Effects of canopy light gap and early emergence on the growth and reproduction of Geranium maculatum

Abstract

Biomass and reproduction of Geranium maculatum L. were measured in an artificial canopy light gap and under a closed canopy. The effects of emergence date were also compared. Increased light in the canopy gap resulted in increased biomass, rhizome mass, and percent of plants flowering in the second growing season after gap formation. The importance of proximate carbon in reducing fruit abortion was determined experimentally. The rate of fruit abortion increased and the number of days required for leaf expansion decreased when the plants were shaded by neutral screens. The reproductive effort of G. maculatum is thus primarily controlled by the previous year's carbon gain, while the proportion of flowers that matured to seeds was a function of proximate carbon gain. Shoots of G. maculatum emerged over a period of approximately 6 weeks in early spring, with canopy closure occurring midway through this interval. Early emergence of plants under the intact canopy permitted exploitation of the increased light levels in the period prior to canopy closure. This led to slightly increased aboveground growth and rhizome storage and a significant increase in the time spent in leaf expansion, though no differences in reproductive output existed between early and later emerging plants. Early emergence was less advantageous for gap plants. The role of gap-phase growth in G. maculatum is in increased genetic recombination and dispersal during this short period when the balance of sexual versus asexual reproduction is altered.