Growth and Reproductive Allocation of Adenocaulon Bicolor Following Experimental Removal of Sunflecks

We used experimental and multiple—regression approaches to test the hypothesis that the availability of direct light (sunflecks) is important for the growth and reproduction of Adenocaulon bicolor, a perennial herb occurring in the understory of a coastal redwood forest. Plants were shaded from direct—beam light (PFD = photosynthetic photon flux density) with shadow bands that still allow receipt of most of the diffuse light. After two growing seasons the shaded plants receiving only diffuse light were significantly smaller than control plants that received both direct and diffuse light. In general, shaded plants decreased in size whereas those receiving sunflecks remaining at the same size. Reproduction was reduced more than vegetative size in the shaded plants. Despite the dramatic effect of removing sunflecks from the light energy available to individual plants, we found no significant relationship between plant performance and the amount of direct PFD available in a microsite, as predicted from hemispherical canopy photographs. Instead, the size and reproductive output of plants growing under natural light regimes was correlated with the average daily diffuse light availability. Although additional direct PFD could be expected to increase carbon gain, the lack of a correlation between the amount of PFD and plant performance may be due to other factors such as water or nutrient stress that increase in microsites with increased direct light.