Forest management alternatives

3. Forest management alternatives

Thornley and Cannell (2000) used a mechanistic forest ecosystem simulator, which couples carbon, nitrogen and water (Edinburgh Forest Model) to mimic the growth of a pine plantation in a Scottish climate according to thinning and harvesting regimes as follows. The model was run to equilibrium (1) as an undisturbed forest, (2) removing 2.5, 10, 20 or 40% of the woody biomass each year (3) removing 50% of the woody biomass every 20 years, and (4) clear-felling and replanting every 60 years as in conventional plantations in this climate.

More carbon was stored in the undisturbed forest (35.2 kg C m -2 ) than in any regime in which wood was harvested. Plantation management gave moderate carbon storage (14.3 kg C m -2 )

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and timber yield (15.6 m -1 ha year ). Notably, annual removal of 10 or 20% of woody

biomass per year gave both a high timber yield (25 m -1 ha year ) and high carbon storage (20 to 24 kg C m -2 ). The efficiency of the latter regimes could be attributed (in the model) to high

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light interception and net primary productivity, but less evapotranspiration and summer water stress than in the undisturbed forest, high litter input to the soil giving high soil carbon and N 2 fixation, low maintenance respiration and low N leaching owing to soil mineral pool depletion. They concluded that there is no simple inverse relationship between the amount of timber harvested from a forest and the amount of carbon stored. Management regimes that maintain a continuous canopy cover and mimic, to some extent, regular natural forest disturbance are likely to achieve the best combination of high wood yield and carbon storage in the context of pure even-aged coniferous plantations but vey few data is available so far for checking this hypothesis (O’Hara et al. 2007).

Management shifts carbon allocation toward commercial timber production. A comparison of the ratio of wood biomass to NPP among a database assembled recently by Janssens and Luyssaert (pers. com.) exemplifies this result for 229 forest sites (Luyssaert et al. 2007). The carbon allocation among biomass compartment is shifted toward the commercial timber at the expense of other compartments like roots, branches and foliage.

( gC.m St oc k -2 ( gC.m

St ock

C Stock

Trees

(gC.m -2 )

Trees

Humus and soil

Humus and soil

2500 C Flux (gC.m Flux -2 .y -1 )

(gC.m -2 .yr -1 )

GPP NPP

0 ''

0 ''

NEE Ra

Time (yrs)

RECO

0 25 50 75 -3000 0 25 50

Figure 2. Course of carbon stock (upper diagrams) and flux (Lower diagram) in a forest ecosystem according to two management class, unmanaged (left) and managed (right).

The question regarding the balance between fossil carbon emissions associated with silvicultural intensification (herbicide, fertilisation, thinning, harvesting, etc.) and the net gain in carbon sequestration in biomass and soils due to productivity enhancement was addressed recently (Liski, 2001; White et al., 2005; Markewitz, 2006; Sonne, 2006). The fossil carbon

emissions could be estimated to 3 Mg C ha -1 over a 25-year rotation in an intensively managed pine plantation in the southeast USA indicating that fossil C emissions from

silviculture would largely counter-balance 75% of the expected gains in soil C (16 Mg C.ha -1 over 100 years) or in pulp products due to added productivity (Markewitz, 2006). In contrast,

the growth, harvest, and utilisation of saw logs as timber appear to provide a clear benefit for

C sequestration, 35 Mg C.ha -1 over 100 years, relative to the C emissions incurred from intensive silvicultural activities, 12 Mg C.ha -1 over 100 years. The major implication of this

analysis is that the fossil C emissions from intensified silvicultural activities can impact the net amount of C sequestered in managed forests but a net C gain should still be realized, particularly if trees are allowed to grow to a saw log category. A comprehensive assessment of the greenhouse gas balance of 408 management regimes of intensive forestry has been recently proposed by Sonne (2006) for Douglas fir planted forests in the Pacific NorthWest (USA). This study is among the first assessments based on a life cycle assessment approach and accounting for upstream as well as on site carbon emissions for such a range of management regime (Johnson et al., 2005). It concludes that carbon emissions associated with management practices are significant, accounting from 6 to 12.5% of the wood carbon storage according to scenarios, longest scenarios being less carbon-expensive. Upstream emissions, associated with e.g. seedlings, fertiliser and phytocide production and transportation, were 16% and on site emissions 84% of the total. The biggest contribution was log transportation followed by harvesting, site preparation and fertilisers. The author concludes there is an opportunity to enhance carbon sequestration in forests through minimising management emissions.

Forest management orientated towards production tends to intensify the stand productivity,

e.g. through fertilisation, efficient species, vegetation management, site preparation. It keeps the forest stand in an actively growing state through thinning and shortened rotation duration.

As compared woth a more ”close from nature” management, the intensification of silviculture shifts the carbon cycle towards a maximisation of harvested products at the expense of soil

and biomass which leads generally to net carbon loss in the atmosphere 1 because the turn over time of wood products is shorter than biomass and soil. The carbon stocks in situ, in soil and

biomass, are relatively low compared to a low intensity scenario.