Academic journal article The American Midland Naturalist

Influence of Seed Characteristics and Site Conditions on Establishment of the Threatened Prairie Milkweed Asclepias Meadii

Academic journal article The American Midland Naturalist

Influence of Seed Characteristics and Site Conditions on Establishment of the Threatened Prairie Milkweed Asclepias Meadii

Article excerpt

Abstract.-

Population restoration and reintroduction are critical aspects of many plant conservation efforts. However, factors affecting the earliest life stages, critical to the establishment of new individuals, are often poorly understood. I investigated the influence of seed characteristics and manipulations of the field environment on seedling emergence and growth in Asclepias meadii (Mead's milkweed), a federally threatened tallgrass prairie species. Seeds of known mass and maternal plant were reared in a greenhouse and also in experimental restoration plots with combinations of pre- planting soil disturbance and spring burning treatments. In both the greenhouse and field, seed mass was positively correlated with emergence but not growth. Maternal relationships with emergence and growth were observed in the greenhouse but were generally undetectable in the field. Lower emergence was associated with field plot soil disturbance and burning, although there was no statistically significant effect of either treatment. Seedling growth did not appear to be affected by soil disturbance, but burning had a significant negative effect. Mass may be a useful metric for evaluating restoration seed stocks and the quality of seeds produced in maturing restoration populations. Pre-emergence manipulations of the restoration site did not facilitate emergence or growth and may have even been detrimental to restoration efforts. High survivorship of seedlings during their first year of growth and overwintering suggests that direct sowing of seeds into the field is an effective restoration technique for A. meadii.

Introduction

The re- establishment of self-sustaining populations is an important component of recovery plans for threatened and endangered plants (Pavlik et al., 1993). A study of recovery plans for federally listed plants found that 87% proposed réintroduction or population augmentation as part of the recovery effort (Kennedy, 2004). However, the conditions necessary for establishment of many target species are poorly understood. Factors influencing réintroduction outcomes include propagule source and quality (Vander Mijnsbrugge et al., 2010), as well as conditions and management of the restoration site (Wendelberger and Maschinski, 2009). Lessons learned from experimental re- introductions can also be used to encourage recruitment in natural populations and mature restorations (Pavlik et at., 1993).

The physical quality of propagules, in part reflected by mass, can affect germination rates and seedling growth (Morse and Schmitt, 1985), influencing restoration outcomes. Restorers can attempt to facilitate establishment by selecting high quality propagules and directly manipulating environmental conditions at restoration sites. Species in a variety of ecosystems require fire for their seeds to germinate, and natural or prescribed fires can be necessary to maintain native plant communities (Hulbert, 1986; Bell et al., 1993; Brockway and Lewis, 1997). Soil disturbance also influences recruitment in some grassland systems by reducing competition and providing microsites suitable for seedlings (Hobbs and Huenneke, 1992).

Restoration and re- introduction efforts are particularly important for Aslepias meadii Torrey ex A. Gray (Mead's milkweed), a federally threatened species. Populations are typically small and isolated, limiting gene flow and leaving them vulnerable to inbreeding depression and stochastic extinction (Tecic et ai, 1998; USFWS, 2003). Prior research developed restoration techniques for this species, but it also raised unanswered questions regarding seed ecology and seedling establishment. Germination studies by Betz (1989) and Row et al. (1999) suggest distinctions can be made between viable and nonviable seed but do not provide quantitative criteria for seed evaluation. Betz (1989) mentions seeds appearing nonviable; however, the reported seed germination rate pooled both apparently viable and apparently nonviable seed together. …

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