Yale University School of Forestry & Environmental Studies FES519B - Methods of Ecosystem Analysis 1998

ABOVE-GROUND  BIOMASS  and NUTRIENT ESTIMATES at a MIXED DECIDUOUS  and  HEMLOCK-HARDWOOD FOREST TOTOKET MOUNTAIN,  NORTH BRANFORD, CT 1998

        The results are based on field measurements of 101 plots, each 6 m x 30 m, arranged along two transects.  Plots were established to study the composition and structure of the 70-100 year old mixed deciduous and hemlock/hardwood forest at Totoket Mountain (click here to get more information on the forest composition and structure).  The "Uphill" transect, plots 1-51, ran in a north-west direction, and ranged in elevation from roughly 290 ft to 510 ft above sea level.  The "Countour" transect, plots 52-101, ran in a  north-east direction, and stayed as near as possible the 350 ft contour line.  The "Uphill" transect is considered more xeric, the "Contour" transect is more mesic.

         Please note: Biomass results are expressed in Mg / ha, or metric tons per hectare.  1 Mg = 1,000 kg = 1,000,000 g

• How are biomass estimates calculated?
• Comparison of biomass estimates: 1993 vs. 1998
• Biomass of dominant tree species
• QA / QC analysis of biomass estimates
• Total nutrient analysis
• Biomass modeling
• Species-by-species biomass and nutrient data, 1993 and 1998
• Download Biomass Modeling spreadsheet

     Allometry equations, usually in one of two forms,
               biomass = a x dbh ^b
               biomass = a + b log (dbh)
    allow a reasonable estimation of the above-ground biomass to be made, based solely on knowledge of tree species and tree diameter (the equation coefficients, a and b, are different for different species).

       The following figure illustrates how biomass estimates vary by species of the same diameter at breast height. One reason that different species trees of the same diameter have different amounts of above-ground biomass is that different species have different architectures.  Another reason is that the wood of some species is much more dense than that of other species: for example, hemlock has a wood density that is roughly one-half that of most hardwood species.
 

       Based on data collected in the field at Totoket Mountain, we applied (where possible) the species-specific allometry equations contained in Tritton and Hornbeck's (1982) "Biomass Equations for Major Tree Species of the Northeast" (USDA Forest Service, Northeastern Forest Experiment Station, GTR NE-69).   Generally, Brenneman et al.'s (1978) equations for West Virginia hardwoods were applied.  In the cases where Tritton and Hornbeck did not give an equation for the species in question, then we applied the Monk et al. (1970) "General Hardwoods" equation, or the Monteith (1979) "General Softwoods" (see Tritton and Hornbeck (1982) for Monk and Monteith sources).

         The numbers presented below are estimates: the validity of applying equations derived in West Viriginia to a site in Connecticut can be debated until the cows come home.

         For comparative purposes, 1997 data from the 80 year old forest at Watershed 6, Hubbard Brook, NH, are provided where appropriate.

Based on the above, aboveground live biomass at Totoket Mountain increased at approximately 2,000 kg / ha annually over the five-year period 1993-1998.
 

        We wondered "does it matter whether we use species-specific allometric equations? Or do we get a pretty good estimate of total biomass by just using a general hardwoods equation?"
 
        We compared our "best estimate" of total biomass (using species-specific equations) with  biomass calculated using

•    general hardwoods equation (> 10 cm dbh) (Harris et al. 1973)
•    general hardwoods equation (> 2 cm dbh) (Monk et al. 1970)
•    Acer saccharum equation (Brenneman et al. 1978)

• the nutrient concentration in wood (usually low)
• the nutrient concentration in bark (usually high)
• the total biomass of the tree (derived from allometric equations, as above)
• the bark biomass percentage

        In order for the model to work, we had to make a number of assumptions above mortality, tree growth, and regeneration.  These assumptions are as follows:

Basics of the Model

Mortality

• define an annual mortality rate: we assumed an average annual mortality of 1.0%
• randomly determine individual tree mortality
Tree growth
• each tree allowed to have its own growth rate
• growth rate a random variable
• trees not allowed to shrink in size
• average radial growth rate = 1.2 mm / year (click here to get to more tree ring information)
Biomass calculations
• determine which trees are still alive
• apply annual growth
• calculate biomass of still-living trees (allometric equations)
Estimate regeneration
• allow regeneration in the form of sapling ingrowth
• randomly determine the number of new trees
• average number of new trees = 130 / ha over five year period
• assume new trees have dbh = 5 cm
• calculate biomass accordingly
 

Model Results

Calibration test

• 1993 Total Live tree biomass: 145 Mg / ha
 
• Model run five times to predict 1998 total above-ground live tree biomass:
 

Forecasting 2003 Biomass

• 1998 data used as base
 
• Model run five times to predict 2003 total above-ground live tree biomass:
 

        The model is quite sensitive to the mortality rate and annual growth rate assumed.  On the other hand, the amount of ingrowth is so small as to be more or less insignificant.

        The following figures compare the effect of changes in the level of annual mortality and annual growth rate on the forecasted biomass.

        Click here to see the table of biomass and nutrient data.
          Click here to see the species list.

        You can download our biomass model as a Microsoft Excel spreadsheet.  It may take some time.  Click here to download.

Biomass estimates brought to you by:          Nancy Fresco          Andrew Richardson          Yale School of Forestry and Environmental Studies  With thanks to:          New Haven Regional Water Authority          Tom Siccama          Dan Vogt          and the Plantheads (Phytosociology Crew)