Monday, August 2, 2010 - 3:40 PMOOS 2-7: The rise and fall of eastern hemlock: perspectives from the Northwoods and Appalachia
Michael A. Jenkins, Purdue University, Christopher R. Webster, Michigan Technological University, and Eric J. Holzmueller, Southern Illinois University.
Background/Question/Methods Eastern hemlock (Tsuga canadensis) is a wide-ranging species that occurs throughout the Appalachian Mountains and across the northern Great Lakes region. Using historical accounts, vegetation surveys, field experiments, and remeasurements of long-term vegetation plots, we examined the historic importance and contemporary regeneration dynamics of hemlock at distal points in its range, the Northwoods of the Great Lakes region and the southern Appalachian Mountains. From this examination, we identified historic and contemporary factors that have resulted in the greatly differing, but converging, fates of hemlock in the two regions.
Results/Conclusions In the late 19th century, hemlock was a regional dominant in the Northwoods, but now only occurs across a small fraction of sites it formerly occupied. Regeneration failures have been noted in the Northwoods since the 1940s. In areas with low deer abundance, hemlock is a persistent understory species that is favored by small canopy gaps. However, in areas of higher deer abundance, similar-sized gaps are generally captured by maple species (Acer spp.). In a study that examined regeneration in 39 stands across the Upper Peninsula of Michigan, variation in seedling abundance was associated with the availability of suitable microsites, whereas recruitment into larger size classes was closely associated with the level of winter deer use. The balance of this work suggests that deer use of hemlock stands creates a feedback with maple that is detrimental to the establishment of hemlock.
Conversely, hemlock was widely distributed in forests of the southern Appalachians that survived logging in the early 20th century. Long-term plot data show that hemlock expanded with fire suppression into forest types where it was previously uncommon. In these forests, it created a dense understory layer that excluded competitors and allowed hemlock to persist and create more mesic understory conditions. However, the hemlock woolly adelgid (HWA, Adelges tsugae) was first found in the southern Appalachians in 2002, and by 2008 data from long-term vegetation plots revealed heavy decline of trees and the initiation of a wave of mortality that continues. HWA mortality occurred first in the understory, resulting in a scarcity of hemlock regeneration as trees in the overstory began to decline. This heavy mortality of regeneration suggests that successful chemical treatments of overstory hemlock will only preserve the current generation of trees. If the range of HWA extends northward with predicted warming trends, the long-term persistence of hemlock across much of its range is precarious at best.
Results/Conclusions In the late 19th century, hemlock was a regional dominant in the Northwoods, but now only occurs across a small fraction of sites it formerly occupied. Regeneration failures have been noted in the Northwoods since the 1940s. In areas with low deer abundance, hemlock is a persistent understory species that is favored by small canopy gaps. However, in areas of higher deer abundance, similar-sized gaps are generally captured by maple species (Acer spp.). In a study that examined regeneration in 39 stands across the Upper Peninsula of Michigan, variation in seedling abundance was associated with the availability of suitable microsites, whereas recruitment into larger size classes was closely associated with the level of winter deer use. The balance of this work suggests that deer use of hemlock stands creates a feedback with maple that is detrimental to the establishment of hemlock.
Conversely, hemlock was widely distributed in forests of the southern Appalachians that survived logging in the early 20th century. Long-term plot data show that hemlock expanded with fire suppression into forest types where it was previously uncommon. In these forests, it created a dense understory layer that excluded competitors and allowed hemlock to persist and create more mesic understory conditions. However, the hemlock woolly adelgid (HWA, Adelges tsugae) was first found in the southern Appalachians in 2002, and by 2008 data from long-term vegetation plots revealed heavy decline of trees and the initiation of a wave of mortality that continues. HWA mortality occurred first in the understory, resulting in a scarcity of hemlock regeneration as trees in the overstory began to decline. This heavy mortality of regeneration suggests that successful chemical treatments of overstory hemlock will only preserve the current generation of trees. If the range of HWA extends northward with predicted warming trends, the long-term persistence of hemlock across much of its range is precarious at best.