Evaluating effects of stress on orchid populations with integral projection models

Background/Question/Methods

Environmental conditions impact life cycles of organisms and ultimately affect population growth.  In perennial plants, stressful conditions may result in vegetative regression, stasis, or dormancy while favorable conditions may result in growth and investment in reproduction.   Demographic analysis is useful in assessing the effects of different conditions on populations.   Integral Projection Models (IPM's) are demographic models that evaluate the importance of life cycle stages and other selected attributes to population growth. IPM's are useful for modeling populations with complex life histories because they do not require individuals to be grouped into discrete classes.   Orchids have particularly complicated life cycles, with long life spans and periods of dormancy that can last for several years.  Here, we use an experimental approach and IPM's to determine how stressful environments may affect orchid life histories and population dynamics. We established shaded, defoliated, and control treatments in three populations of Cypripedium calceolus and two populations of Cephalanthera longifolia in Estonia. Shading and defoliation treatments simulated stressful environments. On an annual basis from 2002 to 2008 we monitored individuals for survival, and the number of sprouts, leaves, and flowers. We analyzed the data with IPM's.

Results/Conclusions

Effects of shading and defoliation differed markedly between species. Both treatments decreased population growth in Cypripedium, but had varying effects on population growth among populations of Cephalanthera.  Both treatments caused drastic decrease in fertility of Cypripedium plants, as well as reduced growth to large size classes.  However, the effects of treatments on Cephalanthra were not as clear.   For both species defoliation increased dormancy, while shading did not.  Our results indicate that as a response to environmental stress, orchids reduce growth and reproduction and may go dormant. However, the specific responses to stress may differ among species and populations depending on individual site conditions and population characteristics.