Print PageThe expanding mortality of eastern hemlock (Tsuga canadensis, hemlock) due to an invasive pest insect is often used as a case study of the loss of a foundation species. Foundation species are thought to be the defining driver of locally stable ecosystem processes, which support the populations of co-occurring species. Hemlock therefore defines a specific community state and the loss of this foundation should result in the development of alternate community states. What is less clear is how much divergence there will be among the new states. Conceptually, we believe that hemlock dampens underlying compositional and functional differences at local and regional scales. In dynamic regime or alternate state literature, community states are often depicted as a ball at a certain position within peaks and valleys of a surface. Our hypothesis is that hemlock maintains a community within a broad valley. However, the disturbance of hemlock mortality will push communities across the surface in different directions, depending on environmental context. In other words, multiple community states with distinct functional properties will develop if/when hemlock is eliminated.
Results/Conclusions
We are comparing composition and function across a chronosequence of riparian forests dominated by healthy, declining and dead hemlock in the central Appalachians. Our results indicate that in healthy forests, species functional groups differ between transects at 10, 30, and 50 m from headwater streams. However, these differences are not evident in ecosystem processes. Specifically, light availability in the growing and dormant seasons, rates of decomposition, and leaf littler biomass do not differ with increasing distance from the stream. On a regional scale, functional species groups differ within hemlock ecosystems. Sites in the Allegheny Mountains of West Virginia have lower species richness than the Unglaciated Allegheny Plateau of Ohio. Across the Allegheny Mountains, Rhododendron (Rhododendron maxima) will be a foundational shrub where it is present, regulating forest processes in many areas, but is absent on the Unglaciated Allegheny Plateau. Our results indicate that the hemlock community state encompasses a wide range of physiographic and soil conditions. However, underlying differences will cause re-developing systems to diverge, resulting in multiple community states within and between ecosystems. Understaning the this variance in ecosystem response is critical to inform management and restoration of forests with high hemlock mortality.
