Interactions with Resident Vegetation - Constraints on Establishment

Interactions with Resident Vegetation—Constraints on Establishment 

Fossil pollen in a series of small forest hollows about 10 m in diameter provides a record of hemlock invasion of individual forest stands along a 10-km transect in northern Michigan. The distribution of species within the present-day forest, which has never been clearcut, is patchy—a mosaic of stands dominated by hemlock interspersed with mixed stands and large patches dominated by sugar maple (Acer saccharum). Pollen diagrams from four 

In this example, a portion of the landscape was occupied by resident vegetation that inhibited establishment of a migrating species. At present maple stands make up 12% of the upland landscape. Lakes and wetlands compose another 34% of the area (Pastor and Broschart, 1990), leaving only about half of the landscape available for colonization by hemlock. Wits the rate of hemlock 

FIGURE 5 Hemlock pollen percentages and concentrations plotted against the C-14 age of sediment in five small forest hollows located within hemlock stands in western Wisconsin. Arrows indicate the oldest sediment in which fossil hemlock stomata are found at each hollow. Stomata indicate the presence of one or more hemlock trees within 20 m of the hollow (Parshall, 1999). The increases in pollen concentrations (dashed lines) and percentages (solid lines) in the la.st 200-300 years indicate increasing hemlock population densities within the nearest 1-3 ha of forest. Most records indicate a long establishment phase between the initial colonization of the stand and the population increase. [Modified figure reprinted, with the author s permission, from Parshall (1998)]. 

Vegetation—Competition for Light and Resulting Constraints on Population Growth 

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Interactions with Resident Vegetation—Constraints on Establishment. 171... 

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