In general, about 55% of a tree's biomass is found in the merchantable bole, 

 25% is in the top (leaves and branches), and 20% in the roots (Young et al. 

 1976). Bark, which is included in both bole and top figures, accounts for 

 about 14% of total biomass. Coniferous trees have a greater percentage of 

 their biomass in leaves (needles; 11%) than do deciduous trees (4%). 



Although much of the forest land along the coast of Maine is capable of 

 supporting biomasses comparable to those described above (i.e., 67 to 78 

 tons/acre; 150 to 175 t/ha), the actual average biomass is considerably less 

 (table 9-8). The 3,309,800 acres (1,340,000 ha) of commercial forest land in 

 the three sampling units in coastal Maine support an average of 16 tons/acre 

 (35 t/ha) of merchantable timber (trees >6 inches; 15 cm dbh) or 32 tons/acre 

 (72 t/ha) when total biomass of trees above and below ground is included 

 (Ferguson and Kingsley 1972). The latter figure is about half what is 

 expected. The reason for the discrepancy is that much of the forest land 

 contains immature and understocked forest stands on which forest biomass is 

 much lower than in mature fully stocked stands. Average biomass of 

 merchantable timber is highest in the Washington unit (17 tons/acre; 39 t/ha) 

 followed by the Hancock (15 tons/acre; 33 t/ha) and Capitol units (14 

 tons/acre; 31 t/ha). This is because the Washington unit has a higher 

 percentage of its commercial forest land (63%) in pole-sized or sawtimber- 

 sized stands, compared to the Hancock (57%) and Capitol (38%) units (table 9- 

 9), and the higher percentage of pole- and sawtimber-sized stands is reflected 

 in higher biomass values. 



Primary productivity . Primary productivity is the amount of solar energy that 

 is converted to plant tissue on a unit of area during a unit of time; it is 

 expressed, therefore as a rate. Production is measured by either the weight 

 of plant material grown on a unit of land or by the energy equivalent (kcal) 

 of that tissue when oxidized. In this chapter the terms "forest 

 productivity," "net primary productivity," and "productivity" will be 

 synonomous and will refer to the increment in forest biomass (wt/unit area) 

 during 1 year. 



Forests are more productive than most terrestrial habitats because they 

 support a large biomass and have a large photosynthetic surface. The 

 extensive root systems rapidly absorb moisture and nutrients from the soil and 

 make them available to the photosynthetic process. 



The productivity of Maine's forests is below that of most North American 

 forests, because Maine's soils are unusually shallow and low in nutrients. 

 Productivity estimates for forest types similar to those found in Maine range 

 from 3 to 5 tons/acre/year (6 to 11 t/ha/yr; Whittaker and Woodwell 1968), 

 whereas in Maine forest productivity ranges from 1 to 3 tons/acre/year (3 to 7 

 t/ha/yr; Young 1971). 



The distribution of forest productivity among the layers of a forest community 

 reflects the distribution of biomass. In a New Hampshire hardwood forest 97% 

 of the total above-ground production was due to the trees, 0.7% was due to 

 shrubs, and 2.7% by herbs (Bormann and Likens 1979). This pattern is similar 

 to that reported in other studies (Whittaker and Woodwell 1968, and others). 



9-16 



