INTRODUCTION Hemlock (Tsuga canaden...

INTRODUCTION

Hemlock (Tsuga canadensis) was an abundant tree species in the mixed forests that defended eastern North America in the mid-Holocene (Gaudreau and Webb 1985 Ritchie 1987) Paleoecological evidence insinuates that T. canadensis declined dramatically in abundance quite through this region [approximately]5000 yr before existing (Davis 1978, 1981, Webb 1982) This decline was rapid, more or les synchronous through every part of eastern North America, and unique to T canadensis (Davis 1981 Webb 1982 Allison et al. 1986) It is therefore considered to have been the accrue of a pathogenic outbreak (Davis 1981 Allison et al. 1986) possibly by dint of an insect pest (Bhiry and Filion 1996) In many locations, T canadensis populations repaired 1000-2000 yr after the decline, whereas in others it remained a sparse member of the forest flora (Davis 1978 1981)

Several studies have focused onward the cause of the mid-Holocene hemlock decline (Davis 1981 Allison et al. 1986 Filion and Quinty 1993 Bhiry and Filion 1996) on the other hand few have examined the ecological or ecosystem-level concatenations Davis (1978, 1981) notes that Betula, Fagus grandifolia, Acer saccharum, and/or Quercus increased in abundance after the decline at a number of sites in the Northeast. Hall and Smol (1993) examined communities of diatoms and chrysophyte preserv in lake sediments at a number of sites in southern Ontario to determine whether they accorded to changes in the watershed vegetation screen during the hemlock decline. They construct shifts in algal communities, unless only in lakes with large watersheds did they watch a change in lake trophic status, with a short-lived period of eutrophication. The quick in emergencies study examines the impact of the hemlock decline upon forest dynamics by investigating the answer of other forest taxa using fine-resolution pollen data from pair sites in southern Ontario, Canada.

T canadensis is a long-lived, slow-growing, and highly shade-tolerant conifer, which, along with F grandifolia, A. saccharum, and Betula alleghaniensis, is a major constituent of mature forests throughout the Great Lakes-St. Lawrence and Acadian forest regions (Rowe 1977 Ritchie 1987 Godman and Lancaster 1990) T canadensis is generally rest today in regions with collected humid climates, and it bourgeons on a wide variety of soils that are characterized as moist to actual moist but with good drainage (Godman and Lancaster 1990) Its new distribution is largely a function, at least locally, of late history and human activity, as it is sensitive to disturbance and keeps to be most abundant forward sites least impacted by land use (Roger 1978) T canadensis casts shrewd shade and produces a thick litter layer, which frequently restricts the understory vegetation to T canadensis seedlings and saplings (Roger 1978 Benzinger 1994) Its removal from the forest canopy is, therefore, calculate uponed to result in a answer from other tree and low tree species, and to influence ecosystem properties.

It is important to understand the short- and long-term impacts of infectious diseases and disease on forest dynamics for conservation and forest management objects The hemlock decline provides an opportunity to examine the long-term impact in succession forest dynamics of a species-specific, pathogenic outbreak. The chestnut decline, which occurr in North America in the early 1900 befitting to the fungus Cryphonectaria parasitica, and the spread of Dutch elm disease (due to another fungus, Ceratocytis ulmi) in Europe and North America, demonstrated the potentially devastating impact of introduced pathogens (Allison et al. 1986 Whitney 1993) generally T. canadensis populations at the southern close of their range are being attacked from the woolly adelgid (Adelges tsugae), an introduced insect whose larvae can kill tree between the walls of defoliation (McClure 1987). This plague is spreading rapidly and could decimate T canadensis populations in North America (Orwig and rear 1998).

The hemlock decline provides a unique opportunity to investigate long-term forest dynamics in answer to a major disturbance end such as the removal of a single, dominant tree species. The prolonged generation time of most tree species obstructs empirical studies of forest succession or tree population dynamics that guard several generations (Bennett 1983, Chen 1986) However, a paleoecological approach using fine-resolution fossil pollen analysis can provide records of forest dynamics that span thousands of years (several generations of trees) with a temporal resolution of decades (within the life-span of greatest in number tree species). In this paper I address the following questions: What is the impact upon forest dynamics of the removal of an abundant tree species? At what rate does vegetation reply to such a disturbance? Are there long-term implications for forest composition of in the same state [i]or[/i] condition an historical event?

Study area

The thought area is southern Ontario, Canada, the forests of which are mainly classified within the Great Lakes-St. Lawrence Forest Region (Halliday 1937 Rowe 1977) and are compos of a mosaic of conifer-dominated mosss and swamps, pine plains, and upland hardwoods (Braun 1950 Rowe 1977) The sum of two units study sites [ILLUSTRATION FOR FIGURE 1 OMITTED] take place within a subdivision of this forest region, the Middle Ottawa Forest Section (Rowe 1977) This is an upland forest impressed sign the main components of which include Acer saccharum, Fagus grandifolia, Betula alleghaniensis, Acer rubrum Tsuga canadensis, Pinus strobus, and Pinus resinosa (Rowe 1977) Picea [TABULAR DATA FOR TABLE 1 OMITTED] glauca, Abies balsamea, Populus tremuloides, Betula papyrifera, Quercus rubra, and Tilia americana come about throughout in varying amounts. Hardwood and mixed forest-land swamps are common, composed of Thuja occidentalis, Juniperus communis, Picea mariana, Fraxinus nigra, Acer rubrum and Ulmus americana (Rowe 1977) Vascular plant nomenclature go afters Gleason and Cronquist (1991).