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Using long-term records to investigate watershed nitrogen supply and demand dynamics at the Fernow Experimental Forest, West Virginia, USA Mark Benjamin Burnham Fossil fuel combustion has caused elevated anthropogenic nitrogen (N) deposition onto forests in the northeastern United States since the middle of the 20 century, and has resulted in the supply of N exceeding the ecosystem N demand in many forests across the region. While the supplydemand imbalance is often attributed to elevated N inputs, a reduction in N demand may also make a significant contribution to diminished levels of N retention. Long-term records of N inputs, outputs, and stand dynamics at the Fernow Experimental Forest (FEF) in Tucker County, WV, provide a unique opportunity to study how changes in ecosystem demand can influence N retention. The long-term data at the FEF has also allowed me to assess whether changes in N retention are accurately recorded in the stable isotope record of tree rings. If the isotopes in tree rings prove to be a suitable index of N retention, then it would be possible to expand the temporal and spatial extent of existing records of forest N dynamics. In this dissertation, I examine how forest species composition and soil acidity affect stand N supply and demand, and evaluate the use of tree ring δN as an indicator and recorder of temporal changes in N-saturation. In Chapter 1, I introduce N supply and demand dynamics in the FEF, and describe the study areas used to investigate both stand N demand and the usefulness of tree ring δN as a recorder of N cycling. In Chapter 2, I used long-term records of stand composition and measurements of tree N uptake to determine if a shift in species composition reduced stand NO3 demand, resulting in an increase in stream NO3 discharge. Stand NO3 demand did not decline with a change in species composition, but soil NO3 supply likely increased, contributing to greater levels of NO3 loss in stream water. In Chapter 3, I measured the effect of experimental whole-watershed acidification on soil Al solubility, and evaluated whether elevated Al affected the relative uptake of NH4 and NO3 by trees. My results showed that elevated soluble Al in the soil reduces tree NO3 demand of several important species by shifting their mineral N uptake towards NH4, and that an increase of Al in the soil of the acidified watershed may have increased stream water NO3 discharge by reducing stand NO3 demand. In Chapter 4, I examine how effectively the tree ring δN of four tree species records an experimentally induced increase in stream-water NO3 that was caused by a large, one-time addition of urea. While three of the four species examined recorded the onset of the change in stream chemistry, each species differed in its sensitivity and duration of response, and the fourth species only responded to a later increase in baseline N discharge. In Chapter 5, I assess the ability of tree ring δN of the same four tree species to respond to a greater soil NO3 supply and record the apparent onset of N saturation in an untreated reference watershed using soil nitrification measurements and long-term stream water NO3 records. Similar to the response to addition of urea, the results were mixed. The tree ring δN of two species were associated with changes in N cycling and NO3 loss, and they varied in their sensitivity to N-saturation. Finally, in Chapter 6 I show how long-term records at the FEF provide a unique opportunity to illustrate how ecosystem N demand interacts with N deposition trends to affect stream NO3 discharge. In considering both of my assessments of the potential usefulness of tree ring δN records, I conclude that these records can provide opportunities to expand current N cycle records, but enough uncertainty remains to preclude their widespread application until we achieve a deeper understanding of the mechanisms that control wood N. Thus, continuous measurement records of N cycling remain paramount in elucidating N supply and demand dynamics.