Life History and Environmental Factors Influence Population Density and Stage Structure in Hydrophyllum Brownei
Marsico, Travis D., The American Midland Naturalist
Hydrophyllum brownei is a rare endemic species restricted to the Ouachita Mountains of Arkansas. One objective of this study was to model H. brownie population density by investigating the influence of sun exposure, soil characteristics and crowding. The most parsimonious model including shade alone best predicted population density, in which increasing shade was correlated with greater population densities. Reproductive capacity of different parts of the root was tested and individuals of the species were found to produce vegetative shoots from all portions of the root. This indicates that individuals are prolific vegetative reproducers, especially in circumstances of intense physical soil disturbance that break apart root systems. Leaf number was strongly correlated with number of root swellings and was determined to be a good predictor of individual plant stage. It was found that populations were structured either "normally," with about equal numbers of individuals in all stage classes, or "dynamically," skewed to a greater number of early stage individuals. Levels of shade relate to population density and site disturbances likely influence the density and stage structure of populations due to the life history trait of extensive vegetative reproduction from the roots. Further questions about genetic diversity and the ability to colonize new sites should be investigated to gain a better understanding of limits to H. brownei's distribution.
Hydrophyllum brownei Krai & Bates (Hydrophyllaceae) is a rare endemic plant species known to grow only in eight Arkansas counties in the Ouachita Mountains Natural Division. Krai and Bates (1991) described this species as distinct from its closest morphological congener, H. macrophyttum, based primarily on distinctive rootstock attributes. The U.S. Fish and Wildlife Service expressed interest in this species as a possible addition to the federal Endangered Species Act and contracted FTN Associates to conduct a status survey in 2001 (FTN Associates, pers. comm.). As research continued on the species in 2002, previously unknown populations were discovered and recommendations were made to lower the rarity ranking of H. brawnei (Marsico, 2003). In addition, quantitative measures were taken about location and characteristics of landscapes in which the populations grow (e.g., proximity to streams, amount of shade, associated plant and canopy species) (Marsico, 2003).
In 2002 it appeared that populations varied in size and density and that size of plants and proportion of flowering individuals also differed among sites. The causes of these observed differences are investigated in the present study. The most likely environmental factors affecting growth of individuals in populations were amount of shade, soil texture properties and interspecific competition (Krai and Bates, 1991; FTN Associates, pers. comm.).
Shade is an important factor for species in the genus Hydrophyllum (Beckmann, 1979). FTN Associates (pers. comm.) noted that although the amount of shade is variable at H. brawnei sites, all populations grow under riparian hardwood shade. Soil texture also seemed important because both Krai and Bates (1991) and FTN Associates (pers. comm.) noted that populations grow in silt-loams with varying fractions of sand and clay. Soil texture may be a single measurement for the complicated relationship between soil moisture and mineral content because soils high in clay hold more minerals (due to clay particle composition and negative charge of the particles) and more moisture (due to the small size of the particles and pores and the large surface area of the particles) (Brady and Weil, 2000). Krai and Bates (1991) and FTN Associates (pers. comm.) discussed the frequency with which H. brawnei is found growing in a rocky or gravelly substrate and suggested that this species was well adapted to rocky soils. Several populations considered (subjectively) vigorous in 2002 were found rooted in soil lacking gravel. …