Walker Branch Watershed (WBW)

Walker Branch Watershed (WBW) is a forested watershed on the Oak Ridge Reservation and has been the site of long-term environmental research since the 1960s (http://walkerbranch.ornl.gov/). Hydrological, biogeochemical, and ecological studies in WBW have made important contributions to our understanding of the effects of atmospheric deposition and climate variability and change in this region.

WBW spring field site

Objectives of the long-term observations in WBW were:

1. to quantify responses of an eastern upland oak forest ecosystem to inter-annual and long-term variations in climate and atmospheric deposition of sulfur and N, and

2. to provide integrated, long-term data on climate, forest vegetation, soil chemistry, and hydrologic and chemical fluxes at the catchment scale to support other focused research projects on the Oak Ridge Reservation and elsewhere in the region.

DOE-BER funded research in WBW has been phased out over the past few years. The long-term monitoring network (stream flow, precipitation, NADP, stream chemistry, and climate data) ended in 2010-2013. Synthesis work has focused on completing several watershed ecology and biogeochemistry studies.

Walker Branch research site in spring.

WBW weir site

Recent Publications

Two manuscripts detailing the long-term observations of vegetation and hydrology/stream chemistry in WBW were recently published.

* Kardol et al. (2010) reported on the importance of species, succession, and climate on forest composition and biomass accumulation using the long-term data set (1967–2006) of tree diameter growth and survival. Over the period of study, forest communities underwent successional change and substantially increased in biomass. The results of this study indicated that the direct effects of climate variability on eastern hardwood forest biomass accumulation and composition were small in comparison to changes resulting from natural succession or insect outbreaks.

* Lutz et al. (2012) evaluated the seasonal and long-term trends in watershed hydrology and biogeochemistry using the 40-year record of daily stream flow and precipitation and 20-year record of weekly stream water chemistry. WBW experienced a 20% decline in precipitation and a 34% decline in runoff over the past 20 years. Inter-annual variation in stream water nitrate concentrations was driven by antecedent hydrologic conditions (e.g., multi-year droughts). Overall, this research highlights how climate change may alter both hydrologic and biogeochemical processes at a watershed scale, and emphasizes the necessity of long-term monitoring programs to quantify these changes.

Walker Branch weir.

Stream biogeochemistry and ecology studies recently completed in WBW.

* Griffiths and Hill (2014) detailed the role of consumer excretion in stream N cycling. Excretion by a dominant snail supplied over half of the stream water ammonium concentrations and supported 26% of stream N demand, thus demonstrating an important role of consumers in stream nutrient cycling.

* Another study examined the role of temperature variation (diel, daily, and seasonal time scales) on leaf litter decomposition in streams.

* Recently, limited work involved completing field experiments on stream N and P uptake dynamics.

* One study focused on seasonal changes in nutrient uptake kinetics. The data were used to develop a method to analyze uncertainty in nutrient uptake metrics. The second study focused on the effect of dual N and P releases on nutrient uptake metrics, and fieldwork and initial analysis was completed.

* Lastly, collection of a 10-year-long dataset of daily stream metabolism measurements (gross primary production and ecosystem respiration) was completed.

      WBW snails              WBW N and P experiment

Walker Branch snails having lunch and N and P experiment.

WBW fall field site

Future work

Future work will focus on using the daily metabolism data to develop a stream metabolism model to examine the effects of climate change (changes in forest phenology, stream flow, temperature) on stream C cycling.

Transition to NEON

We are in a transition period during which the WBW footprint on the Oak Ridge Reservation (ORR) will be developed as core terrestrial and aquatic site in the National Ecological Observatory Network (NEON; http://www.neoninc.org/) funded by the National Science Foundation. Ecological data will be collected for 30 years. The majority of the terrestrial measurements will be collected in WBW, with additional sites located across the ORR. The West Fork of Walker Branch will serve as the aquatic monitoring site for NEON. The second-order reach will serve as one of 10 STReam Experimental Observatory Network (STREON) sites across the country (http://www.neoninc.org/our-design/collection-methods/streon). STREON is a 10-year experiment that will examine the ecosystem responses to changes in nutrient concentrations and large-bodied consumers.

Construction of the instrument tower and soil arrays at the WBW NEON site began in fall 2013. Sampling site establishment and instrument deployment are ongoing, and the site will be operational in the coming years. More information about the WBW NEON site can be found at: http://www.neoninc.org/science-design/field-sites/oak-ridge

Recent Publications:

Griffiths, N.A., and W.R. Hill. 2014. Temporal variation in the importance of a dominant consumer to stream nutrient cycling. Ecosystems 17:1169-1185.

Dodds, W.K., S.M. Collins, S.K. Hamilton, J.L. Tank, S. Johnson, J.R. Webster, K.S. Simon, M.R. Whiles, H.M. Rantala, W.H. McDowell, S.D. Peterson, T. Riis, C.L. Crenshaw, S.A. Thomas, P.B. Kristensen, B.M. Cheever, A.S. Flecker, N.A. Griffiths, T. Crowl, E.J. Rosi-Marshall, R. El-Sabaawi, and E. Martí. 2014. You are not always what we think you eat: selective assimilation across multiple whole-stream isotopic tracer studies. Ecology 95:2757-2767.

Lutz, B.D., P.J. Mulholland, and E.S. Bernhardt. 2012. Long-term data reveal patterns and controls on stream water chemistry in a forested stream: Walker Branch, Tennessee. Ecological Monographs 82:367-387.

Kardol, P., D.E. Todd, P.J. Hanson, and P.J. Mulholland. 2010. Long-term successional forest dynamics: species and community responses to climatic variability. Journal of Vegetation Science 21:627-642.

(full list available at: http://walkerbranch.ornl.gov/publications.shtml)