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48 Whole Tree Harvesting in Sitka Spruce. Possibilities and Implications M. L. CAREY Forest and Wildlife Service, Sidmonton Place, Bray, Co. Wicklow.' ABSTRACT A study was carried out in a 50 year old crop of Sitka spruce, Yield Class 20, in order to determine: (i) The extent to which total produ~tion could be increased through implementing complete tree harvesting, and (ii) The effect this would have on site fertility. The result showed that the stemwood and bark represented 62% , the slash 20% and the roots 18% of the total dry matter production. Although inclusion of the slash in the final harvest would result in a very large increase in the amounts of nutrients removed, it is concluded that this would have serious consequences for site fertility only on impoverished mineral soils with low reserves of organic matter. Data are included on the actual value of the nutrients (N, P and K only) in the slash and on its energy content. INTRODUCTION Traditional methods of harvesting forest crops involve the removal of tree stems only and the leaving behind on the site of the other components such as branches, needles, cones and roots. Because the stemwood and bark are known to account for only about 50 per cent of the total dry matter production in coniferous crops (Keays and Hatton, 1976; Young, 1974) considerable interest has centred in recent years on the possibility of increasing 1 Present address: Forest Research Institute, Private Bag, Rotorua, New Zealand. IRISH FORESTRY, 1980, Vol. 37, No.1: 48-63
WHOLE TREE HARVESTING 49 production from forestry through the practice of more complete - tree harvesting systems. Although a more intensive harvesting system than that currently practiced may appear a remote possibility at this point in time, particularly with the recent problems in certain wood processing industries, the interest in whole tree harvesting nevertheless continues to increase on a worldwide scale. It has been added to considerably by the concern regarding future energy supplies and the possibilities for supplying part of these from forest residues (McCarthy, 1979). Advances in technology, both in timber processing and in the mechanics of forest harvesting, now seem certain to make the concept of whole tree harvesting increasingly attractive. Mobile total tree chippers have heen developed in a number of countries and are already in practice on a limited scale in parts of Scandinavia and North America (Anonymous, 1977). The main disadvantage of total tree harvesting is that it involves a considerable increase in the amounts of nutrients and organic matter removed from the site which may have serious long term repercussions for site fertility. This concern was expressed in a number of papers presented at a recent symposium in North America on "The impact of intensive harvesting on forest nutrient cycling" (Leaf, 1979) and was manifested in the results of a recently reported Irish study (Carey and O'Brien, 1979). This showed that the inclusion of the slash and roots at harvesting would increase overall production by nearly 75 per cent. However such a harvesting system would increase the removal of nitrogen, phosphorus and potassium by 68,80 and 65 per cent respectively over and above that in a normal harvesting operation. This is mainly because of the higher concentration of nutrients in the needles and twigs relative to the stemwood components. Because forest crops are known to recycle considerable quantities of nutrients and organic matter mainly through litterfall. brashing and pruning operations as they develop and mature, the results from an unthinned 33 year old crop do not necessarily reflect the position in a crop at the end of a rotation. Consequently it was decided in 1976 to extend the initial studies to a clearfelling situation in which the crop had been managed normally and had reached the age of maximum mean annual increment. This article presents the findings from this study and attempts to draw conclusions on the potential and implications of practicing whole tree harvesting in Sitka spruce in Ireland. The study was confined to the three major nutrients, nitrogen, phosphorus and potassium. Other essential elements such as calcium and magnesium are also removed as a result of harvesting operations but may be of lesser importance.
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WHOLE TREE HARVESTING 49<br />
production from forestry through the practice <strong>of</strong> more complete -<br />
tree harvesting systems. Although a more intensive harvesting<br />
system than that currently practiced may appear a remote<br />
possibility at this point in time, particularly with the recent<br />
problems in certain wood processing industries, the interest in<br />
whole tree harvesting nevertheless continues to increase on a worldwide<br />
scale. It has been added to considerably by the concern<br />
regarding future energy supplies and the possibilities for supplying<br />
part <strong>of</strong> these from forest residues (McCarthy, 1979). Advances in<br />
technology, both in timber processing and in the mechanics <strong>of</strong> forest<br />
harvesting, now seem certain to make the concept <strong>of</strong> whole tree<br />
harvesting increasingly attractive. Mobile total tree chippers have<br />
heen developed in a number <strong>of</strong> countries and are already in practice<br />
on a limited scale in parts <strong>of</strong> Scandinavia and North America<br />
(Anonymous, 1977).<br />
<strong>The</strong> main disadvantage <strong>of</strong> total tree harvesting is that it involves a<br />
considerable increase in the amounts <strong>of</strong> nutrients and organic<br />
matter removed from the site which may have serious long term<br />
repercussions for site fertility. This concern was expressed in a<br />
number <strong>of</strong> papers presented at a recent symposium in North<br />
America on "<strong>The</strong> impact <strong>of</strong> intensive harvesting on forest nutrient<br />
cycling" (Leaf, 1979) and was manifested in the results <strong>of</strong> a recently<br />
reported <strong>Irish</strong> study (Carey and O'Brien, 1979). This showed that<br />
the inclusion <strong>of</strong> the slash and roots at harvesting would increase<br />
overall production by nearly 75 per cent. However such a harvesting<br />
system would increase the removal <strong>of</strong> nitrogen, phosphorus and<br />
potassium by 68,80 and 65 per cent respectively over and above that<br />
in a normal harvesting operation. This is mainly because <strong>of</strong> the<br />
higher concentration <strong>of</strong> nutrients in the needles and twigs relative to<br />
the stemwood components.<br />
Because forest crops are known to recycle considerable<br />
quantities <strong>of</strong> nutrients and organic matter mainly through litterfall.<br />
brashing and pruning operations as they develop and mature,<br />
the results from an unthinned 33 year old crop do not necessarily<br />
reflect the position in a crop at the end <strong>of</strong> a rotation. Consequently it<br />
was decided in 1976 to extend the initial studies to a clearfelling<br />
situation in which the crop had been managed normally and had<br />
reached the age <strong>of</strong> maximum mean annual increment. This article<br />
presents the findings from this study and attempts to draw<br />
conclusions on the potential and implications <strong>of</strong> practicing whole<br />
tree harvesting in Sitka spruce in Ireland. <strong>The</strong> study was confined to<br />
the three major nutrients, nitrogen, phosphorus and potassium.<br />
Other essential elements such as calcium and magnesium are also<br />
removed as a result <strong>of</strong> harvesting operations but may be <strong>of</strong> lesser<br />
importance.
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