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Harvard Forest Symposium Abstract 2005

  • Title: Predicting How Changes in Forest Stand Composition Will Alter Stream Ecosystems at Multiple Spatial and Ecological Scales
  • Primary Author: Betsy Colburn (Harvard Forest)
  • Additional Authors: Michael Bank (Harvard School of Public Health); Emery Boose (Harvard Forest); David Foster (Harvard University); David Orwig (Harvard Forest)
  • Abstract:

    How do large-scale changes in vegetation across the landscape affect such closely related phenomena as fluxes of energy and nutrients, trophic dynamics, and the distribution and accumulation of metals in stream ecosystems? This question is a key challenge in ecology, involving as it does the explicit assessment of coupled terrestrial-aquatic systems at multiple spatial and temporal scales and ecosystem levels. It rests on two fundamental tenets of stream ecology: that streamflow, water quality, and aquatic trophic dynamics are tightly coupled to vegetation and land use in watersheds (Hynes 1975; France 1996, 1997; Price et al. 2003; Allan 2004; England and Rosemond 2004), and that, in forested watersheds, the primary energy sources for stream biota change from headwaters to the mainstem, with upper reaches fueled primarily by allochthonous inputs of leaf litter and woody debris from the adjacent forest, middle reaches experiencing more in-stream photosynthetic production due to decreased canopy closure, and lower reaches depending heavily on materials transported downstream from smaller tributaries (Vannote et al. 1980, Cummins et al. 1989, Bilby and Bisson 1992, Wallace et al. 1997, Peterson et al. 2001, Cole et al. 2003, Bernhardt and Likens 2004). Patterns in biological community composition and functional-feeding-group structure change longitudinally along the stream’s length in parallel with changes in energy sources (Wallace and Gurtz 1986, Bilby and Bisson 1992, Stone and Wallace 1998, Kiffney et al. 2004). The bioavailability and bioaccumulation of metals such as aluminum and mercury in aquatic food webs similarly vary with forest composition, water quality — especially pH and dissolved organic carbon — and food-web structure and may feed back negatively on production, foraging ability, community structure, and survival of biota in some aquatic communities (Gilmour and Henry 1991; Stebbins and Cohen 1995; Sparling and Lowe 1996; Thompson 1996; Evers et al. 2003; Webber and Haines 2003; Wiener et al. 2003 and references therein; Bank et al. 2005a, 2005b).





    We are starting to explore relationships between forest composition and stream ecosystem fluxes of energy, nutrients, and metals in the context of an ongoing natural experiment, in which a dominant conifer, eastern hemlock (Tsuga canadensis), is being killed by an invasive insect and replaced by deciduous trees over a large landscape in eastern North America (Orwig and Foster 1998, Orwig et al. 2002, Stadler et al. 2005). The study will investigate relationships between forest characteristics (stand composition and structure, soil characteristics, C:N:P ratios, metals) and the nature of headwater streams (carbon and nutrient sources and ratios, aluminum and mercury concentrations and bioaccumulation/ biomagnification, invertebrate and vertebrate functional feeding groups, and food web structure), at landscape and catchment scales. Preliminary data show differences in decomposition rates of hemlock and deciduous leaf litter. In addition, we will conduct reach-scale experiments to test hypotheses that litter composition and light availability are causal factors behind observed differences between streams in coniferous and deciduous forests. Our overall objective is the prediction of changes in stream ecosystems over time as hemlock forests decline and are replaced by deciduous forests.





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