Factors affecting survival, flowering and population growth for a moth pollinated prairie orchid

Background/Question/Methods

Eastern Prairie Fringed Orchid (Platanthera leucophaea) occurs in tallgrass prairies, sedge meadows, fens and sphagnum bogs of the central and eastern Midwest.  This species is Federal threatened and Illinois endangered largely due to habitat conversion to agriculture.  Pollination is by hawkmoths and seedling establishment requires development of a mycorrhizal relationship with soil fungi.  To increase the number of viable Eastern Prairie Fringed Orchid populations in Illinois, a volunteer network, coordinated by the US Fish & Wildlife Service, was established to monitor and cross-pollinate plants in 36 Illinois natural and introduced populations differing in severity of management needs.  We evaluate management effects on survival, fecundity and population viability.  Demographic monitoring data were collected annually, during peak flowering season in June or July and again when fruits were mature in September. Demographic data comprise 5943 permanently marked plants from 1998-2012.  Monitoring data included presence/absence, height, leaf number, length of the longest leaf, number of flowers, number of hand-pollinated flowers, number of fruits, herbivory severity, and whether the plant location was burned since last growing season.

Results/Conclusions

Logistic regression indicated that survival was significantly higher for plants in burned habitat (R_N² = 0.105, P <0.001) and with low management needs (R_N² = 0.011, P <0.001), but was lower for plants with severe herbivory damage (R_N² = 0.004, P = 0.004).  Hand pollination increased fruit production (t = 2.034, P = 0.042), but did not affect survival (R_N² = 0.001, P = 0.100). Fecundity was significantly higher for plants in burned habitat (F = 4.023, P <0.001) and lower for plants with severe herbivory damage (F = 93.555, P = 0.045).  Population viability analysis using demographic matrix models indicated that population growth rates (λ) were significantly higher for plants in burned habitat (λ_burn = 1.78, λ_unburn = 1.22), normal rainfall (λ_drought = 0.75, λ_normal = 1.27, λ_wet = 0.82), crossing between populations (λ_{outcrossing btw pops} = 1.79, λ_{outcrossing within pops} = 1.13, λ_selfing = 0.91), but did not differ among management needs or herbivory levels. These results suggest that prescribed burn and hand pollination among populations can improve Eastern Prairie Fringed Orchid population viability.