Variation in leaf traits of Apocynum cannabinum in the Quarry Bottom
Beth Drewes, Betsy Feighner, and Dana Dudle
Biology Department and Science Research Fellows Program, DePauw University
The nature park contains a large limestone quarry pit, abandoned in the mid-1970s and largely untouched since then. Today the quarry bottom is a harsh and variable environment, undergoing primary succession and lacking extensive vegetation cover or organic soil matter. Apocynum cannabinum is a perennial, herbaceous clonal weed native to the midwest. Throughout the quarry, A. cannabinum grows in a variety of water and light environments, and shows a high degree of phenotypic plasticity. A. cannabinum has great variation in leaf traits such as size, shape, hairiness, and angle to the stem. The purpose of our study was to identify relationships between leaf traits of A. cannabinum and the environmental conditions where they grow. We predicted that these traits would correlate with the plants' surrounding environment.
To conduct this study, we measured the percent water content in ground cover, we recorded the material at the base of plants, we measured the size of leaves, we counted the trichome and stomatal densities of leaves, and we measured the change in leaf inclination.
We quantified significant environmental heterogeneity in the quarry. Out of 246 plants, 45 percent were growing in sites covered by rocks > 2 cm. However, Apocynum grows in a variety of other habitats in the quarry bottom. Also, water availability varies significantly across microsites and over the growing season. For our plants designated as "shady" and "sunny", we found that plants in the shade have a higher proportion of water in the soil early in the growing season. In addition, shady sites lose more water because the ground cover of grasses and leaf litter does not protect against desiccation.
The leaf inclination was dictated more by overall light than time of day. Plants growing in the sun had greater incident light and leaves were closer to their stems, despite a circadian pattern in leaf movement. trichome density, stomata density, and leaf thickness did not show any significant relationships with the environmental factors. Light availability affects leaf length, with plants in the shade having significantly longer leaves. Plants with longer leaves were more likely to flower and produce more fruits.
Many questions remain about the relationship between these leaf traits and their fitness. Morphological variables do not sufficiently explain reproductive success of these plants. While 50 percent of the plants were flowering, it is still unknown which factors contribute to flowering. Other factors that may affect flowering include photosynthetic rate, water-use efficiency, and herbivory. Many traits also may have a strong genetic component that overrules other environmental influences.