Succession and disturbance affect the population dynamics of many rare plants. The goal of this research was to investigate how succession and disturbance impact the globally rare and declining fen plant Trollius laxus (Ranunculaceae) by focusing on the effects of light intensity, hydrology, and forest canopy gaps.
I examined the relationships between light transmittance and hydrology and the vigor of 30 T. laxus subpopulations in a forested fen. Although T. laxus emerges and blooms early in the spring, spring and summer light had similar positive relationships with subpopulation vigor. Diffuse light was more strongly associated with vigor than direct light. High spring groundwater level was related to greater reproductive output. The positive relationship between light and follicle production was moderated by post-leafout, mean groundwater levels.
A greenhouse experiment gave further evidence that light intensity and water depth interactively affect T. laxus growth. Highest plant performance occurred at high light intensity and low water depth. Plants showed no response to differences in water depth when highly shaded. Increased light caused greater biomass allocation to roots versus shoots, but water depth did not influence allocation patterns. These results were compatible with predictions that rare plants may be excluded from drier, more fertile wetlands by plants that are better competitors for light.
Using stage-based matrix modeling, I examined the subpopulation dynamics of T. laxus in a forested fen. On average, T. laxus subpopulations responded to experimental canopy gaps with first increasing, then declining stem and flower production and population growth rates (λs). The declines, which increased in severity with greater light transmittance, were likely caused by herbaceous layer competition and litter accumulation. There was a unimodal relationship between diffuse light and λ, indicating that T. laxus will persist at forested fens in small gaps that transmit low to intermediate levels of light. These results showed that canopy gaps can have nonlinear and indirect effects on understory plant demography. The similarities between the demography of T. laxus and European fen species suggest that active management will be needed to conserve the rare and threatened plants that are characteristic of North American fens.