PS 18-254 - Increased light availability slows population growth of the Neotropical understory herb Geoppertia marantifolia

Abstract: Many Neotropical understory herbs are present across the gradient in light from the deep shade to the relatively open canopy of tree-fall gaps. The influence of light on individual demographic rates is not always positive and it is unclear how overall population growth correlates with light availability. We investigated the population dynamics of the Neotropical understory herb Goeppertia marantifolia across a continuous light gradient within a secondary forest. Demographic data (growth, survival, fruit production and vegetative offspring production) and estimates of canopy openness for individuals were recorded in Corcovado National Park, Costa Rica from August 2004 until March 2007 (N = 2168). We collected data for six census intervals, including three August-to-March and three March-to-August seasonal periods. Data were used to determine the effect of light and size on demographic rates using multiple regression techniques. The impact of light on individual demographic rates varied by light and by plant size. The impact of light and plant size was most dramatic for survival, where light had a strong positive effect on survival of small plants but a negative effect on survival of large plants. We evaluated an integral projection model of population dynamics using the regression parameters at four light levels. Across all years we found the rate of population growth (λ) was faster in low light conditions and declined with increasing light availability. These results suggest that the negative impact of light on the survival of large plants may lead to a negative correlation between light and population growth across light gradients.