Human influences

8.6.1 Forest clearance and timber harvesting

The removal of timber or even whole forests has major implications as far as nutrient distribution is concerned. Original clearing for pasture has in many cases been accompanied by the use of fire, which renders many nutrients available to the plants but also involves considerable losses through leaching. If logs are hauled to a landing site before de-limbing, a 'bird's nest' of branches and needles will accumulate and nutrient concentration in the rest of the forest is lowered.

A number of studies in northern temperate areas and Australia are reported by Attiwill and Weston (2001) and in round terms the nutrient loss associated with whole-tree harvesting (where the whole tree - trunk, branches and sometimes the roots - is taken away rather than just taking the trunk and leaving everything else on site) results in a loss from the total pool of < 3% of P, Mg and K and < 8% of N and Ca (rising to 13-19% of Ca loss in oak-hickory forests). In eucalypt forests losses of N and P were similar: 2% in stem-only harvesting and up to 8% in whole-tree harvesting. These losses are relatively small and easily replaced. A detailed study by Yanai (1998) at Hubbard Brook compared potential nutrient loss on W5 that was whole-tree harvested (see Box 8.1) with another watershed that was clearcut, removing just the trunks. Table 8.2 shows that while 70% of the above-ground biomass is held in the trunk, it is poor in nutrients and contains around half or less of the

Table 8.2. Biomass and nutrient content of above-ground parts of a 70-year-old hardwoodforest at Hubbard Brook Experimental Forest (HBEF). The data can be used to work out how much biomass and nutrient loss there will be if forestry operations remove just the trunk in clear-felling or remove all of the tree (except the leaves) in whole-tree harvesting. In reality, some of the material that could be harvested is inevitably left behind because it is too small or inaccessible. The harvest removal ratio shows how much in reality was taken from two watersheds at HBEF when they were actually felled. The ratio shows how much more biomass or nutrient was removed by whole-tree harvesting compared with traditional clear-felling.

Content of tree parts as a percentage Above- of above-ground total Harvest ground total - removal kg ha-1 Trunk Branch Twig Leaf ratio

Biomass

1 999000

70

Phosphorus

52

31

Nitrogen

509

40

Potassium

219

48

Calcium

584

51

Magnesium

54

55

Sulphur

59

49

29

0.6

2

2.7

56

1.0

12

5.4

45

0.6

15

3.9

36

0.5

15

3.5

45

0.3

4

3.5

35

0.4

1

3.0

36

0.4

10

2.8

Source: Data from Yanai, 1998. Forest Ecology and Management 104.

nutrients. By contrast, branches and leaves contain a greater proportion of nutrients per unit of biomass. This is particularly so for phosphorus as the leaves are high in phosphorus but low in biomass while the trunk is low in phosphorus and high in biomass. Consequently the branches hold the greatest reservoir of phosphorus (56% of the tree total).

Other nutrients are distributed in different ways: calcium and magnesium are concentrated in the trunk while for nitrogen and potassium a greater proportion than other nutrients occurs in leaves. From the harvest removal ratios it can be seen that whole-tree harvesting removes disproportionately more phosphorus than other nutrients. Almost three times as much biomass and over five times the amount of phosphorus were removed compared with a clearcut. This whole-tree loss equates to the removal of 50 kg of phosphorus per hectare, equivalent to 71% of the total phosphorus in the living vegetation (70 kg ha-1) and 59% of the P in the forest floor (85 kg ha-1). Although the amount of phosphorus removed by the harvest was only 3.1% of the total phosphorus in the mineral soil (1600 kg ha-1), using the data on phosphorus budgets for the same site given in Fig. 8.7, the whole-tree harvest removed 32% of the most readily available phosphorus pool. The fact that phosphorus is a remarkably immobile nutrient is underlined by the negligible loss to streams (0.2 kg ha-1 over 3 years). This is similar to the pattern seen in ammonium after felling in W2, W4 and W6 at Hubbard Brook in an earlier study (Likens et al, 1970). By contrast, nitrate showed a hugely increased concentration in stream water in the same study: it increased from a pre-felling concentration of0.85-0.94 mg l-1 of water to 53 mg l-1 (a 60-fold increase) 2 years after cutting, reaching a peak of 82 mg l-1 (a 90-fold increase) in W2 with the most drastic felling regime (see Box 8.1). The biggest concern is for calcium depletion (potentially linked to acid rain - see above) which may prove a long-term problem for sustained production.

The different timber-felling treatments on the watersheds of Hubbard Brook (see Box 8.1) provide useful insights. Comparing commercial whole-tree harvest (W5) with felling without any wood removal but with subsequent herbicide treatment for 3 years (W2), streamflow losses of both potassium and calcium were much larger for W2 than W5 during the first 8 years following felling. This is attributed in equal parts to slower decomposition of the smaller amount of logging debris and greater uptake by the regrowing vegetation on W5. The difference between the watersheds was most pronounced in years 1-3 since in year 4 vegetation began to recover following 3 years of herbicide suppression.

Timber removal in the tropics should, according to traditional views (where all nutrients are tied up in the vegetation) be far worse for nutrient depletion. The real problem, however, is not so much where the nutrients are held but that they are in short supply and not rapidly replaced. Removal of a few selected trunks, especially if the bark is left behind, does little since most nutrients are in the branches, twigs and leaves. But commercial logging can have important effects. Table 8.3 shows that removal of the timber makes a significant deficit to nutrients. Moreover, further nutrients are lost by extra leaching. This is caused jointly by: (1) reduced evapotranspiration due to tree removal so more water is available for downward leaching, and (2) there is insufficient vegetation to absorb the flush of soluble nutrients released by decomposition of the remaining debris. There is very little supply of nutrients from the parent rock (the soil is too deep) so rain and dust inputs are the major way of replenishing lost nutrients. It can be seen from Table 8.3 that it will take up to 60 years to restore above-ground nutrients to their former amounts.

8.6.2 Fertilizers

In Northern Europe, major forest fertilization commenced in the early 1950s-60s. Correction of deficiency by addition of fertilizer is a simple and

Table 8.3. An above-ground nutrient budget estimated for commercially logged lowland dipterocarp rain forest in Bukit Berembun, Malaya. Logging is assumed to remove 60 m of timber per hectare, equivalent to 10% of the total biomass of all stems over 5 cm diameter.

Nutrient concentrations (kg ha-1) K Ca Mg Total N

Outputs

Nutrient concentrations (kg ha-1) K Ca Mg Total N

Outputs

Removal of timber

45

200

20

70

Extra leaching

75

30

15

?

Total outputs

120

230

35

70+

Annual inputs

5.7

3.8

0.7

11.3

Years to recover

20

60

50

5-10

Source: Based on Bruijnzeel, 1992. Data from Whitmore, 1998. An Introduction to Tropical Rain Forests (2nd edn). Oxford University Press.

Source: Based on Bruijnzeel, 1992. Data from Whitmore, 1998. An Introduction to Tropical Rain Forests (2nd edn). Oxford University Press.

relatively economic business. However, it is used on only a minute fraction of the world's forests because of the perception that nutrient shortage or depletion is rarely a major problem over the long term (compared with large inputs required by the annual crops of agriculture) and is therefore an unnecessary expense. While loss of nutrients is often asserted to lead to loss of productivity (see Section 10.1.3), Attiwill and Weston (2001, p. 176) state that they 'have not found any definitive quantitative evidence in the literature in support of this assertion, nor are fertilizers used regularly in most of the world's forests used for timber production'. They add, however, that fertilizers are used regularly and increasingly in plantations around the world. This makes economic sense since plantations often use species planted well outside their natural range (e.g. pines planted in Australasia, eucalypts in South America and Europe) on soils marginal for tree growth. Moreover, their rotation length is often shortened, putting more pressure on the soil resources. In such cases nutrient deficiencies and imbalances are usually readily seen and rectified. Potassium appears to be the limiting element in plantation forestry and the cost of applying this from the air is really quite low, as it is with boron, which is required at only 6-8 kg ha-1. Agricultural soils require a high pH, but the liming they often require is usually unnecessary on forest soils because most trees, and pines in particular (the staple of temperate forestry), grow well on acid soils. When calculating the amounts of fertilizer required, analyses are made of the pools of both total and available nutrients. There is a dynamic equilibrium between the two pools, of which the second is the one of direct importance to the plants. Nutrient budgets are required to estimate long-term balances between inputs and outputs so that productivity can be sustained indefinitely. Gains result from atmospheric deposition, leguminous and non-leguminous nitrogen fixation, weathering of rocks, groundwater ingress and fertilizer additions. Losses are caused by crop removal, soil erosion, leaching and volatilization including that resulting from burning (Maclaren, 1996, p. 62).

In north-west Europe, Heliovarra and Vaisanen (1984) review the effect of fertilization on microbes and invertebrates. With added nutrients, bacteria dominate the microbes and there is a transitory increase in microfauna such as nematodes but large arthropods and above-ground invertebrates show little response. The authors conclude that most of these effects are due to the decrease in acidity rather than fertilization directly, but this is not the whole story since the number of earthworms decrease despite increased pH.

Soil deterioration, particularly on very large commercial sites, can result from bad forest practice and the gradual loss of mineral nutrients even though the latter can be replaced without much difficulty. The question of whether trees such as radiata pine Pinus radiata are soil degraders or soil improvers is discussed in Sections 2.2.3 and 6.6. With ancient woodlands in Great Britain, the threat is that of eutrophication as a result of fertilizer drift (especially NPK) from neighbouring agricultural fields (see Section 11.4.3 as well). This drift influences the margins of major woods but in smaller sites such as Cantryn Wood, Bridgnorth, where much of the woodland clothes the steep margins of a stream, a great deal of the area is subject to its influence. In these areas the increase in stinging nettle Urtica dioica, hogweed Heracleum sphondylium and sundry other coarse species since a major woodland survey of 1971 is obvious to the casual observer. The decline suffered by 17 ancient woodland species, revealed by a pilot resurvey in 2000 of 14 of the original sites was even worse and in 2003 there was a major resurvey led by English Nature (now called Natural England) and the Woodland Trust. The species concerned included early dog-violet Viola reichenbachiana, yellow pimpernel Lysimachia nemorum, polypody fern Polypodium vulgare, lady fern Athyrium filixfemina, three-veined sandwort Moehringia trinervia and the nettle-leaved bellflower Campanula trachelium. Wood-sorrel Oxalis acetosella, wood crane's bill Geranium sylvaticum and sweet violet Viola odorata were amongst the less uncommon species that had also diminished.

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