Interactions between climate and ecosystems with complex topographic gradients generate unique source and sink habitats for water and nutrients as a result of precipitation, energy, and chemical redistribution. Here, we examine these phenomena for a high elevation site in the Colorado Front Range. Current changes in climate and atmospheric deposition of nitrogen to these systems are causing rapid changes in some portions of this system but not in others.  A conceptual model is presented that links terrestrial ecosystems to each other and to aquatic ecosystems. We report how atmospheric inputs as well as endogenous resources can be amplified or attenuated by transport processes.  High elevation lakes and the alpine tundra-forest ecotone are locations expected to receive the brunt of anthropogenic inputs obtained from the redistribution of exogenous materials from the regional environment, and from endogenous sources originating from other montane areas.

Increases in inorganic nitrogen (N) deposition from the atmosphere have also been detected (Burns 2003; Figure 3). Annual deposition of inorganic N in wetfall at the Niwot Ridge National Atmospheric Deposition Program (NADP) site showed an increase of 0.3 kg/ha/yr for the 1984-1996 interval (Williams and Tonnesson 2000).  The annual increase in deposition is caused by an increase in the volume-weighted concentration and from the absolute increase in precipitation that has occurred during this interval (Williams et al. 1996).

The Patterns

Nitrogen export per unit area declines with elevation (figure 7).

Inorganic nitrogen and organic carbon (and organic N) show inverse relationships with elevation (figure 9).

Soil characteristics fail to exhibit consistent patterns with either the age of the soil or with landscape position (figure 10).

Resolving the patterns

The landscape continuum model integrates terrestrial and aquatic components of high elevation ecosystems.  Three levels of shading represent block fields and talus on top, alpine tundra in middle regions, and subalpine forest beneath the alpine. Streams and lakes are labeled on the left side of the figure. Precipitation and dryfall containing dissolved inorganic nitrogen (DIN) with low amounts of dissolved organic nitrogen (DON) and other materials are deposited across a landscape that varies from zero plant cover to 100% plant cover, and from bare rock to deep organic soils. The absolute amount of atmospheric deposition of water and nutrients is greatest at the highest elevations.  Redeposition via mass flow and wind scour moves materials downhill and downwind.  Runoff that occurs over bare rock or frozen surfaces travels directly to aquatic systems, bypassing terrestrial biota that function as filters. Water transported through soils may contribute some DON to aquatic systems but provides very little DIN. Predictions of the ratios of net primary production (P) to animal and microbial respiration ( R) rates vary within different communities depending upon the sources of organic production (in situ or imported) relative to local respiration and export of this organic material.

Amplification and attenuation of water and nutrient inputs to ecosystems occur in all systems that experience directional physical forces of wind and gravity.  Wind scoured, heterogeneous landscapes are found on all continents.  These transport processes are significant at local scales, as represented by examples presented here, or at regional/global scales where materials from one continent are redeposited upon another land mass (Swap et al. 1992; Chadwick et al. 1999) or upon marine ecosystems (Garrison et al. 2003). 

The ability of natural terrestrial landscapes found adjacent to human dominated systems to persist in their present composition and functioning is a growing concern. This proximity causes modifications to microclimate and atmospheric inputs and alters natural disturbance regimes.  Our research indicates that what may appear to be relatively small changes in environmental drivers can be amplified within more distant ecosystems due to the significance of transport processes in redistributing materials across the landscape.