Howland Research Forest

Climate

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.

Soils

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.

Vegetation

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.

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.

Research, Past and Present

Photo Credit

Courtesy photo by Aaron Weiskittel.

Eddy covariance tower at Howland Research Forest in Maine.

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 two research towers in similar vegetation led to the development of a method for determining the uncertainty in flux tower measurements.

Research Opportunities

Research opportunities include ecological, soil, nutrient cycling, remote sensing, and meteorological studies. Ongoing 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.