Title = Thule_Biocomplexity_NMSDsf_NEE_archive

Data Directory = Terrestrial Ecosystems

PI = 		Dr. Jeff Welker
		Enivironment and Natural Resources Institute
		707 A Street
		University of Alaska Anchorage
		Anchorage, AK, 99508

Co PIs =	Dr. Josh Schimel
		Department of Ecology, Evolution and Marine Biology
		Bren Hall, room 4304
		University of California, Santa Barbara
		Santa Barbara, CA 93106

		Dr. Ron Sletten
		University of Washington Quaternary Reseach Center
		Johnson Hall 363
		4000 15th Avenue NE 
		Seattle,WA 98195-1310

Funding Source = NSF Grant # 0221606

Data Set Overview = 
	Ecosystem carbon dioxide flux measurements were taken at a High Arctic 
	prostrate dwarf-shrub, herb tunda near Pituffik (Thule), Greenland 
	(76 29'N, 68 26'W; elevation 245 m asl) northwest Greenland.  
	Between 1978 and 2004, mean annual air temperature and precipitation
	for the site were -11.4C and 12.5 cm respectively. Over the same
	period, growing season (June, July and August) air temperatures averaged
	3.8C and approximately 50% of precipitation fell between October and 
	April as snow.  The soil surface of the study site was approximately 40%
	covered by vascular plants and 60% unvegetated. Dominant vascular plants 
	include the deciduous dwarf-shrub Salix arctica, the graminoid Carex
	rupestris and the wintergreen dwarf-shrub Dryas integrifolia.  The
	dataset covers the growing seasons of 2006 and 2007.
	The treatments were three levels of winter snow accumulation; ambient 
	snow (0.25 m), intermediate snow (0.55 m) and deep snow (1.1 m).  
	The treatments were accomplished by the erection of a 1.2 m snow fence
	placed perpendicular to prevailing winds.  Snow accumulated behind the 
	fence in a continuously tapering drift and permanent experimental plots
	were located on the ground under the snow depths listed above.

Instrument Descriptions = 

	Midday carbon dioxide fluxes were measured in each plot (n = 6 per treatment)
	using a closed system clear acrylic chamber with an in-line LiCor 6200
	infra-red gas analyzer (IRGA; Li-Cor Inc., Lincoln NE) which continuously
	monitors CO2 concentration (Vourlitis et al. 1993). Chamber air temperature 
	and relative humidity were also measured.  The clear-chamber flux measurement
	technique has previously been used successfully at the study site 
	(Arens et al. 2008).  The IRGA was calibrated using a gas of known CO2 
	concentration (500  2 ppm) on a bi-weekly basis to avoid instrumental error.
	Instrumental drift did not exceed a 2 ppm departure from the calibrated value.   
	Plot-level measurements of 5 and 10 cm soil temperature and volumetric soil
	moisture were taken with a VWR Traceable Temperature Probe 
	(VWR Instruments, West Chester, PA) and a Hydrosense time-domain reflectometry
	(TDR) soil moisture probe (Campbell Scientific, Logan, UT), respectively, 
	during each flux measurement.
Data Collection and Processing = 

	Carbon dioxide flux data were recorded using the LiCor 6200's internal memory
	and were later corrected for changes in chamber microclimate conditions using
	the method of Hooper et al. (2002).  Net Ecosystem Exchange (NEE) and Ecosystem
	Respiration (ER) values were directly measured, while Gross Ecosystem 
	Photosynthesis (GEP) values were calculated as the difference of NEE and ER
	(GEP = NEE - ER).  All data were tested for normality and homogenous variance
	and were appropriately transformed when these criteria were not met.

Data Format =

	The data are tab-delimited ASCII plain text formatted.

References =

Arens SJT, Sullivan PF, Welker JM (2008) Nonlinear responses to nitrogen and strong
interactions with nitrogen and phosphorus additions drastically alter the structure
and function of a high arctic ecosystem. Journal of Geophysical 
Research - Biogeosciences, 113, 1-10.

Hooper DU, Cardon ZG, Chapin FS III, Durant M (2002) Corrected calculations for
soil and ecosystem measurements of CO2 using the Li-Cor 6200 Portable Photosynthesis
System. Oecologia, 132, 1-11.

Vourlitis GL, Oechel WC, Hastings SJ, and Jenkins MA (1993) A system for measuring
in situ CO2 and CH4 flux in unmanaged ecosystems: an Arctic example. Functional 
Ecology, 7, 36-379.