Dutch elm disease and the vegetation composition of Manitoba's bottomland forests
Essenburg, Carol B.
In Manitoba's bottomland forests, elms infected with Dutch elm disease are either left standing or removed. Removal creates gaps of various sizes in the canopy whereas dead trees left standing create smaller openings. In order to determine the impact of Dutch elm disease on the forest, 31 5-year old removal gaps, 43 openings beneath dead elms (openings were < 15 years old) and a 15 ha clearing (13 years old) were located along the Red River near Winnipeg, MB. In each of these areas and the adjacent undisturbed forest, vegetation was sampled using 5x5m plots and the point-quarter technique for trees, and 2x2m quadrats for shrubs and herbs. Woody species taller than 10cm and a diameter at breast height of less than 5cm were considered shrubs, and separated into three height classes (< 1m, 1-2m, > 2m). The herb stratum included all non-woody and woody plants less than 10cm tall. The 82 vascular understorey species with total cover values greater than 5% and bare ground were subjected to correspondence analysis. The closed forest consisted primarily of Ulmus americana, Acer negundo and Fraxinus pennsylvanica. Saplings of these species (< 1m tall) made up 70% of the shrub stratum in both disturbed and undisturbed areas. Fraxinus pennsylvanica saplings dominated all areas except the clearing, where a few young A. negundo occurred. All areas had approximately 50 vascular herb stratum species. Native taxa such as Laportea canadensis and Smilacina stellata predominated everywhere except in the clearing. The clearing was colonized by non-native weedy species such as Arctium lappa and Sonchus arvensis. It was clearly separated from closed forest plots in correspondence analysis, but the gap and dead elm forest plots were not. Gap and closed forest plots clustered by location, possibly reflecting the patchy nature of the forest or its past history. Dutch elm disease may cause the current Ulmus-Acer dominated forest to become dominated by F. pennsylvanica and A. negundo with U. americana present as only small, short-lived individuals. The impact of the loss of mature U. americana trees is dependent on its importance in the stand. Canopy openings created by the removal or death of a small number of U. americana have an understorey composition similar to the undisturbed forest. Tree regeneration is not occurring in the large clearing created by elm removal. "Weedy" herbs and Matteuccia struthiopteris are dominating the clearing.