Woods Hole Research Center

PAR cell phenology

Differences in FPAR for coniferous and deciduous tree stands.

The Fraction of Photosynthetically Active Radiation (FPAR) absorbed by the forest canopy has been the subject of a great deal of research in the past two decades. In simple terms, FPAR is a measure of the amount of incident visible light absorbed by plant tissues, or the amount of light harvested by a canopy. Because light absorption drives the photosynthetic process, FPAR is directly related to photosynthesis and the rate of carbon fixation. Therefore, measurements of FPAR are often used to drive models of net annual carbon uptake or net primary production (NPP).

Measuring light interception with PAR cells in a Spruce stand.

Calibration of PAR cells.

Measuring light interception with PAR cells in a Spruce stand.

FPAR varies in both spatial and temporal extent. Spatially, FPAR exhibits a large amount of variability between stands due to differences in species composition and canopy structure. Temporally, FPAR varies throughout the growing season due to spring leaf growth and fall leaf litter as well as changes in sun angle. Because coniferous species hold their needles year-round, coniferous stands do not show the same seasonal variability in FPAR as deciduous stands.

In the Delta Junction study region, Woods Hole Research Center scientists use arrays of PAR cells to measure both the temporal and spatial variability in FPAR across a number of different stand types under differing environmental conditions. Because the PAR cells measure transmitted light, PAR above the canopy must also be measured and FPAR based on the difference between above canopy and below canopy PAR values must be calculated:

FPAR = 1 - (PARbelow canopy / PARabove canopy)

The FPAR data obtained are then used to parameterize models of NPP and validate remotely sensed estimates of FPAR.

Leaf Area Index (LAI)

A hemispheric photo of a boreal forest. These photos are used to calculate estimates of LAI.

LAI is a fundamental characteristic of vegetation relating directly to the exchange of energy, mass, and water between the plant canopy and the atmosphere. It is an index of canopy density that compares the foliage surface area to the ground area beneath the canopy. It is strictly defined as one-half the total green leaf area per unit of ground surface area. Estimates of LAI are used as an ecophysiological measure of the effective photosynthetic and transpirational surface within a canopy, which has important implications for modelling net primary production and evapotranspiration between forests and the atmosphere. It is important to note however, that LAI is less frequently used to drive ecosystem carbon models due to the more direct association of FPAR with photosynthesis.

In the Delta Junction study region, coordinated ground and remotely based measurements of LAI are carried out throughout the growing season. These data are used to compare different methods of field-based measurement (using the LAI-2000 and AccuPar instruments), and to assess the accuracy of ongoing remote estimates of FPAR and leaf area index.

The data exhibit a large amount of variability between stands. Inter-stand variability is generally a result of differences in species composition, stand age, and the chemical and physical characteristics of the landscape, such as water and nutrient availability.

Flux towers in the Delta Junction region record meteorological data and surface-air carbon dioxide exchange. (a) A tower set in a typical boreal black spruce stand. (b) A tower set in the 1999 Donnelly Flats burn.

Flux Towers

In order to directly monitor how fire and changes in climate will affect net ecosystem carbon exchange, Woods Hole Research Center scientists make use of eddy covariance measurements to estimate local carbon exchange. Eddy covariance essentially measures 3-dimensional wind velocity and gas exchange simultaneously and results in an estimate of the vertical carbon dioxide exchange. Eddy covariance measurements have been collected by collaborators continuously since early 2000 at two towers in the study area, one in the 1999 burn, and another in the 1920 burn now occupied by black spruce forest. An additional tower was established in 2001 and is operating in the 1987 burn site, now occupied predominantly by aspen.

Changes in vegetation cover following fire as seen from the height of the flux towers: counter-clockwise from upper left, area that burned around 1920; one year after 1999 burn; 1987 burn (in leaf-off condition); 1987 burn - one week later following leaf flush.