Jerald J. Dosch

Seed rain during initial colonization of abandoned pastures in the premontane wet forest zone of southern Costa Rica

Understanding tropical succession requires insight into propagule availability, which constrains possible woody plant recruitment, yet seed rain composition in multiple post-agricultural sites has seldom been examined. We monitored seed rain for 60 wk in five abandoned pastures in southern Costa Rica, collecting a total of 1 140 688 seeds of 165 morphospecies. Most seeds (80.1%) arrived during the wet season. Species richness was highest in the wet season and greater in forest than in pasture. Seed rain density was greatest at the forest/pasture edge and decreased drastically just a few metres into pastures. In and near the forest, animal-dispersed seeds were more abundant than seeds dispersed by other means, while wind-dispersed taxa increased in relative importance at greater distances from the forest. Total seed input to pastures did not reflect size of adjacent forest fragments, although seed rain density varied more than threefold among sites. Among-site variation in density of regenerating woody seedlings was roughly proportional to among-site variation in seed rain. Morphospecies composition differed significantly among sites. Also, seed rain and woody plant colonists were rather dissimilar in composition, suggesting that while propagule availability is necessary for early woody plant establishment, it is a poor predictor of successional trajectory.

The Population Dynamics and Ecological Effects of Garlic Mustard, Alliaria petiolata, in a Minnesota Oak Woodland

Abstract Garlic mustard, Alliaria petiolata (M. Bieb.) Cavara & Grande, is an introduced biennial forb that has commonly been referred to as highly invasive and as having substantial negative effects on other plants in the eastern deciduous forests of North America. However, several recent studies have documented only modest effects on other plant species, raising questions as to the extent of the threat really posed by A. petiolata. Alliaria petiolata often exhibits an alternating two-year life-history cycle, with high rosette years alternating with high flowering stem years. It has been proposed that this cycle is partly driven by intraspecific competition between the stems and the rosettes. In a two-year study, we extensively sampled A. petiolata in a Minnesota woodland at two spatial scales, including 6.5 km of belt transects in a 6.8 ha study grid (20 × 20 m cells) and 90 small sampling quadrats (1.0 × 0.5 m) within the grid. At the large scale, we compared seed bank abundance and diversity of other herbaceous plants with A. petiolata abundance. Using the monitoring data we also investigated whether this population was exhibiting an alternating two-year life-history cycle, consistent with the intraspecific competition hypothesis for this phenomenon. At the small scale, we compared A. petiolata abundance with the abundance of other plants, including herbs, ferns, shrubs, and tree seedlings. We also conducted an ex-situ pot experiment in which we planted seeds of six tree species in soil collected from dense A. petiolata patches and soil collected where A. petiolata was absent and recorded emergence rates and seedling growth over an 8 wk period. Overall, we found little evidence that A. petiolata was negatively affecting other plant species. This is consistent with other recent studies and indicates that, despite earlier claims to the contrary, A. petiolata seems to be more a product than an agent of change in eastern North American deciduous forests. We also documented an alternating two-year life-history cycle, providing additional evidence to support the hypothesis that this cycle is at least partly being driven by intraspecific competition.

Population and plant community dynamics involving garlic mustard (Alliaria petiolata) in a Minnesota Oak Woodland: a four year study.

Abstract Garlic mustard, Alliaria petiolata (M. Bieb.) Cavara & Grande, a European biennial, has become common in many Midwestern and eastern North American forests. Researchers have sometimes documented a two-year (two-point) population cycle in which the vegetative and reproductive phases alternate in abundance. However, little research has been conducted on the scale dependence of the two-point cycle and the extent to which the cycle is dependent on the density of A. petiolata. In addition to this interest in A. petiolatas population dynamics, researchers have investigated A. petiolatas effects on other plant species, often concluding that it displaces native herbs and inhibits tree seedling growth and survival through the production of allelochemicals. In order to examine both the population dynamics of A. petiolata and its possible effects on North American native plants, we conducted a four year study in which we monitored A. petiolata in a 6.2 ha portion of a Minnesota oak (Quercus) woodland at two spatial scales (0.5 m2, 400 m2). We also conducted an indoor seedling pot study to investigate whether any soil inhibitory effects associated with A. petiolata may be exacerbated under drought conditions. We found that a two-point cycle was not sustained during the four year study throughout the entire study site. However, the two-point cycle was exhibited in high A. petiolata density areas in the study site, providing support for the hypothesis that the cycle is driven by competition between first-year (rosette) and second-year (stem) plants. Both the monitoring study and the pot experiment yielded little evidence that A. petiolata is a major agent of ecological change in the woodland. Rather, the data indicate that over the four years of this study A. petiolata and the other plant species in the woodland we studied are changing in abundance largely independent of one another.

Effects of urbanization on the population structure of freshwater turtles across the United States: Urbanization Effects on Turtles

Landscape-scale alterations that accompany urbanization may negatively affect the population structure of wildlife species such as freshwater turtles. Changes to nesting sites and higher mortality rates due to vehicular collisions and increased predator populations may particularly affect immature turtles and mature female turtles. We hypothesized that the proportions of adult female and immature turtles in a population will negatively correlate with landscape urbanization. As a collaborative effort of the Ecological Research as Education Network (EREN), we sampled freshwater turtle populations in 11 states across the central and eastern United States. Contrary to expectations, we found a significant positive relationship between proportions of mature female painted turtles (Chrysemys picta) and urbanization. We did not detect a relationship between urbanization and proportions of immature turtles. Urbanization may alter the thermal environment of nesting sites such that more females are produced as urbanization increases. Our approach of creating a collaborative network of scientists and students at undergraduate institutions proved valuable in terms of testing our hypothesis over a large spatial scale while also allowing students to gain hands-on experience in conservation science.