Sunfleck effects on physiology, growth, and local demography of American ginseng (Panax quinquefolius L.)

Abstract

The light environment of the forest understory is heterogeneous, characterized by low levels of diffuse light punctuated by short periods of direct solar radiation known as sunflecks. Less well understood is the degree to which sunflecks are responsible for spatially varying performance of understory plants. We studied natural variation in the light environment, as well as physiology, growth, and microsite demographics of American ginseng (Panax quinquefolius L.) to test whether clustering of plants within a population was related to sunfleck variation. HOBO dataloggers were positioned in 10 clusters within a single ginseng population to measure the local understory light environment during mid-season (June 27 to August 9 2011). Photosynthesis and transpiration were measured with a Li-Cor 6400 portable photosynthesis system. Population census data were used to obtain measurements of the relative growth rate (2011–2012), seed production efficiency (2011) and seed germination (2009–2012) of plants in the same subpopulations. All measured dependent variables were regressed against the percentage of mean photosynthetic photon flux density contributed from sunflecks and mean length of the longest sunfleck. Light saturated photosynthetic rates of ginseng showed a positive linear relationship to both sunfleck characteristics, while transpiration was unaffected. The relative growth rate of established ginseng plants also had a positive linear relationship with the length of the longest sunfleck. However, germination rate was negatively affected by an increasing percentage of photosynthetic photon flux density from sunflecks. Seed production efficiency was not influenced by either sunfleck characteristic. Due to negative effects on germination, initial ginseng cluster formation is likely not associated with sunflecks, however photosynthetic and growth responses suggest that fitness effects are positive for established individuals. A spatially and temporally varying light environment is beneficial for ginseng population growth.