Douglas H. Boucher

Tree species-richness and topographic complexity along the riparian edge of the Potomac River

Abstract We studied the riparian edge of a central portion of the Potomac River in order to test the hypothesis of a positive relationship between small-scale topographic complexity of the riverbank profile and tree species-richness. A total of 153 5xa0m-wide transects established at 530xa0m intervals, containing 2568 trees, were measured along 97xa0km of the river corridor between Harpers Ferry, WV and Hancock, MD. Standardization of species-richness between plots was done by rarefaction. Small-scale topographic complexity was calculated using the standard deviation of slope measurements along the length of each transect. The tree species dominating the community are: silver maple ( Acer saccharinum ), boxelder ( Acer negundo ), and sycamore ( Platanus occidentalis ), together comprising 62% of the stems and 80% of the basal area. A statistically significant positive relationship was observed between tree species-richness and topographic complexity. Analysis of covariance indicated that adjacent land use and bedrock geology had no statistically significant impact on species-richness, and that the simple linear-regression model was adequate to describe the effect of topographic complexity on species-richness. A significant negative relationship was observed between tree species-richness and the width of the riparian zone; this relationship was consistent across geology. Width of the riparian zone was the best overall predictor of tree species-richness. Tree-species distributions suggest that this is an edge effect, due to a greater number of tree species being able to take advantage of a combination of an increased light gradient and a stable, well-drained substrate (the C & O Canal towpath) directly adjacent to the river, in narrower riparian sections.

MULTIPLE BASINS OF ATTRACTION IN A TROPICAL FOREST: EVIDENCE FOR NONEQUILIBRIUM COMMUNITY STRUCTURE

We analyzed a data set accumulated over a 12-year period subsequent to the catastrophic disturbance of a rain forest by a hurricane. We analyzed the data to determine whether observed trajectories correspond to expectations from an equilibrium or nonequilibrium model of community structure. The equilibrium case, implying a single basin of attraction, suggests that trajectories should become ever more similar over time. The nonequilibrium case, implying multiple basins of attraction, suggests the opposite. The data from this particular forest, located on the eastern seaboard of Nicaragua, strongly support the multiple basin hypothesis, and thus the nonequilibrium model.

Post-agriculture versus post-hurricane succession in southeastern Nicaraguan rain forest

Wecompared five-year old forests developing after agriculture to those recoveringfrom Hurricane Joan (1988) and to the pre-hurricane forest, at two sites intropical rain forest in southeastern Nicaragua. We used non-parametric clusteranalysis to group transects by their species compositions, and compared theirspecies richness, estimated total species richness, dominance, density andbasalarea. Post-agriculture transects showed distinctive species compositions andlower diversity than post-hurricane transects, which were in turn more similarto the pre-hurricane forest. These results are similar to those found by otherresearchers in the Amazon and in Puerto Rico. Land use history was moreimportant than proximity in the landscape in determining the composition andstructure of post-disturbance forests in this region.

Recruitment and decay of a pulse of Cecropia in Nicaraguan rain forest damaged by Hurricane Joan : relation to mutualism with Azteca ants

In October 1988, Hurricane Joan severely damaged 500,000 ha of rain forest in southeastern Nicaragua. By and large, the forest is regenerating directly without passing through a secondary stage. However, an apparent exception to the pattern of direct regeneration was the behavior of Cecropia, a pioneer tree which experienced a dramatic pulse of recruitment in some areas. Nineteen ninety-one was the peak year of recruitment of Cecropia, but since then its pulse has decayed rapidly due to heavy mortality. The decline appears to be related to the unusually low rate of occupation by Cecropias mutualistic Azteca ants: unoccupied trees had significantly higher mortality rates than occupied trees.

Newtonian ecology and beyond

(1998). Newtonian ecology and beyond. Science as Culture: Vol. 7, Natural Contradictions, pp. 493-517.

A regional assessment of white-tailed deer effects on plant invasion

Analyses deriving insights from 23 research sites across the northeastern and east central US reveals that white-tailed deer lower native plant diversity and increase the fraction of the plant community made up of non-native species. Particularly, deer increase the abundance of the invasive plants garlic mustard and Japanese stiltgrass. Deer are known to avoid eating these species in favour of more palatable ones, thereby indirectly facilitating the success of the non-native invasives. Managing deer abundance has implications for forest plant communities. By maintaining lower deer densities, native plants likely will be better sustained and invasions of unpalatable non-native plants limited.