PARENT SESSION
Contributed Oral Session 134: Forest Ecology: Reproduction and Recruitment
Thursday, August 11, 1:30 PM - 5:00 PM, Meeting Room 516 D, Level 5, Palais des congrès de Montréal

Potential effects of plant spacing and stand identity on herbivory and fruit set in Dirca palustris L.

Schulz, Kurt^*,1, Burleyson, Travis¹, Turner, Bryan¹, Mattson, William ², Zasada, John³, ¹ Southern Illinois University Edwardsville, Edwardsville, Illinois² USDA-Forest Service, North Central Forest Experiment Station, Rhinelander, Wisconsin³ USDA-Forest Service, North Central Forest Experiment Station, Grand Rapids, Minnesota

ABSTRACT- Eastern leatherwood (Dirca palustris L.) is a medium-sized shade tolerant shrub of eastern North America. It achieves significant abundance on mesic sites in the Western Great Lakes Region. The distribution pattern of leatherwood across the Michigan Upper Peninsula and northern Wisconsin landscape is surprisingly complex: many ostensibly suitable habitats contain few or no leatherwood. Local populations are sometimes quite large, but exhibit strong patterns of aggregation at scales up to ca. 0.3 ha. Limited seed dispersal is very clearly a cause for local abundance and scarcity. The fleshless drupes produced by leatherwood are apparently not transported, but immediately opened by rodents for the large seed inside. Roughly 60% of established seedlings are found beneath their putative mothers. Fruit production is strongly related to shrub size, which creates a strong demographic inertia once a population is founded. The leafminer Leucanthiza dircella regularly damages leatherwood leaves in midsummer; leafminer attack can be correlated with reductions in branch extension growth the following year. Multi-year records of branch growth show patterns consistent with insect attack as opposed to variation in weather. We examined how proximity to neighboring shrubs affects the extent of leaf miner attack and rates of fruit set in 120 shrubs sampled across twenty sites in the Ottawa NF, MI. The proportion of leaf area lost to miners was not related to shrub spacing within stands, but showed strong differences across stands (range 6-58% loss). Rates of fruit set were also not related to spacing, but to stand identity (range 2-47% set). Rates of herbivory in nearby sites are more similar than those at greater distance; this pattern is less marked for fruit set. This study suggests that leatherwood population dynamics may differ strongly across the landscape if hotspots of leafminer attack persist year to year at the same location.

Key words: Dirca palustris, leafminer, spatial pattern, Leucanthiza dircella