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Howland Cooperating Experimental Forest

Bangor, Maine

[photo:] Pit and mound topography and a variety of tree size classes are typical of Howland forests.The Howland Experimental Forest is located about 60 km north of Bangor, Maine. The forest was established in 1984 on land owned by the International Paper Company, Ltd. and is presently owned by the Northeast Wilderness Trust. Land surrounding the core forest is commercial softwood timberlands owned by GMO Renewable Resources, LLC.


The climate is chiefly cold, humid, and continental and the region often carries a snowpack of up to 2 m from December through March. The mean annual temperature (1950-2000) for Millinocket, ME (50 km to the north) is 5.4°C. Summer maximums of 30 °C are common and winter minimums can reach -30 °C. The mean annual air temperature (1996-2010) at the Howland tower is 6.7 °C. Average annual rainfall in Millinocket (1950-2000) is 1050 mm with similar amounts at the Howland Forest.  Precipitation is spread fairly evenly throughout the year and about a quarter of it falls as snow. The growing season (active vegetation period) averages about 235 days.


The landscape around the forest varies from flat to gently rolling, with a maximum elevation change of less than 68 m within 10 km.  Due to the region's glacial history, soil drainage classes within a small area may vary widely, from well drained to poorly drained.  Generally, the soils throughout the forest are glacial tills, acid in reaction, with low fertility and high organic composition. Upland soils are fine sandy loams, classified as Aquic Haplorthods.


Forest stands are dominated by red spruce  (Picea rubens Sarg.) and eastern hemlock (Tsuga canadensis (L.) Carr.), which together account for over 70% of the tree biomass, with lesser quantities of other conifers (primarily balsam fir, Abies balsamea (L.) Mill., white pine, Pinus strobus L., and northern white cedar, Thuja occidentalis L.) and hardwoods (red maple, Acer rubrum L. and paper birch, Betula papyrifera Marsh.).  The forest in the core research area has been only lightly disturbed over the last 140 years as evidenced by the age distribution of canopy dominant spruce trees, cut stumps, and the size class distribution of trees found on a series of forest inventory plots.

Research, Past and Present

Howland was established as part of the Mountain Cloud Chemistry Program, which constructed and instrumented a 95 ft walk-up tower for meteorological and air pollution studies.  Several additional towers were later added in different parts of the forest to support carbon sequestration and ecological studies.  Research at the site initially focused on nutrient cycling and soil ecology studies. In 1996 measurements of forest carbon uptake and loss (carbon sequestration studies) were begun as part of the AmeriFlux program, and Howland now has one of the longest records of carbon flux measurement in the world. Howland is an important land validation site for NASA, first serving as a field site for the Forest Ecosystem Dynamics project and now operating as a core EOS and MODIS validation site as well as part of AERONET (aerosol optical depth network).  It is also a test site for ongoing remote sensing studies of tree biomass and soil moisture.
Howland is presently part of EPA’s Clean Air Status and Trends Network (CASTNET), which records trends in dry deposition of air pollutants.  An above-canopy webcam records daily images as part of the PHENOCAM phenology network, and the site records soil moisture as part of the COSMOS program.

Major Research Accomplishments and Effects on Management

Howland has contributed many insights into forest nutrient and carbon cycling as well as serving as a test bed for remotely sensing forest structure and function. An important result from the carbon flux studies is the finding that this “over-mature” forest is still actively sequestering large quantities of carbon from the atmosphere.  Related studies on adjacent commercial forestland showed that the reduction in carbon uptake following a shelterwood harvest only lasted about 7 years. The presence of 2 research towers in similar vegetation led to the development of a method for determining the uncertainty in flux tower measurements.


Collaborators include the University of Maine, the Woods Hole Research Center, Harvard University, NASA (Goddard Space Flight Center and Jet Propulsion Laboratory), University of Connecticut, University of East Carolina, NOAA ATDD, EPA, and many others.

Research Opportunities

Research opportunities include ecological, soil, nutrient cycling, remote sensing, and meteorological studies.  On going research at Howland includes studies of the impact of climate change on forest growth, CO2 and other trace gas exchange, the mechanics of tree sway, studies of soil respiration, and remote sensing of biomass and soil moisture.


A small air-conditioned field station is located at Howland.  Although there is power and internet, the buildings are without water or bathroom facilities. 
Lat. 45° 12' N, long. 68°44′ W, 60 masl.

Contact Information

David Hollinger
USDA Forest Service
271 Mast Road
Durham, NH 03824
Tel: (603) 868-7673

Related Publications

Note: More than 60 publications concerning the Howland Forest were published prior to 2001, including a special issue of Remote Sensing of the Environment (Vol. 47(2), 1994).

Gea-Izquierdo, G., A. Mäkelä, H. Margolis, Y. Bergeron, T.A. Black, A.Dunn, J. Hadley, K.T. Paw U, M.s Falk, S.a Wharton, R. Monson, D.Y. Hollinger, T. Laurila, M. Aurela, H. McCaughey, C. Bourque, T. Vesala and F. Berninger. 2010. Modeling acclimation of photosynthesis to temperature in evergreen conifer forests. New Phytologist 188: 175-186.

Hollinger, D.Y., S.V. Ollinger, A.D. Richardson, T.P. Meyers, D.B. Dail, M.E. Martin, N.A. Scott, T.J. Arkebauer, D.D. Baldocchi, K.L. Clark, P.S. Curtis, K.J. Davis, A.R. Desai, D. Dragoni, M.L. Goulden, L. Gu, G.G. Katul, S.G. Pallardy, K.T. Paw U, H. Schmid, P.C. Stoy, A.E. Suyker, and S.B. Verma. 2010. Albedo estimates for land surface models and support for a new paradigm based on foliage nitrogen concentration. Global Change Biology 16:696-710.

Richardson, A.D., T.A. Black, P. Ciais, N. Delbart, M.A. Friedl, N. Gobron, D.Y. Hollinger, W.L. Kutsch, B. Longdoz, S. Luyssaert, M. Migliavacca, L. Montagnani, J.W. Munger, E. Moors, S. Piao, C. Rebmann, M. Reichstein, N. Saigusa, E. Tomelleri, R. Vargas and A. Varlagin. 2010. Influence of spring and autumn phenological transitions on forest ecosystem productivity. Philosophical Transactions of the Royal Society B 365:3227-3246.

Richardson, A.D.  M. Williams, D.Y. Hollinger, D.J. P. Moore, D. B. Dail, E.A. Davidson, N.A. Scott, R.S. Evans, H. Hughes, J.T. Lee, C. Rodrigues and K. Savage. 2010.  Estimating parameters of a forest ecosystem C model with measurements of stocks and fluxes as joint constraints. Oecologia 164:25-40.

Dail, B., D. Hollinger, E. Davidson, I. Fernandez, H. C. Sievering, N. Scott. 2009. Distribution of 15N tracers applied to the canopy of a mature spruce-hemlock stand, Howland, Maine, USA. Oecologia 160:589-599.

Medvigy, D., S.C. Wofsy, J.W. Munger, D.Y. Hollinger, and P.R. Moorcroft. 2009. Mechanistic scaling of ecosystem function and dynamics in space and time: the Ecosystem Demography model version 2. Journal of Geophysical Research 114, G01002, doi:10.1029/2008JG000812.

Richardson, A.D., B.H. Braswell, D.Y. Hollinger, J.P. Jenkins, and S.V. Ollinger. 2009. Near-surface remote sensing of spatial and temporal variation in canopy phenology. Ecological Applications 19:1417-1428.

Richardson, A.D., D.Y. Hollinger, D.B. Dail, J.T. Lee, J.W. Munger, and J. O’Keefe. 2009. Influence of spring phenology on seasonal and annual carbon balance in two contrasting New England forests. Tree Physiology 29:321-331.

Savage, K., E.A. Davidson, A.D. Richardson, and D.Y. Hollinger. 2009. Three scales of temporal resolution from automated soil respiration measurements. 2009. Agricultural and Forest Meteorology 149:2012-2021.

Sierra, C.A., H.W. Loescher, M.E. Harmon, A.D. Richardson, D.Y. Hollinger, and S.S. Perakis. 2009. Interannual variation of carbon fluxes from a tropical, a temperate, and a boreal evergreen forest: the role of forest dynamics and climate. Ecology 90:2711-2723.

Williams, M., A.D. Richardson, M. Reichstein, P.C. Stoy, P. Peylin, H. Verbeeck, N. Carvalhais, M. Jung, D.Y. Hollinger, J. Kattge, R. Leuning, Y. Luo, E. Tomelleri, C.M. Trudinger, Y.P. Yang. 2009. Improving land surface models with FLUXNET data. Biogeosciences 6:1341-1359.

Ollinger, S.V., A.D. Richardson, M.E. Martin, D.Y. Hollinger, S. Frolking, P.B. Reich , L.C. Plourde, G.G. Katul, J.W. Munger, R. Oren, M-L. Smith, K.T. Paw U, P.V. Bolstad, B.D. Cook, M.C. Day, T.A. Martin, R.K. Monson, and H.P. Schmid. 2008. Canopy nitrogen, carbon assimilation and albedo in temperate and boreal forests: functional relations and potential climate feedbacks. PNAS 105:19335-19340.

Piao, S., P. Ciais, P. Friedlingstein, P. Peylin, M. Reichstein, S. Luyssaert, H. Margolis, J. Fang, A. Barr, A. Chen, A. Grelle, D. Hollinger, T. Laurila, A. Lindroth, A.D. Richardson, T. Vesala. 2008. Net carbon dioxide losses of northern ecosystems in response to autumn warming. Nature 451(7174):49-53.

Prabha, T.V., M. Y. Leclerc, A. Karipot, D. Y. Hollinger and E. Mursch-Radlgruber. 2008. Influence of Nocturnal Low-level Jets on Eddy-covariance Fluxes over a Tall Forest Canopy. Boundary-Layer Meteorology 126:219-236.

Sims, D.A., A.F. Rahman, V.D. Cordova, B.Z. El-Masri, D.D. Baldocchi, P.V. Bolstad, L.B. Flanagan, A.H. Goldstein, D.Y. Hollinger, L. Misson, R.K. Monson, W.C. Oechel, H.P. Schmid, S.C. Wofsy, and L. Xu. 2008. A new model of gross primary productivity for North American ecosystems based solely on MODIS EVI and LST. Remote Sensing of the Environment 112:1633-46.

van Wijk, M.T.,  B. van Putten, D.Y. Hollinger and A.D. Richardson. 2008. Comparison of different objective functions for parameterization of simple respiration models. Journal of Geophysical Research 113, G03008.

Gaige, E; Dail, D.B.; Hollinger, D.Y.; Davidson, E.A.;  Fernandez, I.J.; Seivering, H.; White, A.; Halteman W. 2007. Changes in canopy processes following whole-forest canopy nitrogen fertilization of a mature spruce-hemlock forest. Ecosystems. 10: 1133-1147, DOI: 10.1007/s10021-007-9081-4

Prabha T.V., M.Y. Leclerc, A. Karipot, and D.Y. Hollinger. 2007. Low-frequency effects on eddy-covariance fluxes under the influence of a low-level jet. Journal of Applied Meteorology and Climatology 46:338-352.

Richardson, A.D. and D.Y. Hollinger. 2007. A method to estimate the additional uncertainty in gap-filled NEE resulting from long gaps in the CO2 flux record. Agricultural and Forest Meteorology 147:199-208.

Richardson, A.D., D.Y. Hollinger, J.D. Aber, S.V. Ollinger and B.H. Braswell. 2007. Environmental variation is directly responsible for short-but not long-term variation in forest-atmosphere carbon exchange. Global Change Biology 13:788-803.

Davidson E. A., A. D. Richardson, K. E. Savage, and D. Y. Hollinger. 2006. A Distinct Seasonal Pattern of the Ratio of Soil Respiration to Total Ecosystem Respiration in a Spruce-Dominated Forest. Global Change Biology 12:230-239.

Gove, J. H. and D. Y. Hollinger. 2006. Application of a dual unscented Kalman filter for simultaneous state and parameter estimation in problems of surface-atmosphere exchange. Journal of Geophysical Research 111, D08S07, doi:10.1029/2005JD006021.

Hagen S. C., B.H. Braswell, E. Linder, S. Frolking, A.D. Richardson, and D.Y. Hollinger. 2006. Statistical Uncertainty of Eddy-Flux Based Estimates of Gross Ecosystem Carbon Exchange at Howland Forest, Maine. Journal of Geophysical Research 111, D08S03.

Heinsch, F.A., M. Zhao, S.W. Running, J.S. Kimball, R.R. Nemani, K.J. Davis, P.V. Bolstad, B.D. Cook, A.R. Desai, D.M. Ricciuto, B.E. Law, W.C. Oechel, H. Kwon, H. Luo, S.C. Wofsy, A.L. Dunn, J.W. Munger, D.D. Baldocchi, L. Xu, D.Y. Hollinger, A.D. Richardson, P.S. Stoy, M.B.S. Siqueira, R.K. Monson, S.P. Burns, L.B. Flanagan. 2006. Evaluation of Remote Sensing Based Terrestrial Productivity from MODIS Using Regional Tower Eddy Flux Network Observations.  IEEE Transactions on Geoscience and Remote Sensing 44(7):1908-1925.

Karipot, A., M. Y. Leclerc, G. Zhang, T. Martin, G. Starr, D. Hollinger, J. H. McCaughey, and G. R. Hendrey. 2006. Nocturnal CO2 exchange over a tall forest canopy associated with intermittent low-level jet activity. Theoretical and Applied Climatology 85:243-248.
Loescher, H. W., B. E. Law, L. Mahrt, D. Y. Hollinger, J. Campbell, and S. C. Wofsy. 2006. Uncertainties in- and interpretation of carbon flux estimates using the eddy covariance technique. Journal of Geophysical Research 111, D21S90.

Matross, D.M., A. Andrews, M. Pathmathevan, C. Gerbig, J.C. Lin, S.C. Wofsy, B.C. Daube, E.W. Gottleib, V.Y. Chow, J.T. Lee, C. Zhao, P.S. Bakwin, J.W. Munger, and D.Y. Hollinger. 2006. Estimating regional carbon exchange in New England and Quebec by combining atmospheric, ground-based and satellite data. Tellus 58B344-358.

Richardson, A.D., B.H. Braswell, D.Y. Hollinger, P. Burman, E.A. Davidson, R.S. Evans, L.B. Flanagan, J.W. Munger, K. Savage, S.P. Urbanski, and S.C. Wofsy. 2006. Comparing simple respiration models for eddy flux and dynamic chamber data. Agricultural and Forest Meteorology 141:219-234.

Richardson A. D., D. Y. Hollinger, G. G. Burba, K. J. Davis, L. B. Flanagan, G. G. Katul, J. W. Munger, D. M. Ricciuto, P. C. Stoy, A. E. Suyker, S. B. Verma, and S. C. Wofsy. 2006. A multi-site analysis of random error in tower-based measurements of carbon and energy fluxes. Agricultural and Forest Meteorology 136:1-18.

Hollinger D. Y.  and A.D. Richardson. 2005. Uncertainty in eddy covariance measurements and its application to physiological models. Tree Physiology 25:873-885.

Richardson, A. D. and D. Y. Hollinger. 2005. Statistical modeling of ecosystem respiration using eddy covariance data: Maximum likelihood parameter estimation, and Monte Carlo simulation of model and parameter uncertainty, applied to three simple models. Agricultural and Forest Meteorology 131: 191-208.

Lai C.-T., J.R. Ehleringer, A.J. Schauer, P.P. Tans, D.Y. Hollinger, K.T. Paw U, J.W. Munger and S.C. Wofsy. 2005. Canopy-scale d13C of photosynthetic and respiratory CO2 fluxes: observations in forest biomes across the United States. Global Change Biology, 11:633-643.

Sims D.A., Abdullah F. Rahman, Vicente D. Cordova, Dennis D. Baldocchi, Lawrence B. Flanagan, Allen H. Goldstein, David Y. Hollinger, Laurent Misson, Russell K. Monson, Hans P. Schmid, Steven C. Wofsy, and Liukang Xu. 2005. Midday values of gross CO2 flux and light use efficiency during satellite overpasses can be used to directly estimate eight-day mean flux. Agricultural and Forest Meteorology 131:1-12.

Xiao X., Q. Zhang, D. Hollinger, J. Aber, and B. Moore III. 2005. Modeling seasonal dynamics of gross primary production of an evergreen needleleaf forest using MODIS images and climate data. Ecological Applications, 15:954-969.

Hollinger D.Y., J. Aber, B. Dail, E.A. Davidson, S.M. Goltz, H. Hughes, M. Y Leclerc, J.T. Lee, A.D. Richardson, C. Rodrigues, N.A. Scott, D. Achuatavarier, and J. Walsh. 2004. Spatial and temporal variability in forest-atmosphere CO2 exchange. Global Change Biology, 10:1689-1706.

Lai C. -T., J. R. Ehleringer, P. Tans, S. C. Wofsy, S. P. Urbanski, and D. Y. Hollinger. 2004. Estimating photosynthetic 13C discrimination in terrestrial CO2 exchange from canopy to regional scales. Global Biogeochemical Cycles 18: GB1041.

Lin J.C., C. Gerbig, S.C. Wofsy, A.E. Andrews, B.C. Daube, C.A. Grainger, B.B. Stephens, P.S. Bakwin, D.Y. Hollinger. 2004. Measuring fluxes of trace gases at regional scales by Lagrangian observations: Application to the CO2 Budget and Rectification Airborne (COBRA) study. Journal Geophysical Research 109:D15304, 2004

Xiao X., D. Hollinger, J. Aber, M. Goltz, E. A. Davidson, Q. Zhang, and B. Moore III. 2004. Satellite-based modeling of gross primary production in an evergreen needleleaf forest. Remote Sensing of the Environment 89:519-534.

Wilson K. B., D. D. Baldocchi, E. Falge, M. Aubinet, P. Berbigier, and others 2003. Diurnal centroid of ecosystem energy and carbon fluxes at FLUXNET sites. Journal of Geophysical Research Atmospheres 108 (D21):4664.

Davidson, E.A., K. Savage, P. Bolstad, D.A. Clark, P.S. Curtis, D.S. Ellsworth, P.J. Hanson, B.E. Law, Y. Luo, K.S. Pregitzer, J.C. Randolph, and D. Zak. 2002. Belowground carbon allocation in forests estimated from litterfall and IRGA-based soil respiration measurements. Agriculture and Forest Meteorology 113:39-51.

Falge, E., D. Baldocchi, J. Tenhunen, M. Aubinet, P. Bakwin, P. Berbigier, C. Bernhofer, G. Burba, R. Clement, K. J. Davis, J. A. Elbers, A. H. Goldstein, A. Grelle, A. Granier, J. Guðmundsson, D. Hollinger, A. S. Kowalski, G. Katul, B. E. Law, Y. Malhi, T. Meyers, R. K. Monson, J. W. Munger, W. Oechel, K. T. Paw U, K. Pilegaard, Ü. Rannik, C. Rebmann, A. Suyker, R. Valentini, K. Wilson, and S. Wofsy. 2002. Seasonality of ecosystem respiration and gross primary production as derived from FLUXNET measurements. Agricultural and Forest Meteorology 113:53-74.

Falge, E., J. Tenhunen, D. Baldocchi, M. Aubinet, P. Bakwin, P. Berbigier, C. Bernhofer, J. Bonnefond, G. Burba, R. Clement, K. J. Davis, J. A. Elbers, M. Falk, A. H. Goldstein, A. Grelle, A. Granier, T. Grünwald, J. Guðmundsson, D. Hollinger, I. A. Janssens, P. Keronen, A. S. Kowalski, G. Katul, B. E. Law, Y. Malhi, T. Meyers, R. K. Monson, E. Moors, J. W. Munger, W. Oechel, K. T. Paw U, K. Pilegaard, Ü. Rannik, C. Rebmann, A. Suyker, H. Thorgeirsson, G. Tirone, A. Turnipseed, K. Wilson and S. Wofsy. 2002. Phase and amplitude of ecosystem carbon release and uptake potentials as derived from FLUXNET measurements. Agricultural and Forest Meteorology 113:75-95.

Law, B. E., E. Falge, L. Gu, D. D. Baldocchi, P. Bakwin, P. Berbigier, K. Davis, A. J. Dolman, M. Falk, J. D. Fuentes, A. Goldstein, A. Granier, A. Grelle, D. Hollinger, I. A. Janssens, P. Jarvis, N. O. Jensen, G. Katul, Y. Mahli, G. Matteucci, T. Meyers, R. Monson, W. Munger, W. Oechel, R. Olson, K. Pilegaard, K. T. Paw U, H. Thorgeirsson, R. Valentini, S. Verma, T. Vesala, K. Wilson, and S. Wofsy. 2002. Environmental controls over carbon dioxide and water vapor exchange of terrestrial vegetation.  Agricultural and Forest Meteorology 113:97-120.

Davidson, E.A., K. Savage, L.V. Verchot, and R.I. Navarro. 2002. Minimizing artifacts and biases in chamber-based measurements of soil respiration. Agriculture and Forest Meteorology 113:21-37.

Thornton, P.E., B. E. Law, H. L. Gholz, K. L. Clark, E. Falge, D. S. Ellsworth, A. H. Goldstein, R. K. Monson, D. Hollinger, M. Falk, J. Chen, and J.P. Sparks. 2002. Modeling and measuring the effects of disturbance history and climate on carbon and water budgets in evergreen needleleaf forests. Agricultural and Forest Meteorology 113:185-222.

Wilson, KB, Baldocchi, DD, Aubinet, M, D. Hollinger and others. 2002.  Energy partitioning between latent and sensible heat flux during the warm season at FLUXNET sites. Water Resources Research 38 (12): art. no.-1294.

Baldocchi, D., E. Falge, L. Gu, R. Olsen, D. Hollinger and others. 2001. FLUXNET: A new tool to study the temporal and spatial variability of ecosystem-scale carbon dioxide, water vapor and energy flux densities. Bulletin of the American Meteorological Society 82:2415-2434.

Falge, E, Baldocchi, D, Olson, R, et al. Gap filling strategies for long term energy flux data sets. 2001. Agricultural and Forest Meteorology 107:71-77.

Falge, E., D. Baldocchi, R. Olson, P. Anthoni, M. Aubinet, C. Bernhover, G. Burba, R. Ceulemans, R. Clement, H. Dolman, A. Granier, P. Gross, T. Grünwald, D. Hollinger, N. Jensen, G. Katul, P. Keronen, A. Kowalski, C.T. Lai, B. Law, T. Meyers, J. Moncrieff, E. Moors, J. Munger, K. Pilgaard, U. Rannik, C. Rebmann, A. Suyker, J. Tenhunen, K. Tu, S. Verma, T. Vesala, K. Wilson, and S. Wofsy. 2001. Gap filling strategies for defensible annual sums of net ecosystem exchange. Agricultural and Forest Meteorology 107:43-69.

Gaudinski, J.B., S.E. Trumbore, E.A. Davidson, A.C. Cook, D. Markewitz, and D.D. Richter. 2001. The age of fine-root carbon in three forests of the eastern United States measured by radiocarbon. Oecologia 129:420-429.

Savage, K.E., and E.A. Davidson. 2001. Interannual variation of soil respiration in two New England forests. Global Biogeochemical Cycles 15(2):337-351.

Last Modified: 06/15/2011