Seedling Quality

With the quality of trees cultivated in plantation being critical for the marketing of the resultant oil, then the quality of the propagation material becomes equally critical. Most plantation trees have so far been propagated from seed as the higher costs and massive numbers of trees planted out makes the logistics of propagation by tissue culture or cuttings unmanageable. The latter methods have been tried successfully on a small scale and do guarantee more consistent quality. With propagation by seed, an oil quality check on the mother tree will not necessarily determine the oil quality of the progeny as fertilisation from a poor quality father may have occurred. Hence unless seed is collected from a region where cross pollination from poor quality father trees is impossible, there is a chance the seed could be of poor quality.

In addition, seed has often been bought from merchants unaware of the chemical varieties available within individual Melaleuca species. This has been seen with attempts to establish tea tree plantations in at least two overseas countries. In one of these, of the eight clones described (Kawakami et al. 1990) only one (Clone II) met the requirements of the International Standard 4730. Clone I, although containing acceptable quantities of terpinen-4-ol and 1,8-cineole also contained excessive concentrations of sabinene which, along with the high a-thujene/a-pinene ratio (Southwell and Stiff 1990) suggested M. linariifolia. Clones III to VIII contained insufficient terpinen-4-ol with excessive concentrations of 1,8-cineole and Clones III and VIII contained excessive quantities of terpinolene as well.

Hence there was a need for the determination of oil quality in seedlings to prevent the large scale planting of substandard quality trees. Such a method was developed at the Wollongbar Agricultural Institute (Russell et al. 1997) by using the extraction analysis method described above.

With careful analysis of the first dicotyledon leaves to emerge from a seedling, it was possible to predict whether the variety in question was the terpinen-4-ol, cineole or terpinolene chemotype. Although oil yields were very low at this stage, ethanolic extraction with microwave irradiation followed by GC analysis, gave a sufficiently intense profile to type each variety. A misleading aspect of this method was that the commercial terpinen-

4-ol variety contained substantial percentages of terpinolene (approx. 12%), a-pinene (12%) and ^-pinene (14%) and no terpinen-4-ol. This early stage analysis gave the wrong idea that the terpinen-4-ol variety was actually the terpinolene chemovar because of high terpinolene concentrations. The cineole variety contained a similar proportion (approx. 10%) while the terpinolene chemovar had much more (approx. 25%) terpinolene (Table 5).

The key component for measurement is 1,8-cineole. In the commercial terpinen-4-ol variety, cineole is usually low (0.5-5%). In the terpinolene variety it rises to approximately 12% whereas in the cineole variety it reaches values as high as 40-70%. Although the measurement of terpinen-4-ol means nothing in the dicotyledon leaves (all varieties read zero), values increase as the seedling ages. With, for example, a ten week old leaf set ten analysis (i.e. ethanolic extract analysis of the tenth leaf pair to emerge when tested ten weeks after emergence) terpinen-4-ol proportions are approximately 36% with the commercial variety and only about 1% with the other chemical varieties. Terpinen-4-ol first appears as cis- and trans--sabinene hydrate (ratio approximately 7:1 with M. alternifolia and 0.7:1 with M. linariifolia) (Southwell and Stiff 1989, 1990) in bright green flush seedling growth as it does with the flush growth on mature plants. These investigations of the ontogenetic variation in oil constituent percentages (Figure 8) indicate that different biogenetic pathways (e.g. the pinene, the cineole/ limonene/a-terpineol, the terpinolene and the a-thujene/sabinene/sabinene hydrate/ y-terpinene/terpinen-4-ol pathways are initiated at different stages of the plant's ontogeny. Clearly the cineole, pinene and terpinolene pathways appear at earlier stages of development in seedling growth than the sabinene hydrate/terpinen-4-ol pathway. Consequently producers can reliably assess the quality of their seedlings at both early and late stages of the seedlings development.

Leaf Set Number

Seedling age (weeks)

Figure 8 The concentration of key consitituents in M. alternifolia seedling leaves at emergence

Seedling age (weeks)

Figure 8 The concentration of key consitituents in M. alternifolia seedling leaves at emergence

BIOGENESIS

Although little work has been done specifically on Melaleuca oil biogenesis, the pathways in other genera producing similar monoterpenoids have been investigated. For example, Croteau (1987) summarised the current thinking concerning the metabolism from acetyl-CoA through mevalonic acid pyrophosphate and isopentenyl pyrophosphate to geranyl pyrophosphate. Recent investigations now suggest that isopentenyl pyrophosphate is formed, not from mevalonic acid but via the alternative triose phosphate/pyruvate pathway (Lichtenthaler et al. 1997; Eisenreich et al. 1997). Croteau's review (1987) continued to examine the cyclisation reactions of geranyl pyrophosphate (34,35) and linalyl pyrophosphate (36,37) that produce cyclic monoterpenoids such as limonene, a-terpinene, 7-terpinene, sabinene, 1,8-cineole, a-pinene, terpinen-4-ol, a-terpineol etc. Cyclase enzymology is complex in that different cyclases can produce the same product and an individual cyclase can produce multiple cyclic products. Definitive investigations involving partially purified enzymes, cellfree extracts, isotopic labelling and substrate substitution are adding gradually to our knowledge of these pathways. Some conclusions can also be drawn by studying the cooccurrence and concentration variation of significant metabolites at various stages of ontogeny in different chemical varieties.

With the high 1,8-cineole variety of Melaleuca species, the 1,8-cineole concentration increases concomitantly with limonene and a-terpineol suggesting that all three cyclic products are derived from a linalyl pyrophosphate derived, enzyme bound moiety containing (31) (Figure 9). Conversely, the terpinen-4-ol variety is richer in congeners a-terpinene, 7-terpinene, terpinolene and terpinen-4-ol, all derived from a moiety such as (32) which is easily obtained from (31) by 1,2-hydride shift. Evidence for such a hydride shift has been provided following investigations on the formation of cis and trans sabinene hydrate in marjoram (Hallahan and Croteau 1989). The product of this shift is now available for further cyclisation to form the cyclopropane moiety (33) which has been implicated in pathways to the thujanes in both marjoram (Hallahan and Croteau 1988) and tea tree (Southwell and Stiff 1989). Tea tree flush growth contains similar monoterpenes to the sabinene hydrate variety of sweet marjoram, Majorana hortensis (Southwell and Stiff 1989). The difference is that in marjoram, the sabinene hydrates remain and do not appear as terpinen-4-ol and y-terpinene in the mature leaf. The fact that steam distillation causes the hydrates to convert to

Figure 9 Chemical structures of likely intermediates in the biogenetic pathways to Melaleuca oil constituents

Figure 10 Possible biogenetic pathways for the formation of Melaleuca terpinen-4-ol type monoterpenene constituents o

Figure 10 Possible biogenetic pathways for the formation of Melaleuca terpinen-4-ol type monoterpenene constituents

Copyright © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Harwood Academic Publishers imprint, part of The Gordon and Breach Publishing Group.

terpinen-4-ol and 7-terpinene has been advantageous for the tea tree oil industry. Distilling flush growth does not give a different quality oil as the precursor sabinene hydrates convert to the end product terpinen-4-ol and 7-terpinene during the distillation in a similar way to the change that takes place as the leaf matures. The inability of marjoram to carry out this conversion as the leaf matures has meant that the quality of marjoram products varies according to the processing method. Consequently extraction methods favour the hydrates and distillation methods terpinen-4-ol (Fischer et al. 1987). The biosynthetic capacity of marjoram seems confined to the synthesis of the sabinene hydrate skeleton in contrast to that of Melaleuca which converts the thujane skeleton to the menthane skeleton as the leaf matures. Whether this change in tea tree is a secondary enzymic transformation of thujanes to terpinenes or the closing down of one biogenetic pathway and the concomitant initiation of the other is yet to be confirmed. The comparative concentration of the metabolites however suggests the former.

Cornwell et al. (1995) distilled young leaves of M. alternifolia in 18O-labelled water to obtain label in the resultant terpinen-4-ol and a-terpineol. This indicated that if terpinen-4-ol (14) was formed directly from cis-sabinene hydrate (24), the process involved hydration of the terpinen-4-yl cation (32) rather than a 1,4-hydroxyl-shift in cis-sabinene hydrate (24) which does not account for the non-oxygenated byproducts. In addition the ratio of these products is similar to the ratio of the same products obtained by the acid catalysed rearrangement of sabinene hydrate. Hence these authors conclude that the sabinene hydrate degradation during ontogenesis is a purely chemical breakdown rather than enzymic secondary metabolism as suggested by Southwell and Stiff (1989). This does not however, address the comparison with marjoram where the sabinene hydrates are retained even in the mature leaf (Fischer et al. 1987, 1988).

By applying these findings to the commercial terpinen-4-ol chemical variety of tea tree oil, biogenetic pathways like those outlined in Figure 10 are likely to be appropriate for monoterpenoid formation.

In addition, Melaleuca oil contains up to approximately 12% of sesquiterpenoids, mostly sesquiterpene hydrocarbons. The flush growth extracts also indicated the presence of sesquiterpene precursors which convert to aromadendrene (16), ledene (17), 5-cadinene (18) etc. as either the leaf matures or the leaf is distilled. Although these precursors have not been isolated and positively identified, a biogenetic precursor like bicyclogermacrene (38) as suggested by Taskinen (1974) for marjoram is plausible (Figure 11) (Brophy et al. 1989b; Ghisalberti et al. 1994).

The terpinen-4-ol chemical varieties of both M. linariifolia and M. dissitiflora would be expected to display similar biogenetic pathways to M. alternifolia. The GC difference in the a-thujene/a-pinene ratios (see above) for the oils of M. alternifolia and M. linariifolia suggests that the thujane pathways differ because of the greater concentration of a-thujene in the latter. A similar difference was also seen in the GC traces of the ethanolic extracts of the flush growth of both species. In addition to the a-thujene/ a-pinene ratio, M. alternifolia gave a cis: trans sabinene hydrate ratio of approximately 7:1 whereas in M. linariifolia the ratio was approximately 0.7:1. These ratios are very similar to those obtained by Hallahan and Croteau (1989) for the sabinene hydrate ratios when both the "natural" (-)-(3R)-linalyl pyrophosphate (36) and the "unnatural" (+)-(3S)-linalyl pyrophosphate (37) precursors respectively were used as substrates for

Figure 11 Possible biogenetic pathways for the formation of Melaleuca terpinen-4-ol type sesquiterpene constituents

sweet marjoram (Majorana hortensis) partially purified cyclase. This suggests that the difference between the two species is significant at the enzyme level where for M. linariifolia the "unnatural" (3S)-linalyl pyrophosphate is better accommodated even though there is minimal difference at the end of the biogenetic pathways especially with the steam distilled oil constituents (Southwell and Stiff 1990). Thus the sabinene hydrate cyclase from M. linariifolia should bind preferentially to the right handed screw form of geranyl pyrophosphate (35) which isomerises to the bound (+)-(3S)-linalyl pyrophosphate (37) in contrast to the M. alternifolia cyclase which should bind to the left handed screw form of geranyl pyrophosphate (34) isomerising to the bound (-)-(3R)-linalyl pyrophosphate (36) (Figure 10).

Determination of oil quality in tea tree seedlings by extraction has thrown some light on the progressive initiation of biogenetic pathways in Melaleuca. At the dicotyledon leaf stage of development, the pinene, cineole and terpinolene pathways are evident with no initiation of the thujane sabinene and sabinene hydrate or the terpinene terpinen-4-ol and 7-terpinene pathways (Russell et al. 1997). a- and fi-Pinene appear to be the products of one cyclase. Croteau (1987) suggested a possible association with myrcene which is not supported by Melaleuca seedling ontogeny where myrcene concentrations increase only after the formation of a- and ^-pinene. 1,8-Cineole is also formed immediately and varies little in the course of development of the seedling. The other products which, like cineole, are possibly derived from moiety (31) (Figure 9) are a-terpineol (15), terpinolene (13) and limonene (8). Terpinolene, which can also be derived from moiety (32) (Figure 9), is present in the dicotyledon leaves in higher proportions than in the mature leaves or the distilled oil. This decreasing proportionality is in contrast to the constant proportion of cineole suggesting some differences in formation pathways. On the other hand, along-the-branch analysis of the terpinolene variety shows an inverse proportion relationship between cineole and terpinolene suggesting that, in this variety, the two are similarly derived (Southwell et al. 1992). Limonene (8) and a-terpineol (15), known to increase in concentration as cineole increases, only appear later in seedling ontogeny (Figure 8).

The most remarkable concentration changes in seedling development occur, as in mature tree along-the-branch analysis, with the thujanes sabinene, cis- and trans-sabinene hydrate and the terpinenes a-terpinene, 7-terpinene, terpinolene, and terpinen-4-ol. These biogenetic pathways seem to be initiated in earnest at a seedling age of around three weeks when the third leaf set begins to emerge (Figure 8). From three weeks to ten weeks both cis-sabinene hydrate and terpinen-4-ol concentrations increase. After ten weeks, the more mature leaves have greater concentrations of terpinen-4-ol than cis-sabinene hydrate (Figure 6).

MELALEUCA OIL STABILITY

Health authorities require shelf life measurements to be made so that "use by..." dates can be printed on health care product labels. For these purposes an understanding of the chemical changes taking place as an oil ages is essential. The first and most obvious chemical change in oil composition to be noted is an increase in p-cymene concentration as the menthadienes a-terpinene (7) 7-terpinene (12) and terpinolene (13) oxidise (Figure 12). The rate at which this change occurs is variable as some oils are remarkably stable over ten years whereas others can oxidise after two years if storage conditions are poor (Table 7) (Southwell 1988; Brophy et al. 1989b). Storage in cool, dark, dry, inert atmosphere (or minimum surface area/volume ratio) and inert containers (stainless steel or tinted glass) ensures optimum stability.

Some aged oils have been reported to be deep yellow in colour before depositing small amounts of oil insoluble crystals. These crystals were found to be 1S,2S,4S-trihydroxy-p-menthane (39) from both M. linariifolia (Jones and Oakes 1940; Davenport et al. 1949) and M. alternifolia (Brophy et al. 1989b) terpinen-4-ol type oils. The formation of this oxidation product occurs concurrently with but by no means as extensively as, the formation of p-cymene from the terpinenes (Figure 12).

Shelf-life trials conducted in our laboratories have shown that the antimicrobial activity of aged oils is not decreased as long as terpinen-4-ol levels are maintained. Indeed some researchers have reported increased activity with aged oils in which terpinen-4-ol concentrations have been maintained (Markham 1996). Consequently the antimicrobial stability of aging oils can be assessed by measuring terpinen-4-ol content.

Table 7 Monoterpenoid composition comparison of aged oils of M. alternifolia

Sample no.

1

2

3

4

5

Age, years

10

10

5

2

1

Rei. deterioration rate

slow

rapid

tapid

rapid

moderate

Composition, %

a-thujene

0.6

0.2

0.8

a-pinene

2.2

3.2

tr

2.0

2.5

sabinene

0.1

tr

tr

/J-pmene

0.6

0.3

tr

0.4

0.7

myrcene

0.5

0.2

tr

0.1

0.7

cr-phellandrene

0.2

tr

tr

0.4

a-terpinene

5.8

0.2

0.1

6.6

p-cymtnt

4.3

32,0

21.7

35.3

8.0

jS-phellandrene +

dneole + limonene

0.4

7.3

2.1

3.1

4.0

y- terpinene

15.0

tr

tr

tr

17.6

terpinolene

2.7

tr

tr

tr

3.1

terpinen-4-ol

41,6

31.5

45.9

23.8

37.3

o-terpineol

3.7

6.4

9.6

8.2

2,9

1,2,4- trih jdroxymenth anc

tr

4.6

2.5

3.6

tr

ISO Standard Status

pass

fail

fail

fail

fail

The overall chemical stability however, is best assessed by measuring a-terpinene, 7 terpinene and ^-cymene in addition to terpinen-4-ol. Chemical stability trials at the Wollongbar Agricultural Institute indicated that well-stored oils, 9-13 years old, only show a little oxidation and actually increase in terpinen-4-ol content. Poorly stored oils however, can oxidise rapidly (even in 12 months) and one 47 year old oil contained 37.7% _p-cymene and 3.2% 1S,2S,4S-trihydroxy-p-menthane (39) while still retaining 26.7% terpinen-4-ol (Table 8). 1,8-Cineole content remains remarkably constant during aging as it does during the different stages of plant ontogeny.

PROCESS MODIFIED OILS

Typical Melaleuca oil compositions are based on steam or hydro-distillations of several hours duration for either laboratory or commercial scale operations. It has already been seen that this composition changes for solvent extraction (especially when flush growth or early seedling leaf is included) (Southwell and Stiff 1989, 1990), for supercritical fluid extraction (Wong 1997), and for headspace analysis (Southwell 1988; Kawakami et al. 1990). The compositions of these process-modified oils are shown in Table 9.

Of commercial significance for oil quality is the compositional variation that can occur using different distillation times. For example Brophy et al. (1989b) found that the first 30 minutes of a laboratory scale-distillation gave a 55.9% terpinen-4-ol oil whereas the remaining 60 minutes gave only 25.1% terpinen-4-ol (Table 9). This trend also exists in commercial-scale distillations albeit within a reduced time-frame (Russell et al. 1997). This variation in

7,12,13 menthadienes 10 p-cymeiie

Figure 12 The oxidation of oil of Melalecua, terpinen-4-ol type constituents

7,12,13 menthadienes 10 p-cymeiie

Figure 12 The oxidation of oil of Melalecua, terpinen-4-ol type constituents

Table 8 Shelf-life tests for tea tree oil chemical stability (25-35°C) at the Wollongbar Agricultural Institute (italics indicate composition of freshly distilled oil)

Age

Percentage composition

( years)

terpinen-4-ol

dneole

p-cymene

a-terpinene

y-terpinene

1S\Z?,45-tribydroxy-p-mentbane

47

26.7

2,2

37.7

0

0

3.2

13

38.7

1.2

5.0

7.4

18.4

0

34.5

13

3.7

9.8

21.2

0

11

34.6

8,9

5.7

7.1

19.3

0

318

9.3

3.4

9.8

225

0

9

40.4

3.5

7.5

5.2

13.4

0.1

39.6

3.1

16

10.4

20.1

0

1

32.0

15.7

11.1

1.9

7.1

0.3

29.0

14.8

5.8

6.1

14.3

0

constituent percentages showed that the more polar constituents distil over before the less polar, lower boiling components suggesting that hydrodiffusion acts to extract more polar components first (Koedam, 1987). More detailed studies of a similar nature were reported by Stiff (1996) for hydrodistillations and by Johns et al. (1992) for steam distillations. Consequently, distillation for reduced periods of time will enhance terpinen-4-ol content and reduce the percentage of terpene hydrocarbons that occur in a completely distilled oil, the former enhancing the concentration of the active ingredient in Melaleuca and the latter reducing the concentration of some possible allergenic fractions (Southwell et al. 1997).

Table 9 Percentage composition of process modified M. alternifolia products compared with a typical terpinen-4-ol type oil

Typical

Head

Flush

Mature

Early

Late

Interface

Water

oil*

space*

extract^

extractc

dist.à

dist.A

fractionc

solublec

Ci-pinene

2.1

4.2

1.5

3.0

1.4

3.5

2.0

tr

Sabinen«

1.6

12.3

5.6

1.5

0.2

0.1

0.2

tr

«-terpinene

10.4

19,8

0.3

9.5

7.8

14,0

4.7

tr

limonene

0.6

2.8

0.5

1.9

un

un

un

tr

^i-cymene

3.8

10.0

0.3

2.8

1.3

1.4

4.7

tr

1,8-cineole

0.5

8.8

4.3

2,2

5.7

4.1

3.8

1.6

/-terpinene

19.4

24.6

6.3

26.4

15.6

29.1

9,9

tr

terpinolene

4.1

4.7

1.4

3.7

2.6

4.8

2.2

tr

terpinen-4-ol

42.9

3.2

7.6

32.9

55.9

25.1

19.2

76.5

a-terpineol

3.9

1.2

0.6

2.3

3.8

2.1

4.3

13.1

aromadendrene

0.2

tr

tt

1.6

0.3

1.2

7.8

tr

ledene

0,2

tr

tr

1.5

0.5

1.5

6.6

tr

5-cadinene

0.5

tr

tr

1.6

0.3

1,2

6.0

tr

/rtttw-sabinene

tr

tr

7.6

hydrate

w-sabinene

tr

tr

39.0

tr

-—

hydrate

aKawakami et al. (1990), "Southwell and Stiff (1989), cRussell et al. (1997), "Brophy et al. (1989b). un, unresolved; tr, trace.

aKawakami et al. (1990), "Southwell and Stiff (1989), cRussell et al. (1997), "Brophy et al. (1989b). un, unresolved; tr, trace.

Some of the more polar components of an essential oil will always dissolve in the condensate. For terpinen-4-ol type Melaleuca oils, these are the more polar 1,8-cineole, terpinen-4-ol and a-terpineol components. Extraction of several batches of condensate with diethyl ether has shown that about 0.07% of oil can be recovered from the water containing the dissolved oil. This oil was made up of approximately 73.7% terpinen-4-ol, 13.2% a-terpineol, 1.6% 1,8-cineole and traces (<1%) of the other alcohols (cis-, and trans-menth-2-en-1-ol, cis-, and trans-piperitol) (Russell et al. 1997). Although distillation condensate is dilute, it may well provide a suitable source of terpinen-4-ol for spray application (e.g. horticultural plant pathogens as investigated by Bishop (1995) and Bishop and Thornton (1997)).

In addition, the interface of the oil and aqueous layers of the condensate sometimes contain distillation debris and is kept separate from the commercial oil. Analysis of this interface oil showed enhanced sesquiterpenes and reduced terpinen-4-ol typical of the "dregs" of a distillation (Russell et al. 1997). Extreme conditions of distillation, especially time and pressure, can "overcook" an oil and give similarly substandard results (Table 9).

As with any distilled oil, composition can be modified by further fractional distillation. This can be the method of choice for preparing 98% pure terpinen-4-ol, for enhancing the terpinen-4-ol content of an oil by removing outlying volatile constituents and for removing the potentially allergenic sesquiterpenoid fraction. 1,8-Cineole however can not be removed in this way without adversely effecting the composition of the oil by removing quantities of desirable constituents as well (Russell et al. 1997).

METABOLISM OF TEA TREE OIL

Little is known about the metabolism of tea tree oil in mammals including humans. The metabolism of the oil by the tea tree plantation pest Pyrgo beetle, Paropsisterna tigrina, has been investigated by examining the frass volatiles from larvae and adults (Southwell et al. 1995). At first, no obvious metabolites were observed when the beetles fed on commercial terpinen-4-ol type Melaleuca plantation tea tree. When higher 1,8-cineole tea tree leaf was the sole diet, (+)-2^-hydroxycineole (40) was isolated as the principal metabolite (Figure 13). Reinvestigation of the frass dropped when the commercial terpinen-4-ol variety was fed, showed that traces of (+)-2^-hydroxycineole occurred in the frass. Other paropsine beetles when fed high cineole diets also metabolised cineole but to other isomers of hydroxycineole. Chrysophtharta bimaculata hydroxylated cineole in the 3a (41), 9 (42) and 2a (43) positions, Faex nigroconspersa larvae in the same positions in different proportions and F. nigroconspersa adults in different proportions again (Table 10, Figure 13). Investigations now need to establish whether these seemingly species-specific metabolites are being used as pheromones for insect communication.

40 2ß-hydroxycineole (35%)

41 3a-hydroxycineole (16.5%)

40 2ß-hydroxycineole (35%)

41 3a-hydroxycineole (16.5%)

42 9-hydroxycineole

CH2OH 43 2a-hydroxycineole (27.4%)

42 9-hydroxycineole

42 9-hydroxycineole (36.2%)

CH2OH 43 2a-hydroxycineole (27.4%)

42 9-hydroxycineole (36.2%)

Figure 13 The chemical structures of metabolites of 1,8-cineole

Table 10 The proportions (%) of 2a(43), 2fi (40), 3a(41) and 9 (42) hydroxycineoles detected in the frass volatiles of paropsine beetles feeding on high cineole Melaleuca leaf

Paropsisterna

P. tigrina

Faeax nigro-

F. nigro-

Chrysophtharta

tigrina

larvae

conspersa

conspersa

bimaculata

adults

adults

larvae

adults

2a-hydroxycLneole (43)

1.7

2.0

11.4

27.4

0.6

2^-hydroxycineole (40)

38.7

39.8

0.5

0.5

0.8

3 a-hydroxycineole (?) (41)

1,6

1.9

6.7

4.3

16.5

9-hydroxyrineole (42)

1.7

0.7

36.2

5.2

3.4

OTHER USES OF TEA TREE

OTHER USES OF TEA TREE

General Uses

Most of the other additional uses of Melaleucas are based on the physical or aesthetic rather than the chemical properties of the wood (used for construction), branches (for broom fences), bark (for bark paintings, art) or entire tree (in windbreak, landscaping, swamp reclamation) (Wrigley and Fagg 1993).

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