frequency increased population persistence. It also revealed an ecologically driven demographic switch, with wetlands more dependent upon survivorship than fecundity, and uplands more dependent on fecundity than survivorship. Our results facilitate an understanding of environmental and management effects on the population demography of P. leucophaea in the prairie region of its distribution. Parallel studies are needed in the other habitats such as wetlands, especially in the eastern part of the range of the species, to provide a more complete picture.

Highlights

The orchid family (Orchidaceae) is one of the world’s largest plant families, with species represented in tropical, temperate, and boreal regions
We address long-term interactive effects of habitat, fire management, and climatic variation, as well as pollination crossing effects, on population demography of P. leucophaea in 42 populations in fragmented grasslands and wetlands of the Chicago region of northeastern Illinois in the United States (Figure 1)
Our analysis indicates a demographic switch between habitat types, in which wetland populations depend more upon survivorship than fecundity, and upland populations depend more on fecundity than on survivorship especially in response to fire

Summary

Introduction

The orchid family (Orchidaceae) is one of the world’s largest plant families, with species represented in tropical, temperate, and boreal regions Because of their complex requirements for pollinators and mycorrhizal fungi, orchids are often habitat-specific and adapted to narrow environmental and climatic conditions [1]. In the U.S and Canada, precipitation thresholds during different phenological stages affect the flowering and reproduction processes of Platanthera praeclara Sheviak and Bowles [10,11] and the closely related Platanthera leucophaea [12]. As these species occupy fire-dependent grassland vegetation, additional monitoring is needed to assess fire impacts and interactions with precipitation [13]. Chase in the UK [16]

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Conclusion