Biomass and productivity

The sheer size of forest and woodlands is what people often comment upon. The weight or mass of organic material present is referred to as the biomass (or sometimes the standing crop). It should be borne in mind that this can be a somewhat loose term since it may or may not include dead wood or litter. Table 8.1 shows that the biomass above ground increases from the boreal forest towards the tropics, starting from very low levels at the Arctic treeline and reaching in excess of 9401 ha-1 in the Amazon basin. However, there are exceptionally large forests outside the tropics, the record-holders being the temperate forests of the Pacific Northwest of North America including stands of huge Douglas fir (1600 tha-1) and coastal redwoods, the tallest trees in the world (trunk biomass of 34501 ha-1 with total net primary productivity (NPP) possibly approaching a staggering 45001 ha-1 y-1). In most forests, more than

Box 8.1 Hubbard Brook Experimental Forest

The name Hubbard Brook appears in many studies on nutrients and productivity in forests and these have provided some of the best information on the functioning of temperate hardwood forests. Hubbard Brook Experimental Forest in the White Mountain National Forest of New Hampshire, eastern USA (3160 ha) was established in 1955 as a centre for hydrological research. In 1963 the Hubbard Brook Ecosystem Study was set up within the forest by Herbert Bormann, Gene Likens, Noye Johnson and Robert Pierce specifically to study linkages between hydrology, nutrient dynamics and cycling in response to natural and human disturbances, such as air pollution, forest cutting, land-use changes, increases in insect populations and climatic factors. The forest is composed of a large watershed with all minor streams draining into the Hubbard Brook subdivided into a number of discrete watersheds underlain by impermeable bedrock of schist and granite (Fig. 8.1 below). The forest is second-growth hardwood forest dominated by sugar maple Acer saccharum (from which maple syrup is tapped), American beech Fagus grandifolia and yellow birch Betula alleghaniensis.

Much of the original work done here was based on inputs and outputs. It was assumed since the rock underlying the watersheds is impermeable, that looking at what arrived in precipitation and dry deposition and what nutrients came out in stream flow gave a good indication of what was happening in the forest in terms of nutrient budgets (mass balances).

The following are the notable uses of the different watersheds.

W6 Untouched control. No. 6 has been used for energy and nutrient studies (see Sections 8.2 and 8.6.1) and has been undisturbed by fire or cutting since 1919. The vernal dam work of Muller and Bormann (1976) was carried out here (see Section 8.4.1).

W2 (1965) All trees were cut down but not removed, followed by herbicide use for 3 years to prevent regrowth. Used for nutrient loss studies (see Sections 8.4.1 and 8.6.1).

W4 (1970) Trees cut and removed in 25-m strips, every third strip being cut in 1970, second set cut in 1972 and the final strips cut in 1974. Used for nutrient studies (see Section 8.6.1).

W101 (1970) Trees cut and removed in blocks as a comparison to No. 4.

W5 (1983) All trees cut and removed to mimic commercial whole-tree harvesting conditions, using heavy machinery on the flat and chainsaws on inaccessible slopes. Partly used to study fine root biomass (Section 8.1.1) and nutrient loss studies (Section 8.6.1).

W1 (1999) Control untouched until 1999. Calcium has been declining in the forest over the last 50 years probably due to acid rain. In 1999 fifty tonnes of calcium was added to the watershed to bring calcium levels to pre-acid rain levels.

Energy and nutrients Box 8.1 (cont.)

Energy and nutrients Box 8.1 (cont.)

Hubbard Brook Nutrient Cycling
Figure 8.1 Hubbard Brook. Above is a view of Watershed 2 and below is a view of Watershed 2, 4 and 5 (from left to right). Descriptions of the watersheds are given above. (Photographs courtesy of USDA Forest Service, Northeastern Research Station.)

85% of the biomass is usually contained in the above-ground portion of the woody plants. The extra biomass present in roots is often unmeasured but Jackson et al. (1996) in a comprehensive review show that root/shoot ratios increase from 0.19 in tropical evergreen forests (i.e. 0.19 tonnes of roots to

Table 8.1. Above-ground biomass and net primary productivity (NPP) in four forest types. The data give the usual range encountered but inevitably examples of outliers above and below the figures given can be found. Figures from Reichle (1981) quoted in Packham et al. (1992) are considerably lower, but more recent work justifies the data given below.

Above-ground biomass (t ha"1) NPP (t ha"1 y"1)

Boreal conifers 10-160 1-10

Temperate broadleaved deciduous 150-300 7-12

Temperate rain forest (Pacific Northwest) 500-1000 15-25

Tropical montane evergreen 450 10-24

Tropical broadleaved evergreen 450-700 17-32

every tonne of biomass above-ground) to 0.23 in temperate areas to 0.32 in boreal forest. Although a greater proportion of the biomass is below-ground away from the tropics, above-ground biomass decreases along the same gradient, so there is still less biomass below-ground away from the tropics (49tha-1 in tropical evergreen forests; 40-421 ha-1 in temperate and tropical deciduous forests; 29tha-1 in boreal forests). Even fewer studies of fine roots (< 2 mm diameter) have been conducted: in the hardwood forest of Hubbard Brook (mostly in W5 - see Box 8.1) fine roots were calculated to add 4.7 tha-1 by Fahey and Hughes (1994). The sclerophyllous shrubs of dry Mediterranean environments, which are evergreens with small, hard, thick leaves, resistant to hot, dry conditions, put more of their growth below ground than above (root/shoot ratio of 1.2), presumably as a defence against frequent fire.

Although useful, biomass is a static measure of how much there is at any one time and gives no indication of how quickly new growth is being added or lost, and so gives little insight into how the forest is functioning. More useful are estimates of the productivity of the forest, how much new material is being added. Most primary productivity in a forest (the manufacture of organic complex compounds from simple inorganic substrates) is carried out when green plants use light in photosynthesis. By adding up all the photosynthetic activity in a forest the gross primary productivity (GPP) can be calculated in terms of the carbon fixed or sugars created. Plants have, however, to expend some of this energy in respiration (energy used to keep existing tissue alive). Gross primary productivity minus the respiratory costs gives the net primary productivity (NPP), the actual increase in growth that can be measured in tonnes of new material per unit area per year. For the forest to grow in size, GPP has to be bigger than the respiratory costs so that the NPP or carbon balance is positive. In practice, GPP is very difficult to measure since it is almost impossible to trace all carbon uses and losses in a complex forest. But estimates have been made which show that respiratory losses account for between a third and three-quarters of the GPP. Such estimates are useful since they give insight into how energy is flowing in a forest. For example, Kira (1975) found that a 25-year-old beech forest in Denmark which expended around 40% of photosynthate in respiration, had an NPP comparable to that of a mature lowland tropical rain forest, which, although having a GPP more than double that of the beech forest, had to support respiratory losses of 75%. Both GPP and respiration (and hence NPP) are dependent upon factors in the environment such as temperature, light and water, as described in Chapter 3. Nutrients are also vitally important to growth and so form the subject of part of this chapter.

Forest ecologists are particularly interested in the NPP since this is what other organisms have to feed upon, and it relates to how much carbon is being stored in a forest (discussed further in Chapter 11). Table 8.1 gives average rates of NPP for different forests, ranging from 1 tha-1 y-1 in boreal forests to over 30tha-1 y-1 in tropical rain forests, and a maximum of 36.2 tha-1 y-1 in a 26-year-old western hemlock Tsuga heterophylla stand in the Pacific Northwest.

There have been comparatively few studies that have investigated how the new material produced in NPP is distributed within the forest. Figure 8.2 shows such a study for a mixed-oak forest in Europe. It can be seen in this example that 85% of the new growth is above-ground and that only 15% is below-ground. If just the woody plants (trees and shrubs) are considered, the ratio is still 83% above-ground to 17% below-ground. This ratio may well be closer in some forests, possibly even reaching 50:50. Within the woody plants, 26% of the production goes into leaves, 65% into woody material (twigs, branches and trunks) with the remainder being such structures as flowers and fruits which, with dead wood and twigs, are absorbed into the non-leaf litter (Fig. 8.2).

It should be noted that as a tree gets bigger it becomes more or less fixed in size and so holds a maximum number of leaves. This creates an upper limit on GPP for an individual tree. At the same time, as the trunk gets thicker, the respiratory burden grows from keeping more living tissue alive. Thus, NPP of a tree tends to decline with age (see Section 10.1.3 for further information). If all the trees in a stand are of the same age, it can be expected that the overall stand NPP will also decline with age (Fig. 8.3). This decline would be less pronounced in a large uneven-aged stand where the proportion of different-aged trees stays more constant over time.

HORNBEAM OAK BEECH

HORNBEAM OAK BEECH

Peter Duvigneaud
1000. (Based on data from Duvigneaud and Denaeyer-De Smet, 1970. From Archibold, 1995. Ecology of World Vegetation. Chapman & Hall, Fig. 6.3.1. With kind permission of Springer Science and Business Media.)

Peter Attiwill (see Attiwill and Weston, 2001) describes three stages that a eucalypt forest goes through in growing to maturity which are undoubtedly widely applicable. In the first stage, a few decades long, the forest goes through a building phase where the major proportion of the NPP goes into building biomass, with a net uptake of nutrients from soil reserves. The second stage sees the rate of growth slowing and an increasing proportion of NPP stored as dead heartwood (see Fig. 1.2). Internal recycling of nutrients becomes more important and uptake from soil reserves decreases. In the final stage, the trees

40 60 80 Age (years)

40 60 80 Age (years)

100 120

Figure 8.3 Changes in net primary productivity (•) and annual litter-fall (~) with age in two Russian forest types, (a) southern taiga pine forests (see Section 1.6.1 for more information on taiga), and (b) oak forests near Moscow. The graphs show that net primary productivity reaches its highest levels at 40-50 years and 25 years old, respectively, in the pine and oak stands. (From Rodin and Bazilevich, 1967. Production and Mineral Cycling in Terrestrial Vegetation. Oliver and Boyd.)

reach maturity and the amount of added biomass approaches zero; the major proportion of NPP is shed as litter and coarse woody debris (CWD), and nutrients are increasingly immobilized in the litter.

8.1.2 Ecosystem productivity

It is important to differentiate between the productivity of the component plants and the total productivity of the stand. Though the NPP of the individual trees declines with age, in a large uneven-aged stand young trees replace dead trees, so in the absence of any major disturbance, NPP should remain more or less constant with time. When all the components of a forest including the soil are considered, however, the total productivity of a stand will inevitably decline with age. As shown in Fig. 8.4, gross stand production increases rapidly with age, matching almost exactly the pattern of increase in biomass of leaves. As the forest stand gets denser, leaf biomass and gross stand production reach a peak followed by a slight drop as the stand self-thins to a sustainable level. Thereafter, gross stand production and leaf biomass settle down almost to a constant, reflecting the more or less constant NPP. Over time, however,

Figure 8.4 Changes in productivity in a hypothetical forest stand with age. (Based on data from Duvigneaud, 1971. In: Symposium on the Productivity of Forest Ecosystems. Ed. P. Duvigneaud. UNESCO.)

the respiratory cost of running the stand increases. This is partly because of the build-up of living biomass (as trunks get bigger and the flora builds to a maximum) and which forms part of the NPP calculation as discussed above. Also, however, there is a build-up of organic matter in the soil and dead woody material which decomposes, causing a build-up of respiration needs in the decomposer subsystem. As total respiratory losses of the stand increase for both these reasons, there will come a point where the total stand respiration equals the gross productivity (Fig. 8.4). At this point the net stand productivity (gross productivity minus respiratory costs) approaches zero; the stand stops getting bigger because much NPP is shed as dead material and decomposes. All the biomass produced in a year is matched by an equal loss of biomass used in respiration above and below the soil surface. To avoid confusion with NPP, this net stand production is referred to as the ecosystem productivity. As the stand reaches maturity the ecosystem productivity approaches zero.

As will be seen in Chapter 11 the distinction between NPP and ecosystem productivity is crucial when it comes to looking at locking up carbon in forests to mitigate climate change. The NPP figure suggests that a large amount of biomass (half of which is carbon) is being stored in a forest every year. However, the ecosystem productivity shows that if all parts of the stand are considered, the forest is growing significantly only in size (and thus soaking up carbon) until it reaches maturity. Thereafter no more carbon is stored because as much is lost each year as enters. As will be seen in Chapter 11, in practice this is a slight oversimplification but the principle stands.

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