Ongoing Research
D. Warren

Influence of woody debris on stream nitrogen dynamics      

Collaborators:
Dr. Darren Bade
Dr. Kristin Judd
Dr. Gene Likens
Dr. Clifford Kraft

      Headwater streams can be important areas of stream nitrogen retention. My dissertation research focuses on evaluating the influence of wood in streams on the uptake and retention of nitrogen (as nitrate) in forested headwater streams. Wood in streams is has been shown to increase the retention of both organic matter and fine inorganic sediments which should both directly and indirectly increase in nitrogen uptake and retention.  I am conducting research in three watersheds across the northeastern United States Hubbard Brook, NH; Sleepers River, VT; and Frost Valley (the Model Forest), NY. At Sleepers River we have a pair of first-order streams with second-growth mixed hardwood-conifer riparian and upland forests. Measurements of stream Nitrate uptake were conducted in both streams three times during summer 2005 using a modification of the LINX II protocols for 15N-NO3- tracer additions. The first stream acted as the reference stream for our study and will remain unmanipulated.  In the second stream nearly all wood in the stream channel was removed in September 2005, after the final pre-treatment nitrate uptake measurement was conducted (excluding 4 pieces that were key in bank stability).  In summer 2006, and 2007 we will repeat the nitrate uptake measurements and compare the difference between SR-S and SR-N uptake before versus after the manipulation.  At Hubbard Brook, we have a wood removal and a wood addition stream.  The three streams in this study are all small streams with north-facing watersheds.  Wood removal was conducted in mid-October 2005 [before and after pictures].  Beginning in Summer 2006 an addition set of three streams were established in the  in Frost Valley, NY at the Model Forest.  Pre-treatment data were collected in summer 2007 and fall 2007.  Wood manipulations were conducted in fall 2007 after surveys were conducted.  Post treatment data will be collected in summer 2008 and fall 2008.













Woody debris dynamics in northeastern stream systems 

Collaborators:
Dr. Clifford E. Kraft
Dr. William S. Keeton
Dr. Gene E. Likens

      Wood serves multiple functions in stream ecosystems and the relative importance of wood in a stream can depend upon multiple biotic and abiotic factors.  For the purposes of my research, I have focused on "large woody debris" defined as dead wood in the stream channel greater than 10 cm diameter and greater than 1 m length, an operational definition that has become well established in the literature.


1) Wood movement  
In fall 2000 I tagged 116 pieces of large wood in a second-order section of Rocky Branch, a stream in the eastern Adriondack Mountains of New York.  I returned to this 400 m section of stream in 2001, 2003 and 2004 and I currently have data on wood movement, distribution and recruitment to this stretch of stream over this time period.

2) Wood distribution
Given the importance of wood to stream biota and stream biogeochemical processing, understanding how wood is distributed in a system can provide insights into how organisms and stream processes may be spaced within a stream.  Areas of wood aggregation– debris dams – can be quantitatively assessed using a linear modification of a neighbor K analysis (see Kraft and Warren 2003), and periodicity in wood accumulation in this analysis can reflect regularity in debris dam formation.  I am currently working with Cliff Kraft to develop this analysis further and evaluate patterns in wood distribution across systems.

3) Wood volume in mixed hardwood-conifer forest streams
I am developing a conceptual model to explain the large wood standing stocks in streams running through mixed hardwood-conifer forests.  I suggest that for these streams input rates are most strongly related to riparian forest age and output rates are most strongly controlled by the width of the stream relative to the length of instream wood.  These two features should together explain a large amount of the variability in the volume of large wood in a stream.  I am assessing the conceptual model using data that I have collected in collaboration with other researchers and data from other published studies.  A recent study by Dr. William Keeton and others (Keeton et al. 2007) evaluates the abundance and volume of large wood in streams and riparain zones from mature and old-growth forests in the Adirondack Mountains of New York.  I am currently expanding upon this work to evaluate wood loading in the White Moutains as well.


















Ecology of northeastern headwater stream ecosystems

Collaborators:
Dr. Clifford E. Kraft
      I have conducted research on the influence of woody debris and debris dams as habitat for brook trout and for invertebrates in a headwater stream ecosystem in the eastern Adirondack Mountains of New York.  While woody debris manipulations were closely linked to brook trout abundance in second order streams, brook trout responses to wood manipulation in first and third-order streams were mixed (Warren and Kraft (2003). Invertebrate communities were largely unaffected by the wood manipulations that we imposed on the system (Warren and Kraft 2006).  We suggest that these systems are not habitat limited but rather food limited (food deriving either from allochtonous or autochthonous sources).  We are currently developing studies to evaluate this hypothesis.

  We are also interested in the influence of salamanders on fish and invertebrates and the alternate influence of fish and invertebrates on salamanders, however, we have not conducted studies as yet to evaluate this.  If you are interested in collaborating on such a project, please contact me or Dr. Kraft.    




Brook trout reproduction in Adirondack lakes 

Collaborators:
Dr. Stephen Sebestyen
Daniel Josephon
Jesse Lepak
Dr. Clifford E. Kraft
Peter Stevens
      Brook trout are an important fishery across eastern North American.  Brook trout reproduction in lakes relies on spawning in tributaries or along lake shorelines with upwelling groundwater.  Sizeable tributaries are often limited in small lakes, typical of the Adirondack mountains of New York and other regions.  In these cases, brook trout rely solely on areas of discharging groundwater along the shoreline.  While groundwater is usually well buffered relative to lake ecosystems, we have documented acidic groundwater discharging into brook trout redds in a small lake in the western Adirondackswith poor wild brook trout reproduction (Warren et al. 2005).  How prevalent is this phenomenon?  What are some of the watershed conditions that may promote acid groundwater?  How can it be mitigated?  Studies to address these next questions are currently pursued by Peter Stevens and Daniel Josephson.





Fish populations in streams of the Hubbard Brook Experimental Forest, NH 

Collaborators:
Dr. Clifford E. Kraft
Dr. Gene E. Likens
Donald C. Buso
To date, few studies have evaluated fish in streams of the Hubbard Brook Experimental Forest (HBEF).  What fish are present in streams in HBEF streams and how are the fish distributed?  This project is designed to address these important initial questions that can set the stage for future research on fish in this well studied ecosystem.

     In summer 2005, we conducted initial fish surveys in all Hubbard Brook tributaries and at three locations on the mainstem of Hubbard Brook.  Brook trout (Salvelinus fontinalis) were the only fish species present in all HBEF tributaries except Norris Brook, which also contained slimy sculpin (Cottus cognatus).  In most cases the upstream movement of fish was clearly constrained by a waterfall or other large physical barrier.  In a few cases, the mechanism for a loss of fish as one moved upstream was not immediately apparent.  We are exploring the possibility that episodic acidification during snowmelt moves fish downstream in the spring and that the presence of groundwater seeps are particularly important in maintaining fish in these streams.  One chronically acidified stream, contained no fish.   No surveys were conducted below the gorge on the mainstem of Hubbard Brook.  This feature may act as a barrier to fish recolonization by species such as dace or sculpin following their local extirpation due to stream acidification in the 1980’s.  Norris brook, which does contain sculpin, enters the mainstem below this waterfall.  We currently have a manuscript in press in Northeastern Naturalist, documenting the current distribution of fish in the HBEF system.